classification and ordination of coastal sand dune vegetation along the gulf and caribbean sea of...

Vegetatio 66: 147-182, 1986 147 © Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands

Classification and ordination of coastal sand dune vegetation along the Gulf and Caribbean Sea of Mexico*

Patricia Moreno-Casasola I & Ileana Espejel 2,** llnstitute of Ecological Botany, Uppsala University, Box 559, S-751 22 Uppsala, Sweden. Permanent address." Laboratorio de Ecologia, Facultad de Ciencias, UNAM, Mexico 04510, D.F., Mexico; 2Institute of Ecological Botany, Uppsala University, Box 559, S-751 22 Uppsala, Sweden. Permanent address: Centro de Recursos Bioticos de la Peninsula de Yucatan, INIREB, Apdo. Postal 281, Merida 97000, Yucatan, Mexico

Keywords: Beach vegetation, Caribbean Sea, Classification, Coastal sand dune, Community type, Dune vegetation, Embryonic dune, Foredune, Gulf of Mexico, Mexico, Ordination, Zonation

Abstract

A classification is presented of community types found in the coastal sand dune systems along the Gulf coast and the Caribbean Sea of Mexico. Twenty-eight dune systems were sampled along transects using the Braun-Blanquet approach. A total of 4444 relev6s were subjected to agglomerative classification and table arrangement (program TABORD). Synoptic species values were calculated for the 396 clusters obtained. They were arranged in two new data sets, one comprising the material from Tamaulipas and Veracruz and the other Yucatan and Quintana Roo. These data sets were again subjected to classification and ordination (detrended correspondence analysis) programs. Final community types are based on the basic clusters, 59 types for the Gulf area and 31 for the Caribbean. Their geographical and ecological distribution is interpret- ed.

The description of types includes information on differential and dominant, as well as frequent accompanying species, and also on some structural characters. The results were compared with a parallel study of the central Gulf area, including Tabasco and Campeche.

Six main distribution patterns of the community types were found: 1) along the coast (mostly pioneer communities), 2) northern section of the Gulf, 3) northern section of the Gulf and the Caribbean, 4) mainly restricted to the Gulf, 5) intermediate patterns within Tabasco and Campeche and 6) restricted to Yucatan and Quintana Roo.

The ordinations showed both local zonation patterns directly related to dune physiography and regional patterns produced mainly by soil and climate differences. Both for the Gulf and the Caribbean material axis 1 reflects a sea-inland gradient from pioneer types towards thickets on the fixed dunes. Axis 2 shows a geographical transition from Tamaulipas to Veracruz types in the Gulf material and from Yucatan to Quintana Roo types in the Caribbean case.

The Gulf types can be grouped into: 1) Tamaulipas types with species in common with Texas and the south-

** This research received support from CONACYT PCECBNA 005223 and CONACYT PCECBNA 005238. We are grateful to S. Castillo, E Rodriguez, J. Carabias, S. Guevara, J. Popma, V. Rico-Gray; Facultad de Ciencias (UNAM), Centro Cientifico IBM, Centro de Recursos Bioticos de la Peninsula de Yucatan (INIREB), Centro de Investigaciones de Quintana Roo, Swedish Institute, and Uppsala University for scientific and economic

support. We especially want to thank E. van der Maarel for his constant support and advice. Finally we acknowledge the thor- ough review of the manuscript by H. Doing and an anonymous reviewer, and the stylistic improvements by E. Sj6gren. * Nomenclature follows Sosa e t al. (1985) for the Caribbean and Standley & Steyermark (1946-1977) for the Gulf.

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eastern United States, (Uniolapaniculata). 2) Veracruz and Tabasco types, with many thicket species in common with inland vegetation types (Randia laetevirens, Coccoloba barbadensis and Citharexylum berlan- dien3.

The Caribbean types can be grouped into: 1) Yucatan types with species from drier habitats and characterized by the endemic Enriquebeltrania crenatifolia, Agava angustifolia and halophytes like Bravaisia tubiflora, Pithecellobium keyense and Suaeda linearis; 2) Quintana Roo types with typical Caribbean communities with Erithalis fruticosa, Coccoloba uvifera, Ernodea littoralis and the palms Pseudophoenix sargentii and Thrinax radiata.

Introduction

The Atlantic coast of Mexico extends over ca 2000 km. Coastal ecosystem types include mangroves, brackish marshes, cliffs, shingle and limestone beaches, sandy beaches, ridges and sand dunes. The latter three types make up more than half of the coast line.

Plant communities within these systems vary greatly both locally and regionally. The extent and orientation of the area, mobility of the sand, depth of the water table, management and use of the vegetation are among the more important factors causing a complex of many different plant communities. From north to south there are two additional major large-scale gradients, a climatic and an edaphic one. The latter includes a change in sand from light coloured quartz sands mixed with calcareous elements in the north, via dark, mineral sands in the central region of the Gulf, towards fine calcareous sands, rubble and limestone in the Caribbean (Sauer, 1967). This gradient is associated with a climatic change in precipitation from 700 to 3 300 mm per year and in extreme minimum temperatures from - 3 °C in the north of Mexico to 16 °C in the central areas (Rzedowski, 1978; Direc- ci6n General de Geografia del Territorio Nacional, 1981).

There is little specific information on the plant communities of these coastal sand dunes (Poggie, 1962; Sauer, 1967; Puig, 1976; Moreno-Casasola et al., 1982; Espejel, 1984, 1986a; Castillo, 1984). It was considered interesting from an ecological point of view and urgent from a conservation and land use perspective to systematically describe and clas- sify the plant communities, using specially collected data over most of the dune systems along the Gulf of Mexico and the Mexican Caribbean Sea.

Description of the area

Climate

In general the climate belongs to Koeppen's type A (tropical forest climate; monthly average always above 18 °C). There are some exceptions: the northern region bordering the United States has a type BSx' climate, and part of the northern coast of Yu- catan has a dry BSs climate. Most of the climatic stations involved have records of a mean annual temperature of over 22°C. Only the stations of Tamaulipas show lower average values (18 °C); with occasional decreases to zero (Puig, 1976; Rzedowski, 1978).

Precipitation is seasonal with the summer as the main rainy season, but in several stations winter rains may make up more than 10% of the total. The lowest annual precipitation occurs in the north of the Yucatan Peninsula (450 mm). The state of Tamaulipas, and the northern parts of the states of Veracruz, Campeche and Quintana Roo receive 1000-1 500 mm, and the central and southern part of Veracruz and Tabasco receive 1 500- 3 500 mm of rain each year (Fig. 1).

In most areas a double summer rain maximum occurs: an early peak in June-July is followed by a major peak in September or occasionally October. Only in the central part of Veracruz the highest peak occurs in July. Thereafter, precipitation gradually decreases to a March-April minimum. The dry season is only dry in a relative sense in the more humid areas, since these drier months still receive ca 30-40 mm of rain.

The 'nortes' are atmospheric disturbances, restricted to the Gulf of Mexico and surrounding areas, which modify the climatic characteristics of the cool season. Temperatures can drop as much as

149

'AULIPA$ GULF OF MEXICO

2 2 o

VERACRUZ ~ • I l l /

',}. l/ .,.,D , , f C A M P E C H E ~ " o

V e r s e r u z t ~ . V / ~

Fig. 1. Map of the east coast of Mexico showing study areas. Areas are grouped in sections (Roman numerals) defined by differences in climate, type of sand and other characteristics. 1 = Barra del Tordo, 2 = Barra Chavarria (north), 3 = Barra Chavarria (south), 4 = Bocatoma, 5 = Riachuelos, 6 = El Raudal, 7 = Faro del Morro, 8 = La Mancha, 9 = Rancho E1Quijote, 10 = Paso Dofia Juana, 11 = Playa Chalchihueca, 12 = Rinc6n del Pirata, 13 = Playa Azul, 14 = Las Gaviotas, 15 = Rancho E1Capricho, 16 = Playa La Trocha, 17 = Sontecomapan, 18 = Monte Pio, 19 = Celestfin-Sisal, 20 = San Benito, 21 = Telchac, 22 = Dzilam de Bravo, 23 = Rio Lagartos-Las Coloradas, 24 = E1 Cuyo, 25 = Punta Blanca, 26 = Puerto Morelos, 27 = Capit~in Lafitte and 28 = Sian Ka'an. Full descriptions of the 28 sites are available upon request. For a summary, see Appendix.

10°C and rain fall dur ing this season can a m o u n t to 1 0 - 1 2 % o f the to ta l annua l prec ip i ta t ion . Usua l ly 2 0 - 2 5 'nor tes ' o r f ronta l s torms are ex- per ienced between Oc tober and March (Thom,

1967). W i n d is an i m p o r t a n t aspect o f the ' no r t e s ' .

More t han ha l f o f t hem are a c c o m p a n i e d by winds with velocit ies over 40 k m / h . A l o n g the coast , high energy waves are genera ted by these s torms, and subs tan t ia l m o d i f i c a t i o n o f the beach prof i le occurs (West et aL, 1969). These no r the r ly winds are respons ib le for the moving o f large quant i t ies o f sand. This accounts for the no r the r ly o r i en ta t ion o f dune systems on the no r the rn leg o f C a b o Rojo (Poggie, 1962) and in La M a n c h a (Moreno- Casaso la , 1982, 1986) a m o n g others.

There is also cons iderab le var ia t ion in the water level a long the coast . The d o m i n a n t force affect ing water levels in the Tabasco region is no t lunar t ides bu t seasona l var ia t ions in ra infal l which affect s t ream discharge. H igh water levels occur dur ing the rainy season and last t h rough the 'nor te ' per iod. Dur ing this per iod , ove rbank discharge causes the f lood ing o f in te rd i s t r ibu ta ry bas ins and l agoon

waters spread into ad jacen t low-lying swamps and marshes . Per iodic 'nor te ' s to rms tend to rise the sea level by as much as 1 m. (Thom, 1967). These p h e n o m e n a occur a long mos t o f the G u l f and C a r i b b e a n coasts. Also, the high water sea level caused by the 'nor tes ' injures the p ioneer p lan ts and accounts for the l andward retreat o f the vegeta- t ion (Poggie, 1962). A n o t h e r i m p o r t a n t c l imat ic feature which affects coas ta l vegeta t ion are the hur- r icanes which some years can be very des t ruct ive al- tera t ing coral reefs, mangroves and devas ta t ing dune vegetat ion. Nevertheless, dune vegeta t ion

recovers r e ma rka b ly well in a few years (S toddar t , 1969).

Physiography

The central and no r the rn par t o f Mexico form a

southward extension o f the m a j o r physical units o f the western par t o f the Uni ted States. The Texas Coas ta l P la in has its coun te rpa r t in the Gu l f Coas t - al Plain. The Rockies con t inue into the Sierra M a d r e Orienta l . The t ransverse zone o f volcanic peaks , known as the Sierra Volcanica Transversal

150

terminates this alignment of north-south structural units. Between 18 and 200 N a different pattern is found. A pattern of east-west ranges, island arcs, limestone platforms and all the associated phenomena of tectonic instability appear linking it physically with Central America and tke West- Indies (Butland, 1960).

Physiographically the area can be divided in two provinces (Butland, 1960; Rzedowski, 1978):

a. Gulf Coastal Plain. This province arose during the Tertiary. It stretches with two minor breaks from Florida to Yucatan. The interruptions consist of two volcanic areas, the Sierra Volcanica Trans- versal in the north of Veracruz, and the Volcanoes of Los Tuxtlas, in the south of Veracruz. Many rivers discharge their sediment loads into the coastal lagoons and on the continental shelf. The sand of this region is a light coloured quartz. The central and southern areas in Veracruz are situated in a volcanic region. The sand here is of a dark colour, with numerous mineral elements. Further south in Veracruz as well as in the states of Tabasco and part of Campeche light tan quartz sand is again found (Castillo, 1984; Moreno-Casasola et al., 1982; Sauer, 1967).

b. Yucatan Peninsula*. The province is clearly different from the rest of Mexico. Its origin is more recent since it emerged during the late Tertiary and is still uplifting. The Peninsula forms a fiat limestone region surrounded by coral reefs which end at the border with Campeche (Fig. 1) but again can be found in the Gulf forming reef islands in Veracruz. The northern part of Campeche and Yucatan has coarse calcareous sand with fragments of shells, the eastern coast of Quintana Roo has very fine sand with coral sand and rubble. These coasts are more similar to those of Florida and the Caribbean islands (Espejel, 1984, 1986b; Sauer, 1967, 1982). A strik- ing characteristic of the Peninsula is the absence of superficial rivers. Typical subterranean streams emerge at several points of the coast as fresh water springs (Miranda, 1959; Barrera-Marin, 1964; Rzedowski, 1978).

The coastline changes its orientation, combined with a gradual narrowing and widening of the continental shelf, this results in great contrasts in ex-

*The whole of the peninsula of Yucatan will further be called 'Peninsula' here, the State of Yucatan simply 'Yucatan' (see Fig. 1).

posure to the prevailing winds and waves (Sauer, 1967). These differences in exposure of the coast affect the width of the coastal pioneer habitat (Poggie, 1962) and contribute to the regional diversity of the littoral.

The littoral is also modified by the presence of coastal lagoons and inundated plains, which in many cases limit the eastward or southward extension of the dune systems, forming barrier beaches. These lagoons themselves have mangroves or halophytic herbaceous vegetation. This situation is very common all along the coast except in the central part of Veracruz, where, although several lagoons are present, the dune systems are very wide. The tidal range is small in the whole area.

In the state of Quintana Roo, limestone rocky places known as 'caletas' (inlets) have a mixture of pioneer sand dune vegetation and rain forest. This complex differs considerably from the sand dune vegetation proper and was not included in this study.

Methods

Areas sampled

Twenty eight areas were sampled (Fig. 1). They were selected to include the different types of dune systems occurring in the region: narrow beaches, wide beaches, coastal ridges and extensive sand dune systems in mobile, semimobile and/or fixed state. Systems heavily grazed by cattle or with coconut plantations that were being taken over by the coastal thicket were not sampled; an exception was made for the Quintana Roo coasts because abandoned coconut plantations cover almost all the sandy coasts. The areas have been arranged geographically, defining sections through differences in climate, type of sand and characteristics of the system. (Detailed vegetation analyses are found in Espejel, 1984, 1986a; Garcia, 1982; Garcia, 1984; Moreno-Casasola et al.; 1982; Moreno-Casasola et al., mscr., and Rodriguez, 1984).

Methods o f sampling and analysis

Along the Gulf coast, extensions from 2 to 6 km along the beach were sampled; in the Caribbean region larger areas were considered. Relev6s were

made mainly along transects perpendicular to the coast according to the Braun-Blanquet approach (Mueller-Dombois & Ellenberg, 1974; Westhoff & Van der Maarel, 1978). Care was taken to sample every topographical unit present. Relev6 size varied from 2 × 2 m (in the wet slacks) to 10 × 10 m in the coastal thicket. On the beach, plant cover is scattered. Some species occur in patches surrounded by bare sand; in such cases relev6s were made for each of the patches, usually 4 - 2 5 m 2 in size, for example Sesuvium portulacastrum. In the case of beach pioneers with runners like Ipomoea pes-caprae and Sporobolus virginicus which may cover larger areas, relev6s were of a bigger size. The following data were collected: quantitative species composition for each stratum using an ordinal scale (Westhoff & Van der Maarel, 1978) (see Table 1), number of strata and dominant species in each one, slope and orientation, percentage of bare sand, distance to the sea, depth of water table, topography of the dune or the beach where the relev6 was taken. Notes related to magnitude, degree and type of disturbance for the different areas were taken. We tried to obtain at least ten representatives of each physiognomically distinct vegetation type for each area.

Provisional phytosociologial tables were made for each area and the groups found were checked in the field. I f necessary additional relev6s were taken and included in the data set. The relev6s of each area were subjected separately to the classification and table structuring program TABORD (Van der Maarel et al., 1978). A fusion limit of 0.50 was found the most adequate (only in Monte Pio 0.60 was considered better). A fusion level of 0.70 result- ed into too many small clusters, each showing particular variations for an area. To obtain homogene- ous clusters a threshold of 0.10 to 0.40 was used. Minimal cluster size between two and four was used, as this would not allow very particular cases in each area to come out as a small cluster. Most clusters were formed by 5 to 20 relev6s.

A structured table was then obtained with constant species at a minimum frequency level of 0.60. The structured table obtained previously provided the initial cluster array the program TABORD re- quired. Relev6s moved to the residue were not used further on in the analysis, with the exception of initial clusters with only one relev6, for instance Sesu- vium portulacastrum of which we were sure that

151

they were represented with more relev6s in other areas. Such relev6s were included in the table as 'one-relev6 clusters'. A synoptic cluster value was calculated for each species in each cluster in each of the tables. This 'synoptic cover-abundance degree' was calculated as the product of the mean cover-abundance value and cluster frequency (Van der Maarel et al., 1986). This value was then round- ed off again to an ordinal value from 1 to 9. A new raw table was elaborated for each region using the newly formed basic clusters as units. As the floristic composit ion of the Gulf and the Caribbean appeared to be different, it was decided to treat the two tables separately, thus simplifying computer work.

The number of species in the Gulf data set reached more than 350, in the Caribbean more than 250. Many of them appeared only once or twice in one area. It was decided to reduce the final table by deleting such species and species with a similarly low occurrence as follows: first, species with a synoptic cover-abundance value under 0.05 were already automatically deleted during the calculation. This did not sufficiently reduce the number of species. Second, species that appeared less than four times in a local table and were not restricted to one or two particular clusters were also deleted. This reduced the species number in either regional table to ca 150.

With the provisional structured table as a basis for the initial clusters each regional table was then subjected to a new round of the program TABORD. The synoptic table for the Gulf contained 277 basic clusters including 3 306 relev6s. The Caribbean table contained 119 basic clusters including 1 138 re- lev6s. A fusion limit of 0.50 was used as an overall option to obtain an optimal number of basic clusters. No threshold was applied.

These groupings of basic clusters were then floristically described as community types, by means of one or more characteristic species having a high constancy in the cluster and a lower one in most other clusters. Usually they also have a high cover-abundance value. In addition the physiognomy and vegetation cover of the type and its habitat were described.

No formal hierarchical classification and syntax- onomical system is developed because: 1) there is not enough information on the geographical distribution of species and the geographical variation

152

within community types, 2) the environmental parameters have not been thoroughly studied, and 3) the Mexican coastal dune flora is not well known yet.

Finally, in order to help ecological interpretation of the community types Detrended Correspon- dence Analysis, DCA (Hill, 1979a; Hill & Gauch, 1981) was carried out for each set of basic clusters, Gulf and Caribbean. Community types as well as the geographical regions were superimposed on the resulting diagrams.

Example of the analysis of one site

Table 1 shows an example of a structured table with relev6s forming clusters for the area Monte Pio, with a narrow beach and a coastal ridge in southern Veracruz, represented in Fig. 2. The first four clusters include beach pioneer communities. They mainly inhabit the foreshore and backshore but may climb up the lower part of the ridge. lpomoea pes-caprae is the outmost pioneer species for this area. Croton punctatus and Oenothera

Table 1. Structured table obtained with TABORD for the area of Monte Pio, in the south of Veracruz. 1 = one or few individuals, 2 = occasional and less than 5°/0 cover, 3 = abundant with 1-2°70 cover or less abundant with 3 - 5 % cover, 4 = very abundant and less than 5°70 cover, 5 = 5 - 1 2 % cover, 6 = 12 .5 -25% cover, 7 = 2 5 - 5 0 % cover, 8 = 5 0 - 7 5 % cover and 9 = 7 5 - 1 0 0 % cover. (independent of individuals number) (Westhoff & Van der Maarel, 1978). Species used in names of vegetation types are set in bold-face.

NUMBER OF RELEVE:

BASIC CLUSTER CODE:

Iponmeapee-cRprae Sporobolus virginicus

Iponme8 s to loni fera Oenothera drummondii Croton punctattm Cyperus art iculatus B i d e n s p i l o s a Vigna luteols Verbesina persicifolia R a n d i a l a e t e v i r e n s Chiococca slba

1 1 1 1 1 2 1 2 2 2 2 2 3 3

9 1 2 3 4 1 2 3 4 5 6 7 8 6 0 0 1 2 3 4 5 5 6

1 2 3

8 7 8 2 2 . 2 1 5 . 3 2 6 . . 6 . . . . . . .

. 7 9 4 6 . . . . . . . . . . . . . . 3...

. . . 1 . 5 5 6 6 6 5 5 5 4 6 . . . . . . 7 6

. . 6 3 2 . . . . . 2 . . . . 6 8 8 8 9 7 5 7

. . . . . . . . . . . . . . . . . . . . 5 . .

2 . . 2 1 . . . . 2 5 4 . 4 5 . . . 3 2 5 2 .

. . . . . . . . . . . . . . . 4 . . . . . 9 7

............... 5 ..... 5 1

...................... I

1222233333 1 1 1 4 4 4 5 6 3 3 3 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 6 6 6 6 6 6 6 7 7 7 7 6 6

9 6 7 8 9 0 1 2 3 4 5 7 8 2 3 4 1 3 7 8 9 0 1 5 6 7 8 9 0 2 3 4 5 6 7 8 9 0 1 2 4 5 6 8 0 1 2 3 7 9

4 5 6 7 O

. . . I . . . . . . . . 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 . 2 . . . 4 . . . . . . 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . . . . . . . . . . . . . .

5 . . . . . 3 . . . 1 . . 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 . . . . . . .

6 8 9 9 8 9 4 7 8 5 , . . 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7 . . 1 . . . 2 . . 6 6 6 3 . 2 . . . . 1 . . 5 . . . . . . 1 . . . . . . . . . . . . 3 . 4 . . . .

. . . . . . . . . . . . 8 7 7 7 8 . . . . . . . . . . . . . . . . . . . . . . . ) . . . . . . . .

. . . . . . . . . . . . 1 2 1 4 3 . . . . . 2 3 . . . 2 . . . . . . . . . 6 . 3 . . . . . . . . .

. . . . . . . . . . . . 2 6 5 6 9 8 5 1 7 8 8 9 3 3 4 8 7 7 7 5 9 8 9 8 9 8 9 8 9 8 1 7 . 6 8 7 1

. . . . . . . . . . . . 9 9 8 9 7 9 9 9 9 9 8 9 9 9 8 8 9 8 8 9 5 8 6 7 . 8 8 7 4 7 7 7 6 8 5 . .

. . . . . . . . . . . . . . . 6 . 6 6 5 5 . . . 2 5 . 4 4 4 . . . 6 . 5 5 8 5 3 4 . . . 2 4 . . .

P a p p o p h o r u m p a p p i p h e r u m . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2...

Coccoloba b a r b a d e n s i 6 ................................

C i t h s r e x y l u m b e r l a n d i e r i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

s p e c i e s 7 9 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

P a n i c u m a m a r u m . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Opun t i a 6 t r i c t a . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . .

Arundo donax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Ambrosia Br tem is i i f o l i a . . . . . . . . . . . . . . . . . . . . . . . . . . . I . . . .

Amsranthus greggii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Sm i l ax sp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Pass i f l o rB f oe t i da . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I r es i ne ce l os i a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Legum inosae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

S e r j a n i a T a c e m o s a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . 3 . 5 . 1 . 1 . 6 4 . . . . . . . . . . . . . . . . . 1 . . . . 7 .

. . . . . . . . . . 7 . . 6 . 7 . 6 5 . 1 8 . . 6 5 7 . 6 . 8 8 9 8 9 . 9

. . . . . . . 6 . . . . 4 . 9 5 . . . . . . 4 7 2 . 5 7 . . . . . . . . .

. . . . 1 . . . . . . . . 4 . 1 . . . . . 5 4 . . 5 2 3 . . . . . 4 . . .

. 3 . . 5 . . . . . . . . . . . 2 1 . . . . . 2 . 2 . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . .

. . . . . . . . . . . . . . . . . . . . . . . . 3 . . . . . . . . . . . .

. . . . . . . . . . . . . . 5 4 . . . . . . . . . . . . 2 . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . 3 1 1 . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . ~ 2 . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . 3 . . . . . . . 3 . . . . . . . . . . . . .

10 20

. . . . . . . . . . . . I I []

. . . . . . . . . . . . . I - - 1

3, Of.NOTHE~ DeUMMONRI

4 CROTON pU~C'rAll~

5. CVpEnus AnT~ULATUS

6. RANmA L ~ V ~ N H m E N S ~OSA

7, RANOL~ LAETEVIRENS-VERRESINA PER@I@mOLIA~COCCOLOSA BARBADEN@i@

30 ~0 (m)

[] F - ' l l I

F"'I Z3

[] I I

C] [] [] F"'I

I

Fig. 2. Vegetation profile at Monte Pio showing the distribution o f seven community types along a transect of 45 m. Species composition and cover-abundance values are found in Table 1 and synoptic values in Table 2.

drummondii occupy the upper part of the beach, well above the high tide line. Cyperus articulatus, a species of slacks, occurs here outside its normal range. The remaining two clusters are dense thicket types. Verbesina persicifolia, Randia laetevirens, Citharexylum berlandieri and Coccoloba barbadensis are very abundant shrubs that occupy the slope of the ridge facing the sea; most of the top parts of the plants have been damaged by the action of the wind and sea spray. In the upper part of the ridge C. barbadensis is very common. Vigna luteola and Bidens pilosa appear in the more open parts of the thicket. The back part of the ridge is being used for grazing. Monte Pio is an area with a simple topography with small variations in environment and few species and community types. Fig. 2 shows a topographical profile with the distribution of the types indicated.

Table 2 shows the corresponding basic clusters and their synoptic cover-abundance degrees. While comparing Tables 1 and 2 we may appreciate that the synoptic values are more realistic than either mean frequency or mean cover. Clearly dominant species having high cover values and high frequen- cies, are weighted to the extent that they will out- weight both species that have high cover values but are less frequent, for example Coccoloba barbadensis and Chiococca alba, and frequent species with low cover-abundance such as Vigna luteola in the

Table 2. S y n o p t i c table o f the bas ic clusters f r o m M o n t e P io

o b t a i n e d f r o m Table 1.

BASIC CLUSTER NUMBER: I 2 3 4 5 6 7 O

CLUSTER SIZE: 5 10 8 10 3 5 30 2

Ipomoea pes-esprae 6 2 1 1 Sporobolus vi rg in icus 5 I i Ipmmeastolonifera 1 6 1 1 . . . . OenotheradrLmme~dii 2 I 8 I I I Croton pm~tatua I 8 I Cypertm artieulatus I I I I 6 I i Bidenspil~ 1 8 Vigna luteola I 3 Verbesina persicifolia 1 6 7 Randia l a e t e v i r e n e . . . . . 9 B Chiocoeca alba . . . . . 1 2 Pappophorum pappipherum I I I ½ Coeeoloba barbadensis . . . . . . 2 3 Citharexylum berlandieri . . . . . . I species 791 . . . . . 1 1 P a n i c u m amarum . . . . . 1 1 Opuntia strieta 1 . . . . . A r u n d o d o n s x 1 . . . . Ambrosia artemisiifolia . . . . . . . . Amaranthus greggii 1 Smila~sp. i i i : . . i Passiflora foetida . . . . . . 1 Ir.si . . . . losia i : ~ ~ i i i i Leguminosae Serjania racemosa . . . . . . 1

153

basic clusters 4 and 6. This is considered realistic, since the dominance of a few species is a clear character of these pioneer communities. Such a clear dominance is lost in the thickets more inland.

The cluster with the code '0' corresponds to the residue obtained in Table 1. This and similar residue groups were disregarded further on.

A general framework for describing dune vegetation

Doing (1981) proposed a framework for comparing temperate coastal sand dune zonations. Later he used this scheme to compare temperate and tropical 'foredune complexes' (Doing, 1985). A simplified version of this scheme can also be applied to tropical coastal dune systems to allow com- parisons among them.

Six zones have been defined on the basis of sand movement, salinity and depth of water table. Varia- tions in these factors coincide with topography which has been considered one of the major controlling factors in vegetation distribution (Dansereau, 1957; Judd et al., 1977; Martin, 1959; Van der Maarel, 1966). The distribution and structure of community types along these zonations is shown in Fig. 3A-E.

A. Outer zone in which sand movement and salinity are the major factors. The importance of sand movement has been emphasized by several authors (Barbour et al. , 1985; Martin, 1959; Moreno-Casasola, 1985, 1986; Olson, 1958; Ranwell, 1958, among others). Only specialized species with sufficiently persistent lateral and verti-~ cal growth are able to establish and survive. Air- borne salt is also an important factor in determin- ing plant distribution (Barbour & De Jong, 1977; Boyce, 1954; Etherington, 1967; Martin, 1959; Oosting, 1945; Oosting & Billings, 1942; Randall, 1970; Sloet & Heeres, 1969; Van der Valk, 1974). This area includes several zones:

I. Beach includes foreshore and backshore and is subjected to occasional flooding when overwash occurs; this increases soil salinity. On the beaches there are several factors preventing sand movement; the tide drift material (algal and coral) deposited over the sand, the winds are not very strong in this zone, and as the sand moisture is high the winds are unable to transport the grains. Few annuals consti-

154

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tute the communities that develop on this zone. II. Embryonic dunes and foredunes. These dunes

are formed on the beach as a result of the interac- tion of wind and vegetation occurring both on wide and narrow beaches. Both zones share many grass, herb and even woody species; frequently it is hard to separate them on a floristic basis. Where active dune building takes place, most communities are monospecific (Doing, 1981). At the Gulf coast both zones share the same species, unlike in the Caribbe- an, where embryonic dunes (IIa) are formed by the

presence of succulent creepers forming 'enrich- ment' types and on the main foredune woody species dominate (IIb). If these communities occur on limestone rocky beaches the zone corresponds to a salt-spray scrub.

III. Blowouts (open, sterile sand in secondary or older dunes, cf. Doing, 1981). They develop where the vegetation cover of unconsolidated coastal dunes is destroyed or removed so that sand is no longer held in position. This may lead to parabolic dunes, a frequent situation in the wide dune sys-

156

tems of central Veracruz. The organic matter content is very low (0-0.07°70 in La Mancha, Moreno- Casasola et al., 1982), and temperature fluctuations at the soil surface are very high (with maxima of up to 60 °C, Moreno-Casasola, 1982). Few communities, among them some beach pioneers, are able to grow under these conditions of high sand mobility and low salinity.

IV. Humid and wet slacks. They include zones in which the ground water table never falls more than 1 m below the surface so that plant roots have an adequate moisture supply year round. They also include environments flooded for several months, and sites under the influence of coastal lagoons, in which a saline or brackish environment is main- tained. Overwash areas are not as common at the Gulf and Caribbean coasts as they are along the southeastern coasts of the USA (Bellis, 1980) where a Spart ina community establishes. At the edge of the mangroves the ridges remain low because of a similar phenomenon of 'overwash'. Succulent herbs, grasses, Cyperaceae and mangrove trees, sometimes mingling with beach species form the community types of these zones.

B. Inner zones with soil development where ex- position and moisture are the main abiotic factors. Soil development where succession proceeds, degree and type of disturbance (Sousa, 1984), availa- bility of propagules and vital attributes of the cies (Guevara, 1982; Espejel, 1986; Moreno- Casasola, 1985) are among the main factors affecting the distribution of plant communities.

V. Sheltered zone behind the foredunes. This zone is nearly always developed adjacent to the leeward side of the main foredune ridge; sand which has not come to rest in the first two areas is deposited in this zone because of the reduction of the wind force by surrounding dunes. This zone is often very different from the other zones and its communities are quite distinct, with several characteristic species (Doing, 1981). It is an intermediate zone where environmental factors show transitional values, i.e. between a maximum in the beach and a minimum in the stabilized zones. Grasses, forbs and low shrubs predominate. In the Caribbean region this zone is highly disturbed by roads and coconut plantations.

VI. Fixed dunes. The vegetation is largely independent of the ground water table. Sand movement, air salinity and water table do not influence

plant distribution any longer. As the Caribbean ridge system is low, most of the species forming the fixed dune communities use the ground water, but they are not tolerant to sand movement or salt spray. Doing (1981) gives some examples of communities on stabilized (not characterized by instability of the soil surface) temperate dunes and considers them determined by pr imary or secondary succession and development of the soil profile. We do not have enough information to apply this framework to our systems. Instead our types will be tentatively organized by growth form. Fixed dune vegetation comprises communities of herbs and low shrubs, grasslands and thickets.

Community types of the Gulf region

The Gulf area includes 58 community types, 16 of which are based on only one basic cluster, 6 on two and 36 b y 3 - 1 7 basic clusters. They will be described in the following sections: beach, embryo- dune and foredune, blowouts and active dune, wet and humid slacks and dry dune.

L Beach vegetation

The 4 types of this section (Table 3; Fig. 3 A - C ) establish on the foreshore and backshore. During the nortes and hurricanes they are damaged by inundation with sea water and flooding of closeby rivers. They are composed of annual species. The roots of these species help to bind sand but no embryo dunes are formed.

1. Cakile edentula type. This type is formed by 2 basic clusters (bcl) including a total of 7 relev~s (reD. This type is entirely dominated by the name-giving species. This herb is one of the few annuals on these shores. It is only loosely associated with a few other pioneers like Sporobolus virgin&us, growing as isolated individuals. The type has a simple structure with only one layer, 10 to 25 cm high. It grows close to the high tide mark on the beaches of Tamaulipas.

2. Cakile geniculata type. (l bcl; 9 rel). It is the only species characteristic of the type and forms an open herb cover, 15 to 30 cm high, forming a discontinuous community• It also grows close to the high tide mark in the north of Veracruz and Tamau- lipas, although it is not very common.

3. Aman.;,thus greggii type. (5 bcl; 26 reD. This pioneer type has only one characteristic and dominant species: A. greggii. Its distribution is discontinuous. The dominant species is mostly associated with Ipomoea stolonifera, Sporobolus virginicus and Okenia hypogaea• This community is also one-layered, and 10

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to 40 cm high. It grows higher up on the beach than the Cakile types. 4. Okenia hypogaea type. (3 bcl; 16 rel). This is also a discon- t inuously distributed type with only Okenia hypogaea, a Nyc- taginaceae, as the characteristic species. Okenia is a postrate, annual herb, with stems up to 2 m growing as isolated individuals, al though able to cover a little more than 4 m 2. It is frequently associated with other pioneer species. It forms only one open 5 - 10 cm high stratum. It grows on the backshore and can penetrate towards the foredunes. It can also be found in washovers among the dunes.

II. Embryo dune and foredune vegetation

Most of the types found on embryonic dunes also appear on the foredunes (Table 3; Fig. 3A- C). Their constituent species can tolerate sand accretion and trap sand. Especially during the 'nortes', strong winds produce considerable sand movement (Moreno-Casasola, 1982). Some of these types can also be found closer to the high tide mark (Sesuvi- um portulacastrum) or creeping towards the thickets in the sheltered zone behind the foredune (Canavalia rosen). They tolerate salt spray. Stoloniferous growth forms predominate among sand binding species, but one can also find rhizomatous and shrub forms which grow right down to the back of the beach. This may reflect the lower rates of sand movement by wind and the reduced exposure to waves at the rear of tropical beaches when compared with temperate ones (Davis, 1980).

5. Sesuvium portulacastrum types. (10 bcl; 106 rel). This succulent Aizoaceae is the characteristic, dominant species of the type. Ipomoea pes-caprae and Sporobolus virginicus are frequent associates but generally with low cover values. It is very successful in building embryo dunes, as in the Pacific (cf. John- son, 1977), and frequently grows isolated, forming small hummocks along the beach, with only one 10 -30 cm high stratum. It can also be one of the outmost pioneer types, and much more frequent than the beach types already described.

6. lpomoeapes-caprae type. (16 bcl; 147 rel). A very common and widely distributed type, frequently associated with Sporobolus virginicus and also Sesuvium portulacastrurn, especially when it goes down closer to the sea. Towards the foredunes other species appear as well, such as Palafoxia lindenii (especially in Rinc6n del Pirata and Chalchihueca), Ipomoea stolonifera, Croton punctatus, Amaranthus greggii, Canavalia rosen, Cyperus articulatus and Lippia nodiflora. Ipomoea is a very important builder of embryonic dunes or rather stable beaches. It grows laterally by means of long stolons (up to 30 m) which creep over the sand surface, sending down adventitious roots. In this way it may cover very big areas and form a rather 'continuous' vegetation cover. It can also grow as isolated individuals

forming small hummocks. The structure includes only one stratum, 15 to 35 cm, leaving patches of bare sand in between the runners.

7. Sporobolus virginicus type. (9 bcl; 96 rel). This is a grass- dominated pioneer type. It has only one characteristic species and is commonly associated with other pioneers such as Ipomoea pes-caprae, 1. stolonifera, Oenothera pilosa; occasionally with Croton punctatus. In several areas it shows particular associations. For example in Paso Dofia Juana it is associated with Fimbristylis castanea and Andropogon spp. (A. scoparius var. littoralis and A. muellerO in Gaviotas it is also associated with E castanea. In these cases the habitat is more humid. The type forms an open, herbaceous stratum, 10 -20 cm high. Its rhizomes help to bind the sand.

8. lpomoea stolonifera type. (11 bcl; 106 rel). 1. stolonifera, a creeping herb with long stolons, is the characteristic dominant species of this group. It is most commonly associated with Sporobolus virginicus and Ipomoea pes-caprae. It also shows particular associations in some of the sites. In E1 Raudal it is accompanied by Euphorbia arnmanioides and in La Mancha by Canavalia rosen. In all the sites its main habitat is the backshore and the lower part of the joining ridge. It can not stand so much sand burial as the 1. pes-caprae type. If the beach is wide it can extend considerably.

9. Croton punctatus type. (16 bcl; 200 rel). This Euphorbia- ceae is the characteristic and dominant species of the type and is commonly associated with Sporobolus virginicus, Ipomoea stolonifera and Ipomoea pes-caprae. Occasionally it is found together with Palafoxia lindenii and Cyperus articulatus, in wide, flat and wet beaches. It is a very good dune builder, and tolerates considerable sand accretion (Moreno-Casasola, 1985). Oosting (1945) found that the species distribution and survival in the system is unrelated to salt spray, a l though in some systems in South Carolina, in the U.S.A., it favours sites where salt spray is low (Stalter, 1974). It is a low shrub, forming discontinuous 4 0 - 6 0 cm high, patches of vegetation. Sometimes several individuals grow together forming rather large patches, mostly on the frontal slopes of the foredunes (La Mancha). It also occurs in some mobile systems ma in ly on the crest and leeward slopes of dunes.

10. Palafoxia lindenii type. (10 bcl; 177 rel). The endemic character species, highly dominant, is a 70 cm high shrub belonging to the Compositae. It inhabits the beach, where it forms monospecific patches or grows together with Ipomoea pes- caprae and/or Croton punctatus. It forms hummocks along the beach and also appears on the frontal slope of the foredune. It is also found, though less frequently, in the nude areas of the crest, arms and leeward slopes of mobile dune systems, associated with Chamaecrista chamaecristoides and C. punctatus.

11. Canavalia rosen type. (4 bcl;39 rel). This type has C. rosen (Papilionoidea), as a characteristic species. Frequently associated species are lpomoea pes-caprae and Palafoxia lindenii. La Mancha is the only area where this species was very abundant . At the time of the sampling there were only three individuals o f lpomoea pes-caprae in the zone where one would expect this species and it looked like C. rosen had taken its place. Both species are very similar, physiognomically and ecologically, although L pes-caprae can go farther down the beach and C. rosen can penetrate across the first beach ridge. Four years later, L pes-caprae had become more abundant and C. rosen had diminished. In Moreno-Casasola et al. (1982) four plant com-

munities with Canavalia rosea were described, which differed in their associated species and also in the part of the beach and/or ridge where they were distributed. Such local variation cannot be elaborated in the present overall survey, however. The structure and growth form is similar to the one described for the Ipomoea pes-caprae type.

12. Uniola paniculata type. (5 bcl; 96 rel). This is a second grass-dominated pioneer type. Uniola forms a high herb layer (80-100 cm) and is commonly associated with Sporobolus virginicus, Ipomoea stolonifera and Croton punctatus which form a low herb layer (10-20 cm) with a few small shrubs. It is an important dune builder in the northernmost sites, as on the Texan and southeastern Atlantic coasts of the United States (Judd et al., 1977; Stalter, 1974). It tolerates strong seawinds (Oosting & Billings, 1942).

13. Heliotropium curassavicum type. (1 bcl; 6 rel). This Lype is only found in Chavarria (south), where it is accompanied by Croton punctatus and Ipomoea stolonifera. There is an open herb layer and a low, shrub layer, on the front and back of foredunes.

14. Oenothera drummondii type. (10 bcl; 77 rel). This pioneer type has only one characteristic species, though it may be associated with Sporobolus virginicus. Other frequent species are Ipomoea stolonifera, Ipomoea pes-caprae, Croton punctatus. It constitutes only one herb layer, 25-35 cm high. In North Caro- lina O. drummondii is reported as locally abundant in sheltered areas of the dunes where salt spray is minimal (Stalter, 1973), while in the south of Texas, on Padre Island, Dahl & Goen (1977) mentioned that it becomes dominant after dunes have been started to be built up by pioneer vegetation. On the Gulf coasts the type is found on the beach close to other pioneers tolerant to salt spray. It does not collect much sand around it.

15. Panicum geminatum type. (2 bcl; 17 tel). It is frequently associated with Uniola paniculata, Croton punctatus and Sporobolus virginicus. This small grass grows on the foredunes and is only found in Tamaulipas. It forms an open type with small herbs, 15 - 2 5 cm, and a high herb layer (80 cm) in case U. paniculata is present.

16. Bidenspilosa type. (3 bcl; 25 rel). Bidenspilosa (Compos- itae) is very common in disturbed sites and it is dominant on many of the beaches of Tabasco. In Veracruz it is less common. The species forms an open, herbaceous cover, 2 0 - 4 0 cm, with scattered low shrubs. It is frequently associated with Croton punctatus. It grows on the backshore close to the foredune, on the lower parts of the foredune and in stabilized plains of low beach ridge systems. It appears to prefer humid areas. In La Mancha it is common in humid slacks.

IlL Vegetation of blowouts and active dunes

They are characterized by a high mobility of the substrate and a low amount of organic material (Table 4, Fig. 3A- C). They are very frequent in the central part of Veracruz.

17. Andropogon scoparius var. littoralis type. (6 bcl; 78 rel). This character species is most frequently associated with Palafoxia lindenii and sometimes with Andropogon muelleri. It also appears in two zones, a) It occurs on semimobile dunes

159

Table 4. Basic clusters and community types for the vegetation of the blowouts and active dunes in the Gulf. Andropogon spp. includes A. scoparius var. littoralis and A. muelleri.

BASIC CLUSTER NUMBER:

COMMUNITY CODE:

• ldropogon ~. Palafoxia lindenii Acacia corniqera C b ~ e c r i s t a c h s ~ e c r i s t o i ~ e s Commelina erec ta Ipomoea pes-caprae Trachypogon gouini Pectis saturejoides lephroeia cinerea Schrankia quadrivalvis Maeroptilium atropurpureum Metastelma pringlei Cenchrus tribuloides Porophyllum nummularium Andropogon glomeratus Fimbristylis spathaeea Oenothera drummondii Palafoxia texana Croton punetatus Ipomoea stolonifera Sporobolua virginicus Chamaesyce dioica 8idens pilosa Cyperus articulatus Lippia nodiflora Panicum geminatum Borrichia frutescens Pouteria sp. Uniola paniculata

122 11 111112222 2 789177 911 0159135780046 7 132834 367 9948954281759 0

17 18 19 20

897778 131 . .12131 . . . . . . 3112.3 311 .121 .12 . . . . 11 . . . 1 . . 939 . . . . . . . . . . . . . . 1 . 216 1.1 9889998998888 . 1 . . 1 . . . 1 13 . . . . . . . . . 1. 1 . 2 . . . 1.1 1 . . 1 . . . 1 . 1 1 . . .4 . . . . . . . . . 1 . . . . . . . . . . . . . . 42 . . . . . . . . . . . . . . . . . . . . 3 . . . . . . . . . . . . . . . . . . . . . 1 . . . . 12 . . . . . . . . . 1. 1 1 . 1 . . 1.1 11 .1 .1 . . . . . 1. . . . 1 3 . 1.1 .111 . . . . . . . . 1 . 1 . . . . . . . 11 . . . . . . . 1 . . . . . . 1 . . . 1 1 . 1 . . 1 . . . . . . . . ...5...11 ..1.2 ........ ..3 ....... I...I ....... I ........ I ..... 3...I.. ................ 1411, . 1 . . . 1 1 . . . 1 . 1 . . . 3 M 1 5 3 . 4 . . . . . . . . 41 . . . . 111111. . . 1 . . . . . . 21 . . . . 1 . . 1 1 1 . .1 . . . . . . . 11.1 . . . . . . . . . 1 3 . . 1 . 1.1 . 2 . 1 . . 1 . . 1 . . . . . 1 . . . . . . 3 1 . . . 1 . . . . . . . . . 1 . . . 1 . . 1 1 . . . 1 1 . 1 1 1 1 . . . . . . . . . . . . . . . . . . 111 . . . . . . . . . . . . . . . . . . 1 1 1 . . . . . . . . . . . . . . . . . . . . 1 1 . . . . . . . . . . . . . . . . . . . . . 6 1 . .

where other species have already started to stabilize the sand, for example Chamaecrista chamaecristoides, Trachypogon gouini, Andropogon glomeratus and Macroptilium atropurpureum, b) In the rear part of wide beaches it is found with Ipomoea stolonifera and Fimbristylis spathacea. On the beach it forms an open herbaceous community (80 cm high) with a lower (5-15 cm) understorey. In semimobile dunes it forms patches with a closer vegetation cover with low shrubs and high grasses (50-90 cm high).

18. Acacia cornigera type. (3 bcl; 32 rel). Acacia cornigera is the dominant shrub of this type, commonly associated with Palafoxia lindenii, and on the beach, with Ipomoea pes-caprae. This type inhabits the mobile back dunes and beaches of two neighbouring areas: Rincon del Pirata and Chalchihueca. It can stand considerable sand accretion. It grows discontinuously and forms high, dense shrub patches (1-3 m).

19. Chamaecrista chamaecristoides type. (13 bcl; 272 rel). The Caesalpinaceae C. charnaecristoides is one of the most widespread species in the Gulf dune areas, and the most important dune fixer in the mobile dunes of central Veracruz. The type forms monospecific patches, 5 0 - 6 0 cm, on the windward slopes, arms and crests; on the leeward slopes and arms it is accompanied by Croton punctatus and/or Palafoxia lindenii; when the areas are sufficiently stable other species such as An- dropogon spp. appear. The small shrub tolerates sand deposition and erosion (Moreno-Casasola, 1985). In Tamaulipas it is also found on the foredune with Uniola paniculata. Sauer (1967) mentioned it as an endemic species of the Gulf.

20. Commelina erecta-Chamaecrista chamaecristoides type.

160

(1 bcl; 6 rel). This particular type from La Mancha is poor in species and grows on the crest of mobile dunes. It forms an open layer (5 cm high) with scattered low shrubs.

IE Vegetation of humid and wet slacks

T h e s e t y p e s a p p e a r in areas w h e r e the level o f the

g r o u n d water is c l o s e to the surface , b o t h o n the b e a c h a n d b e t w e e n d u n e s (Table 5; Fig . 3 A - C ) .

21. Erigeron myrionactis type. (6 bcl; 59 rel). The type has only one constant characteristic species but also Schrankia quadrivalvis is a good differential species• In some areas the type forms combinations with species only found there, for example Eustachys petraea in Riachuelos and Bacopa monnieri in E1 Faro• The community forms a continuous cover of dense, low herbaceous vegetation, occasionally with small shrubs like C.

chamaecristoides. The type is common in humid slacks, mainly in Veracruz.

22.Bidens pilosa-Cyperus articulatus type. (1 bcl; 10 rel). This type, only known from La Mancha, has a number of differential species which occur more commonly in the next group of dry dune vegetation, i.e. Macroptiliurn atropurpureum and Metastelmapringlei. Its vegetation is low, closed and herbaceous, with occasional shrubs. It occurs in humid slacks in which the water table during the rainy season is around 15 cm deep.

23. Fimbristylis spathacea type. (4 bcl; 56 rel). The type appears mainly in two areas: Paso Dofia Juana and Chalchihueca, which have a relatively high diversity of humid and wet environments. The characteristic species is commonly found with the wet slack species lva asperifolia and Hydrocotyle bonariensis and the dry dune grass Andropogon scoparius var. littoralis. Sporobolus virginicus is also frequent. It constitutes an open,

Table 5. Basic clusters and community types for the vegetation of the humid and wet slacks in the Gulf. Andropogon spp. includes A. scoparius var. littoralis and A. muelleri. Eleocharis spp. includes E. cellulosa and E. geniculata. Leucophyllum sp. could be identi- fied as Sternodia tornentosa.


COMMUNITY CODE:

Erigeronnyrionactis Schrankia quadrivalvis Axonopus compressus Chmmesyce d i o i c a Bidens p i l o s a H a c r o p t i l i u m atropurpureum Hetastelma p r i n g l e i

P e c t i s sa tu re jo ides F i n b r l s t y l i s spatlmeea Andropogon spp. Cyperaceae no. 186 Lippia nodiflora Cyperus a r t i e u l a t u s Sporobolus virginicus

1122

175644

127867

21

869588

2 2 . . 2 1

. . . . 31"

2 . . . 1 3

1 1 1 . 1 ,

1 1 . . 1 1

2 111 1111222 1111 11 111112 222 1112 1

6 9334 592399144 3665 66 4 5 7 9 2 2 5 7 8 8 12246 1882 7

4 4671 7 6 6 4 1 0 3 9 2 4 8345 06 8 6 7 3 5 1 9 9 1 6 9 24735 24 2024 O

22 23 24 25 26 27 28 29 30 31

1 . . 1 . . . . . . . . . . 1 . . . . 1 . . . . 2 . . . 1 . . . . . . 1 . . . . . . . .

. . . . . . . . . . . . . . 1 . . . . . . . . . 1 . . . . . . . . 1 . 1 . . . . . . . .

. . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . 1 . . . . . . . . 1 . . . . . . 1 . . . . . . . . . . . 1 . . . . . 1 8 . . .

4 . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . 3 . . 6 . . . . . . .

5 , . 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . .

3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . , 1 . . . . . . .

3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . .

. 7378 . . 1 . . . . . . . . . . . . . . . . . 1 , 1 . . . 1 .1 . . . . . . . . . .

.1 . . . . 1 1172 . 1 1 . 1 . . . . . . . . . . . . . . . 11 . . . . . . . . . . . . . . . . .

. . . . . . . . 711 . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 3 1 3 . 5 5 . 1 1 8 7 7 8 8 9 8 8 8 7 .111 52 3 , 1 3 3 2 2 7 3 5 3 1 3 . 5 3 . . . . 1 , 1

3 1 1 1 1 . 5 3311 1 4 1 1 1 . , . 2 1 3115 21 7 6 8 9 7 8 8 7 7 8 8 3 1 . 3 3 11 1 . , . 3

5 1 . . 1 . . 2 1 . 4 2 . 1 1 2 1 . 5 2 2 1 . . . . . 3 . . . . . 1 . 2 1 4 6 2 4 1 . 11 . 1 1 . .

Chamaecrista chamaecristoides 1 . 3 . 3 1 1 .161 . . 1 . . . 6 1 . 3 . . . . . . . . 2 . . 1 1 2 , 1 . . 1 . 1 . . . . 1 . . .

Ipomoea stolonifera Portulaca no. 1479 E]eoc~ris s~o. Panicum repens Scirpus no. 1957 Nymphoides indicus Portulaca oleracea Rhynehospora stellata Panieum agrostoides Ambrosia artemisiifolia Hydrocotyle bonariensis fimbristylis castanea Borriehia fruteacens fimbristylis spadicea Conocarpus ereetus Palafoxia lindenii Ipomoea pes-caprae Commelina erecta Tephrosia cinerea Croton punctatus Palafoxia texana Oenothera drummondii 8aeopa monnieri Ira asperifolia Typha domingensis Flaveria linearis Leucophyllum sp. Panicum geminatum Randia laetevirens Sesuvium portulacastrum

431 . . . . . . . . 2 , 1 . . 1 1 1 1 . . I . . . . 2 . , 2 . . . . . I , 1 1 . 2 , 13 . . . . .

. . . . I . . . . . . . . . . . . . . . 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . I . . . . . . . . . . . . 9781 . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . 1111 . . . . I . . . . . 1911 63 . . . . . . I . . . . . . . . 4 . . . . .

, . 1 2 . . . . . . . . . . . . . . . . . . 1 , . 42 . . . . . . . I . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . 243 19 . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . 169 , I . . . . . . . . I . . . . 4 . . . . . . .

. . . I . . . . 61 . . . . . . . . . . . 2 1 . , 31 . . . . . . . 12 . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . 1 .1 14 . . . . . . . . I . . . . . . . . . . . .

. . . I . . . . . . . . . . . . . . . . . . . . . 11 . . . . . . . 171 . . . . . . . . . . .

1 . . . 4 1 1 .631 . . 1 . . 1 . . . 3 4111 24 . . . . 1 1 1 , 1 3 6 8 8 7 8 6 . . . . .

. . . . . . . . . . 1 1 . . . 1 2 . 1 . . . . . . . . 1 . . . . . . . 1 2 . . . 8 . . 82 .11

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9968

. . . . . 1 1 . . . . . . . . . 1 .1 . . . . . . . 1 . . . . . . . . . 2 . . . . . . . . 6 1 8 .

. . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . 1 . . . . . . . . . . . .

.11 . . . . 1 . . . . 1111 . . . . . . . . . . . . . 1 1 1 1 , 1 . . . . . . . 1 . . . . . .

. . . . . . . 1 . . . . . 1 1 1 1 . 3 . . . . . . . . 1 1 . . . 1 1 . . 1 . . . . . . 1 . . . . .

2 1 . . 1 , 1 . . . . . . . . . . . . . 1 . . . . . . . . . 1 . . . . . . . . 2 . . 1 11 . . . .

1 . . . . 1 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . 1 . 1 . . . . . . 1 . . . . 12121 . . . . . . . . . . 1 .1 . . . . . . . 5 . 11 . . 1 1

• .11 . . . . . . . . . . . . . I . . . . . . 1 .1 . . . . . . . . . . . . . . . . . . . 11 .

111 . . . . . . . . . . . . . . . 1 . . . . . . . . . 1 . . . . . 1 . . . . 1 . . . . 1 . . . .

. 9 . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . 4 . . . . . 1 . . . . . . . . . . 1 .

. . . . . . . . 65 . . . . . 1 . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . 1 .

. . . . . . . . . . . . . . . . . . . . . . . 3 . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 . . . . . . . . . . . . . . .

, . . 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . 1 . •

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I . . . . . .

herbaceous vegetation (20-60 cm). The type occurs in both inundated, humid areas and drier parts on undulating wide beaches. In the drier sites A. scoparius is typical. The beach is occasionally flooded during storms.

24. Lippia nodiflora type. (10 bcl; 153 rel). The general slack species Lippia nodiflora (Verbenaceae) reaches full dominance in this type. It is a small stoloniferous herb. The herb layer is 5 - 10 cm, sometimes with shrubs (P lindenii or C. punctatus) or a high 30-50 cm herb layer (C. articulatus). On wide beaches with humid and wet slacks Sporobolus virginicus and Ipomoea stolonifera are constant companions. Storm waves can flood these beaches. The type is also characteristic of the slacks that are formed inside the mobile dune systems. In the latter case Cyperus articulatus tends to be more common, while the other species diminish. In the sites of Tamaulipas, this type appears associated with Chamaecrista chamaecristoides.

25. Eleocharis spp. type. (4 bcl; 45 rel). This type is mainly found in Riachuelos and Paso Dofia Juana. The main characteristic species are E. geniculata and E. cellulosa. Both species are associated with Cyperus articulatus in all the areas, Rhyn- chospora stellata and Hydrocotyle bonariensis in Paso Dofia Juana and Nymphoides indicus in Riachuelos. It grows in wet slacks which are inundated several months, and it constitutes an open, low herbaceous community with a stratum of open, high herbs when C. articulatus is present. The type is characteristic of wide undulating beaches and flats between low ridges.

26. Panicum repens type. (2 bcl; 37 rel). This type was only found in Riachuelos. Apart from the characteristic species, many relev6s also have Scirpus sp., Hydrocotyle bonariensis, Rhynchospora stellata, Panicum agrostoides and Eleocharis geniculata. The type is found in wet, inundated slacks between low ridges; it forms a low, open herbaceous community. It is very variable in species composition. Therefore, it is believed that more relev6s in other areas are needed to give a better description of the community.

27. Cyperus articulatus type. (11 bcl; 108 rel). This type includes the other common slack species Lippia nodiflora. It is prominent in wet and humid slacks. It is frequently associated with Sporobolus virginicus and Fimbristylis spadicea. In Ri- achuelos it is also associated with Flaveria linearis and Ambro- sia artemisiifolia. The low herbaceous stratum is open, and 5 - 2 0 cm high, while C. articulatus can form a dense, high 70-100 cm herb layer. Algae occur frequently. The type is found on wet and humid slacks between active dunes and ridges, and also on wide beaches with slacks. It is one of the first types to colonize newly formed slacks.

28. Hydrocotyle bonariensis type. (5 bcl; 31 tel). The characteristic species, belonging to the Umbelliferae, can be found with several other slack species. The type is composed mainly of a low herb stratum, 10-20 cm, and a high herb stratum when C. articulatus is present. It inhabits wet slacks that are inundated for several months. 29. Fimbristylis castanea-Chamaesyce dioica type. (2 bcl; 20 rel). This type is very frequent in Las Gaviotas where it is accompanied by lpomoea stolonifera; less frequent are Croton punctatus and Cenchrus sp. It appears as an open, low herb layer, on flat beaches where the ground water table comes near to the surface.

30. Borrichia frutescens-Fimbristylis spadicea type. (4 bcl; 36 rel). The two characteristic species are a composite and a sedge respectively. The type is only found in some northern

161

areas. It forms a high herb layer, 20-40 cm, with patches of bare sand, in sheltered areas behind the foredune, close to the lagoon. It can tolerate saline conditions.

31. Conocarpus erecta type. (1 bcl; 3 rel). The dominating species is a very important component of the mangroves in the Gulf and Caribbean area, but it also appears in the dunes of Ri- achuelos, where it forms part of the vegetation of the humid border areas. Here it forms a few clumps in the border of wet and humid slacks inside the ridge system, probably because of the salinity of the water. In other areas it appears at the border of the lagoon. It forms a 2 - 4 m high ticket.

V. Vegetation o f the sheltered zone behind the foredunes

T h i s is a well d e f i n e d z o n e in s i m p l y s t r u c t u r e d

d u n e sys t ems , w i t h t y p i c a l c o m m u n i t y t ypes res-

t r i c t e d to it. I n m o r e c o m p l e x d u n e s y s t e m s t h e

z o n e is less c o n s p i c u o u s w i t h c o m m u n i t y t ypes a l so

o c c u r r i n g in o t h e r zones . I n t h e G u l f r e g i o n t h e

z o n e is so v a g u e l y d e v e l o p e d t h a t it was i n c o r p o r a t -

ed in t h e s e c t i o n o n f ixed d u n e s .

VI. Vegetation o f f i x e d dunes

T h i s s e c t i o n i n c l u d e s c o m m u n i t y t y p e s o f d r y

d u n e s in a r ea s w i t h a s i m p l e s t r u c t u r e : w i t h o n e o r

t w o b e a c h r idges ( S o n t e c o m a p a n , M o n t e P io ) , w i t h

a ser ies o f low r idges ( R i a c h u e l o s ) , o r w i t h a ser ies

o f m o b i l e , s e m i m o b i l e a n d / o r f ixed h i g h d u n e s

( R i n c 6 n de l P i r a t a , L a M a n c h a ) . S h e l t e r e d z o n e s

o n l e eward s lopes o f f o r e d u n e s a re a l so i n c l u d e d .

See Fig. 3 A - C. T h e t y p e s c o n s i s t m a i n l y o f h e r b a -

c e o u s c l o s e d v e g e t a t i o n , i n c l u d i n g g r a s s l a n d s , low

s c r u b (Tab le 6) a n d h i g h , d e n s e t h i c k e t s (Table 7).

T h e r e is a c l ea r o v e r l a p w i t h c o m m u n i t y t y p e s o f

t h e h u m i d s lacks , w h i c h is r e f l e c t e d b y t h e j o i n t oc-

c u r r e n c e o f spec ies s u c h as Macrop t i l ium atropur-

p u r e u m a n d Lipp ia nodif lora. T h e g r o u n d w a t e r t a b l e is b e y o n d t h e r e a c h o f

r o o t s o f t h e spec ies b e l o n g i n g to t h e s e types . C o m -

m u n i t y t y p e s a re a r r a n g e d a c c o r d i n g to g r o w t h

f o r m . In L a M a n c h a s e m i d e c i d u o u s t r o p i c a l fo res t

b o r d e r s t h e s e m i m o b i l e d u n e s y s t e m a n d t h e r e a re

p a t c h e s o f t h i s c o m m u n i t y in t h e m o r e s t a b i l i z e d

p a r t s o f t h e sys t em. S o m e o f t h e c o m m u n i t y t ypes

d e s c r i b e d , i.e. t h e Diphysa robinioides type , h a v e a

s t r u c t u r e a n d spec ies c o m p o s i t i o n t h a t t e n d to -

w a r d s t h e s e m i d e c i d u o u s t r o p i c a l fores t .

162

Table 6. Basic clusters and community types for the grasslands, herb community types and low scrubs in the Gulf. Andropogon spp. includes A. scoparius var. littoralis and A. muelleri. Aristida spp. includes A. adscensionis, A. curvifolia and A. aff. roemeriana.


COMMUNITY CODE:

Sehrankia quadrivalvis Commelina erecta l~eropt i l im a t r o p u r p u r e t m

Verbesina persicifolia Metastelma pringlei Opuntia stricta T r ~ - h y l ~ 9 ~ n g o u i n i

Aristida app. Andropooon app. Waltheria indica Eustachys p e t r e a

~porobolus jacquelonti Bouteloua repena Porophyllum nummularium Andropogon glomeratus Bidena pilosa Chamaecriata chamaecristoidea Cenchrus tribuloides Tephroaia cinerea P e c t i s s a t u r e j o i d e s P a l a f o x i a l i n d e n i i

Rhynehosia americana Tsmmea curassavica Ambrosia artemiaiifolia Fimbristylis caatanea SteBodia tomentosa Panicum geminatum Borrichia fruteacens P a l a f o x i a t e x a r m v a t . robusta Croton p u n c t a t u s

Ipomoea atolonifeva Sporobolua virginieus Chamaesyce dioica Cyperus articulatus Lippia nodiflora Oenothera drummondii Randia laetevirens Acacia cornigera Crotalaria incana Indigofera suffruticoaa Digitaria runyonii Fimbristylis spadieea Cnidoseolus texanus Chamaeayee ammanioidea Florestina tripteria Oeamodium triflorua Oeamodium incanum Axonopua compressus Paychotria erythrocarpa Psidium guajava Cissus sicyoides Turnera diffuaa D a l e a s c a n d e n a

E u p a t o r i u m o d o r a t u m

C i t h a r e x y l u m b e r l a n d i e r ~

2 11111 2 2 1 2 2222 2 11112 1

124 1 3 5 6 0 0 2 4 4 587 67 5 85 6677 7 ] 78893 5

091 6 0 9 0 0 1 2 2 3 321 12 5 7El 6767 8 5 74791 8

32 33 34 35 36 37 ] 8 39 40 41 42

888 51 . . . . . . . . 11 11 . 1. 1 . 1 . 1 . . . . . . .

321 1 1 1 1 . . 2 1 . 112 33 . 1 . 11 . . . . . . . . . .

. . 1 9 7 5 8 8 3 6 7 7 12 . 2 , . 1 . .1 . . . . . . . . . .

. . . . . . . 131 . . . . . . . . . . . . . . . . . . . . . .

. 2 . . 2 5 1 3 1 2 . . 1 . 2 . 2 . . . 3111 1 . . . . . . .

. . . . 4 4 1 . . . 1 1 1 . . 11 . . . 2 . 1 1 2 . . . . . . .

. . . . . . 1 . . . . . 898 1 . . . . . . . . . . . . . . . .

. . . . . 2 1 . . . 1 . 1 . 3 77 . . . 1111 1 . . . . . . .

. . . . . . 1 2 . . 5 1 111 11 . . . . 1 . . . . . . . . . .

. . . . . . . 2 . . 1 1 1 .1 . 3 . . . 2129 1 . . . . . . .

. . . 1 . . . . . . . . . . . . . 8 , . , . . . . . . . . . . 1

. . . . . . . . . . . . . . . . . . 98 . . . . . . . . . . . .

.1 . . . . . . . . . . . . 1 16 . . . 1 . . . . . . . . . . .

. . . . . 6 1 1 . 1 1 5 . . . 11 . . . . 111 1 . . . . . .

. . . . . 1 1 2 . 1 , . 1 . . . . . . . . . . . . . . . . . . .

13 . . ] 1 . 1 . 2 . . .21 11 1 . . 6 1 . 1 . . . . 4 .

. 1 . . 1 . 1 3 . . 3 1 ] 1 4 12 ] 11 2421 1 . 1 1 1 2 .

11 , 1 1 1 5 . . . 3 2 1 .1 32 . . . . 1 . . . . . . 11

1 . . . . . 1 . . . . 1 1 .1 13 . 22 1131 . . . . . 3

. . . . . 1 2 . . . . . 1 . 5 ] 5 , . . 7 7 5 6 2 . . . . . .

. . . . 1 . 1 1 . . 1 . 1 .1 11 2 . . 11 . . . . . . . .

. . . . . . . . . . . . . . 1 11 , .1 1111 . . . . . . . .

. . . . . . . . . . . . . . . . 1 . . . 1 . 11 8 . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . 9 . . . . .

. . , 1 . . . . . . . . . . . . . . . . . . . . . . 42111

. . . . . . . . . . . . . . . . . . . . . . . . . . 39157

. . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 1 5 .

. . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 1 1 .

. . . . . . . . . . . . . . . . . 2 . . . . . . . . 1 1 1 . 1

1 . 5 1 . . . . . . . . . . . . 1 1 . . . 1 . . . 1 1 1 1 . 1

4 . 1 11 . . . . . . . . . . . . . . . . . . . . 1 . 1111

311 2 . . . . . . . . . . . . . . . . . . . . . . 1 1 . 1 1

2 1 . 2 . . . . . . . . . I . . . . 1 . . . . . . . . . . . . .

I . . 5 . . . . . . I . . . . . I . . . 11 . . . . 1 . . . . I

. . 2 . . . . . . . I . . . . . . I . . . . . . . . 2 1 2 1 . I

, . . 1 . . . . . . . . . . . . . 2 . . . . . . . . . . . . . .

. I . . I . . . 1 5 . . . . . . . . I , 1 1 . . . I I , . 1 1 .

. . . . . . . . 15 . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . 1 .1 . . . I . . . . 3 . 1 . I . . . . . . .

. . . . . . . . . . 14 . . . 31 . I . I . . . . . . . . . . .

. . . . . . . . . . . . . /4 . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . I . . . . . . . . . . . . 11 . . . .

. . . . . . . . . . . . . . 1 11 . . . 4121 2 I . . . . . .

. . . . . . . . . . . . . . . . I . . . 1 . 1 1 1 . . I . . .

. . . . . . . . . . . . . . I . I . . . 1 . 11 1 . . . . . .

. . . . . . . . . . . . . . . . . . . 3 . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . 2 . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . ] . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . I . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . I . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . I . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . 5 . I . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . i . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . .

A. Community types dominated by herbs

32. Schrankia quadrivalvis type. (3 bcl; 26 rel). The type has only one characteristic dominant species belonging to the Mimosoideae which is frequently accompanied by other herbaceous, creeping species such as Commelina erecta, and low shrubs such as Bidens pilosa and Croton punctatus. It forms a continuous cover of 1 5 - 2 5 cm high vegetation, with occasional low shrubs not higher than 5 0 - 70 cm. It is mostly found in flat areas or gentle slopes, on stabilized sites.

33. Macroptilium atropurpureum type. (9 bcl; 102 rel). The name-giving legume species is frequently associated with Opun- tia stricta var. dillenii and Porophyllum nummularium or with shrubs like Acacia cornigera and Lantana camara. The characteristic species is a climber that tends to cover the vegetation under it; the height depends on the species that act as support, but usually it is not more that 1 m tall. It inhabits stabilized plateaus or gentle slopes and forms a continuous close cover of vegetation.

B. Community types dominated by grasses

34. Trachypogon gouini type. (3 bcl; 46 rel). This type has one characteristic dominant, an endemic grass species, that is very frequently found with Chamaecrista chamaecristoides, Palafoxia lindenii, Commelina erecta and Andropogon spp. It forms a low herbaceous layer which can be open or closed and a high grass layer (80-100 cm) with some low shrubs. It grows on plateaus and gentle slopes of semimobile dunes (Rancho E1 Quijote and La Mancha) and dunes in which grasslands predominate (Rancho El Capricho). It colonizes areas in which sand is being stabilized already by other species, particularly Chamaecrista chamaecristoides.

35. Aristida spp.-Bouteloua repens type. (2 bcl; 22 rel). Ar- istida adscensionis, A. aft. roemeriana and A. curvifolia are included in this type. This low grassland type is only present in two areas, where it is very common. It is rich in species, including Commelina erecta, Pectis saturejoides, Tephrosia cinerea, Cenchrus tribuloides, and scattered low shrubs such as ln- digofera suffruticosa, Waltheria indica and Chamaecrista chamaecristoides. It has scattered individuals of other high shrub species (Mimosa chaetocarpa, Diphysa robinioides). It forms a closed vegetation cover with a height of 30 cm, with scattered low shrubs. These perennial grasses grow on plateaus and hollows on stabilized dunes. 36. Eustachyspetraea type. (1 bcl; 11 rel). This community type was found only in Riachuelos, where it is commonly associated with Chamaecrista chamaecristoides and Oenothera drummondii and less frequently with Erigeron myrionactis. It forms a closed, herb layer with scattered low shrubs on the slopes of ridges.

37. Sporobolusjacquemonti type. (2 bcl; 23 rel). This species characterizes a high grassland, which is only found in Rancho El Capricho and La Trocha. The former area is rich in grasslands, which are uncommon elsewhere. The type includes numerous shrubs, occurring isolated or in nuclei, e.g. Randia laetevirens, Lantana camara, Psychotria erythrocarpa, Psidium guajava, Waltheria indica, Florestina tripteris, and other herbaceous plants like Sida rhombifolia, Axonopus compressus and Tephrosia cinerea. The grasses and some of the shrubs are frequently eaten by cattle. The cover is close, the height 4 0 - 6 0 cm and there are patches of shrubs (1 -2 m) in between. The type grows in stabilized areas.

C. Community types with low shrubs

38. Pectis saturejoides-Waltheria indica type. (4 bcl; 39 rel). This community type is only found in the area of La Mancha, where P saturejoides, Compositae, is a very common and widely distributed herb. Associated species are Tephrosia cinerea, Cnidoscolus texanus, Metastelma pringlei and Chamaecrista chamaecristoides. On certain sites Melampodium americanum and Turnera diffusa become dominant. It forms a rather continuous herb cover (15 -20cm) with scattered low shrubs (40- 60 cm); in the dry season much of the foliage dies. It grows on semimobile and stabilized sites, on the arms of dunes, hollows and plateaus.

39. Tamonea curassavica type. (1 bcl; 10 rel). The dominant species, a Verbenaceae, is only found in La Mancha, commonly associated with Cnidoscolus texanus and Pectis saturejoides. It

163

forms an open, low scrub (30-40 cm), with an open herb layer, only found in stabilized areas with gentle slopes, far from the seashore.

40. Ambrosia artemisiifolia type. (1 bcl; 7 rel). This type is only found in Sontecomapan and is sometimes accompanied by Randia laetevirens. It has a herbaceous and low open shrub layer (20-45 cm). It is found in disturbed areas of the foredune.

41. Stemodia tomentosa type. (5 bcl; 25 rel). This type was only found in the area of Tamaulipas and North of Veracruz. This small Scrophulariaceae is associated with Lippia nodiflora and Palafoxia texana var. robusta. The type has a close herb layer with low shrubs in sheltered areas.

42. Palafoxia texana var. robusta type. (1 bcl; 9 rel). This type is only found in Riachuelos, although as a species it is very frequent in Tamaulipas, in the sheltered area behind the foredune. It is associated with Chamaecrista chamaecristoides, Oenothera drummondii, Crotalaria incana and Ipomoea stolonifera. It forms a continuous, open cover of vegetation with scattered low shrubs, 3 0 - 50 cm. It grows on the front part of the stabilized plains of the low beach ridge system in this area.

D. Community types with spiny shrubs

They are mainly found in stabilized areas of the dunes and in the foredunes and sheltered areas of beach-ridge systems (Table 7; Fig. 3A-C).

43. Bromelia sp.-Opuntia stricta var. dillenii type. (1 bcl; 5 tel). This community type has a very characteristic physiognomy and is only found on the backward low ridges in Gaviotas, close to the highway. Both characteristic species have particular growth forms that make the type very conspicuous; although they do not have the typical growth form of a shrub, they form dense thickets 1-1.5 m tall. They are associated with Metastel- ma pringlei, Cnidoscolus texanus, Randia laetevirens, Cissus sicyoides and Macroptilium atropurpureum. The type forms often large patches surrounded by herbaceous, closed vegetation. It grows in backward, stabilized zones of the ridge system.

44. Opuntia stricta var. dillenii-Randia laetevirens type. (10 bcl; 125 rel). The characteristic species, belonging to the Cactaceae and Rubiaceae respectively, can be considered characteristic of the group, although the general frequency of the second species in the total relev6s was 90°7o. Other very common species are: Bidens pilosa, Citharexylum berlandieri and C. ellipticum, Schrankia quadrivalvis, Metastelma pringlei, Cissus sicyoides, Crotalaria incana. The type forms clumps amid other community types. It inhabits dry, stabilized areas and constitutes a rather dense, spiny 1-1.5 m tall thicket. It is one of the types that start the thicket phase in the dunes. The cactus is locally planted to fix dunes, as it can stand sand accretion.

45. Randia laetevirens type. (17 bcl; 264 rel). This is the most widely distributed type of the coastal thickets of the Gulf dunes. It forms very dense, 1 - 3 m tall spiny thickets with other species such as Citharexylum berlandieri, C. eHipticum, Chiococca alba, Verbesina persicifolia, Coccoloba barbadensis, Cissus sicyoides, Bidens pilosa and Iresine celosia. In Tamaulipas it is frequently associated with Borrichia frutescens and Acacia cornigera and with Uniola paniculata. It is found on foredunes of stable beaches (Monte Pio, Sontecomapan) and sheltered areas

164

Table 7. Basic clusters and community types for the thickets along the Gulf coast.


COMMUNITY CODE:

BrQsclia 8p. O p u n t i m m t r i c t m v a t . d i l l e n i i Amphilophium paniculatum Rand ia l ae tev i r ems

A c s c i a n m e r a e a n t h a A c a c i a fmrnesi~ Caesa lp i n i a bond luc

Lippia nodiflora Daloascandens Eulmtorium odbz~ratum L~tanaemata Yerbesinm permici~lim Coocol~bazb~KS~nmim Diphysa z~binioides C h r y m ~ b a l a n t m i c a e o Bumelia retusa Ps id i um gua java

H ib i s cus pe rnabum is

N e c t a n d r a c o r i s c e a e Cissus cisyoides Croemope ta lm= u r a g o g a S c h a e f f m r i a f r u t oaeona

8 u r s e r a simaruba Cssearia nitida Karwinskia humboldtiana Tillandsia 68 Diospyrus sp. Psssiflora foetida Axonopus compressus Croton punctatus Fimbristylis spadicea Borrichia frutescens Panicum geminatum Stemodia tomentosa Uniola paniculata Chiococca aiba Citharexylum berlandieri Dalbergia sp. Sporobolus virginicus Chamaecrista chamaecristoides Porophyllum nummularium Acacia cornigera Metastelma pringlei Bidens pilosa Rscroptilium atropurpureum Iresine celosia Cypetus articulatus Pappophotum pappiferum Crotalaria incana Commelina erects Tephrosia cinerea Pectis saturejoides Waltheria indica Schrankia quadrivalvis Cenchrus tribuloides Cnidoscolus texanus florestina tripte~is Turnera diffusa Mimosa chaetocarpa Cardiospermum halicacabum Guazuma ulmifolia Passiflota holose~ica Eugenia capuli Serjania racemosa Enterolobium cyclocarpum Erythroxylon mexicanum Paullinia tomentosa Psychotria erythrocarpa Bursera biflora Tecoma stans

112222 11111222222 2 11122 112 2 112 122 22 1122 12 11 1 2

1 1268244588 12344814679001238 8 747838 793 3 8 245 238055 56688 1605 62 468 1 8

5 4 7 3 5 5 4 9 0 3 5 3 5 7 8 9 8 1 5 2 6 2 0 5 2 5 2 6 2 474361 550 6 7 363 896225 84504 5991 79 111 3 8

43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

. . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9 4 6 9 7 8 8 8 6 6 32 . . . . . 1 . . . . . . . . 1 1 . 1 . . . 1 . . . . 1 11. 1 . 1 . 1 . 1 . . 1 1 . . . 1 . . . . . . 1 • .1 . . . . . 9 . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . 6 . . . . . 7 . . . . . . . . . . . 2 1 3 8 1 6 1 3 . 1 7 9 7 9 7 9 8 9 9 9 8 9 5 5 9 9 6 9 4 . 1 1 1 7 7 4 . 3 1 1 11. 3 1 1 . 2 . 13353 . 1 . 4 24 1 . 2 2 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 . . . . . 1 . . . . . . . . . . . 2 . . . . . 11 . . . . . . . . . . . . . . . 1 . • 1 . . . . . . . 1 . . 11 . . . . 1 . 1 998888 . . 1 . . . 1 . . . . . . . . . . 11 . . . . . . . . . .

. . . . . . . . . . ; . . . . . . . . . 121 .11 . . . . 111 • 889 . . . . . . . . . . . . . . . . . . . . . . . . 1 .

. . . . . . . . . . . 1 . . . . 1 . 1 . 111 . . . . 1 . . 25 .1 18 . . . . . . . . . . . . . . . . 1 . 1 . . 2 . . 1 .

. . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . 7 . . . . . . . . . . . . . . . . . . . . . . . . . 1

. . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . 5 . . . . . . . . . . . . . . . . . . . . . 1 . . 1

. . 1 . 3 . . 121 . . . . . . . 1 . . . . . . . . 1 1 , 3 . . . 2 . . . . 9 11 . . . 1211 1 . . 11 . . • 2 . . . . .

. . 1 . 11 .2 . . . . 176 . . 1 . . . . . . . . . . . 1 . . . . . . . . . 998 . 1 • . 44 . . . . . . . . . . . 1 . .

. 1 . 1 . • . 1 . 1 . 132 .1 . . . . . . . . . . 1 . . . . . . . . . . 3 . . . . 998786 . . . . . . . . 5 . . 1 . . 1

. . . . . . . . 11 . . . . . . . . . . . . . . . . . 1 . . . . . 1 . . . . . . . . . . . . . 96779 . . . . . . . . . 2

. . . . . . . . . . . . . . . . . . . 1 . 2 . . 1 . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . 9996 . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . 11 .1 . . . . . . 11 2 . . . . . 1 . . 1 . 1 . . . . . . . . . . . . 1 . . 1 . . 99 . 1 .

. . . . . . . . . . . . . . . . 1 . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999

. . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . 1 . 1

1 11 .2 . . 11 . . 26 . . . 1 . . . . . . . . . . . . . . . . . . . . . 1 . 1 . 1 4 . 1 .1 . . . . . . . 1 . 2 . . . . . . .

. . . . . . . . . . . . . . . . . . . . 1 . 31 . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . 6

. . 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . 3 . . . . . 11 . . . . . . . . . . 4

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . 1 . . . 1 . . . . 1 . . . . . . 3

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . 3

. . . . . . . . . 1 . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . 3

. . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

11 . . . . . . . . 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . .

. . 1 . . . 1 . . . . 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 . . . . . . . . . . . . . . . .

1 . . . . . 11 . . 1 . . . 111 . . 1 . . . 111 . . . . 111 . 3 . 2 1 . . . . . . . . . . . . . . . . . 1 . . . . . 1 . .

. . . 1 . . . . . . . . . . . 1 . . . . 1 . . . . . . . . . 111 . . . . . . . . . . . . . . . . . . . . . 11 . 1 . . . . . .

. . . . . . . . . . . . . . . . . . . 41211 . . . . . . 22 . . 21 . . . . . . . . . . . . . . . . . . . . . . . . . 1 . .

. . . . . . . . . . . . . . . . . . . . 1111 .1 . . . . 111 . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . 1111 . . 1 . . . . 2 . . . . 21 . . . . . . . . . . . . . . . . . . . . 1 . . . . .

. . . . . . . . . . . . . . . . . . . . . 2 . 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . 112111 . , 1 . 111131 . . . . . 3 . . . 1 . . . . 111 .11 . . . 11 . 1 . 1 . 1 11 . 1 .

. . . . . . . . . . . . 11 ,11 . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . 11 . . . . . . 1 . 2 . . . . . 1 .

. . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 . . . . . . . . . .

31 .1 . . . 1 . . 21 , . 11 . . . . 1 . . . 11 . . . . 1 . . . . 1 . . . . 11 . . . . . . . . . . . . . . . . . . . .

1 . . . . . 11 .1 11 . . . 1 . . 111121 . . . . . . 22 . . 2 . . 1 . . . . . . . 1 . . . . . 1 . 1 . . 1 . . . . . .

1 . 1 . 161111 . . . . . . . 3 . . . . . . . . . 2 . 6 . . . . . . 1 22 . . . . . 1 . 4 . . 11 . . . 1 . . . . . . .

. . . . 1 . . . . . . . . . . . . . . . 1 . 1 . 12 . . . . . . 1 . . 1 • 1 . . . . . 1 . . . . . 1 . . . . . . 1 . . . . .

2142 . . 1111 22 . . . 1 . . . . . . . . . . 1 1 . . . . . 1 . . 1 1 . 1 1 .121 . 1 . . 11 . . . . . . . . . . .

32122 .2 . . 1 241 .822 .1 . . 54 . . . 1 2 6 . . . . 1 . . 1 2 . . 1 . . . . . . . . 11 21 . . . . 1 . . 1 •

• 131541 . . . . . . . . 1 . 3 . . . . . . . . . . . 3 . . . 1 42 . . . 13 . . 1 . . . 1 . 1

1121 . . 11 . . 111 . . 21 . . . . . . . . . 1 1 . . . . . 1 . . 2 • 1 . . 111 . . . 3 . . 11 . . . . . . 2 . . 1 .

11 .2 . . . . . . 1111121 . . . . . . . . . . . 4 . . . . . . . 1 . . . . . 11 . . . . . . . . . 1 . . 2 . . 2 . . .

11 .1 . . . . . . 1 . 111 . . 1 . 1116 . . 51 1 . . 11 .1 . 1 • . 1 . . 11 . . . . . . . 1 . 1 . . . 1 11 . • •

• . 1 . . 1 . . 1 1 .1 . . . . 11 . . . . . . . . . . . 1 . . . 1 . . • 12 . . . . . . . . . . . . . 1 . . . . . . . . .

31121 .11 . . 111 . . 1 . 1 . . . . . . . . . . . . . . . . . . . . . 1 . 1 . 1 . . 1 . 1 . . . . . . . . . . 1 . . . .

1 . . . . 111 .1 . . . . . . . 1 . . . . . . . 1 . . . . . . 11 . . 1 1 . 1 . . . . . . . . . . . . . . . . . . . . . . .

• . 1 . . . . . 12 . . . . . . . . . . . . . . . . 1 1 . . . . . 1 . . . 1 . . . . . . . . . 1 . . 11 . . . . . . . . . . .

. . . . . 1 . . 11 . . . . . . . I . . . . . . . . . 1 . . . . . . . . . 1 . . . . . . . . . . 1 . . 1 . . . . . . . . . . .

5112 . . 11 . . 21 . . . 3 . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 . . . 1 . . . . . . . . . . . . . . . . .

1 . . 1 . 2 . . . . . 1 . . . 1 . 1 . . . . . . . 1 . • . 1 . . . . . . . . . . 1 . . . . . . . 1 . . . . . . . . . . . . . . .

1 . 11 . . . . 11 1 .1 . . 1 . . . . . . . . . . 1 1 . . . . . 1 . . . 1 . . . . . 1 . . . . . . . 11 . . . . . . 1 . . . .

. . . . . . . . 21 . . . . . . . . . . . . . . . . 1 1 . . . . . . . . . . . . . . . . . . . . . . 1 1 . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . .

. . . . . . . . . 1 . . . . . . . . . . . . . . . . . 1 . . . . . 1 . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . .

. . . . . . . . 11 . . . . . . . . . . . . . . . . . 1 . . . . . 1 . . . . . . . . . . . . . . 1 . 51 . . . . . . . . . . . .

. . . . 1 . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . .

. . 1 . . . . . . 1 . . . . . . . . . . . . . . . . . 1 . . . . . 1 . . . 1 . . . . . . . . . . . 51 . . . . . . . . . . . 1

. . . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . 1 . 1 . . . . 1 . . . . . . . . . . . 2

. . . . I . . . 3 . . . . 1 . . . . . . . . . . . . 1 1 . . . . . 1 . . . . . . . . . . . . . . . . . 31 . . . . . . . . . 2 1

. . . . . . . . 1 . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . 11 . . . . . . . . . . 1

. . . . . . . . . 1 . . . . . . . . . 1 . 1 . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1

. . . . . . . . . 1 . . . . . . . . . . . . . . . . 1 1 . . . . . 2 . . . . . . . . . . . . 1 . . . . 1 . . . . . . . . . . . 1

. 1 . . . . . . . 1 . . 1 . . . . . . . . . . . . . 1 . . . . . . . . . . 1 . . . 1 . . . 21 . . . . . . 3 . 1 . , 31 . 1

. . . . . . . . . . . . . . . . . . . . . . . . . . 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

. , 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 . 1 . . . . . . . . . . . .

in stabilizecl dunes. Many of the component species, including R. laetevirens, form part of the semideciduous tropical forest of the region.

46. Acacia macracantha type. (1 bcl; 18 rel). This type is only found in La Mancha, where it is frequently associated with Ran- dia laetevirens, Lantana camara, Porophyllum nurnmularium, Bidens pilosa, Psidium guajava, Acacia farnesiana, Diphysa robinioides and Paullinia tornentosa. It forms big patches of high, dense, species rich spiny thickets ( 2 - 5 m). It is found on plateaus and flat areas, locally close to humid slacks.

47. Acacia farnesiana type. (6 bcl; 52 rel). This type is frequently accompanied by Randia laetevirens and Lantana camara. In La Mancha it is commonly found with Diphysa robinioides. The herbaceous species that establish under Acacia (which is usually a shrub, sometimes a tree) vary much throughout the study area. Acacia usually forms patches of high thickets ( 2 - 4 m) but in some areas of Tamaulipas (for example Bocato- ma) its branches grow very close to the ground. The type grows in the sheltered area behind the foredune (Tamaulipas) and on sheltered plateaus and in hollows.

48. Caesalpinia bonduc type. (3 bcl; 21 rel). This type was found only in the northernmost areas, with Borrichia frutescens, Palafoxia texana vat. robusta, Croton punctatus, and also Randia laetevirens and Chiococca alba. It forms dense spiny, patchy thickets of 1-2.5 m. It grows mainly on the gentle slopes of the sheltered back part of the foredune and the flat areas towards the lagoon.

E. Community types of high, non-spiny thickets

49. Dalea scandens-Eupatorium odoratum type. (1 bcl; 9 rel). This type was only found in Sontecomapan, frequently associated with Coccoloba barbadensis, Iresine celosia, Randia laetevirens, Dioon edule and Paspalum monostachyum. The species form patches of scrub which are mixed with the Randia and Coccoloba types on the foredune.

50. Lantana camara type. (1 bcl; 7 rel). This type with one characteristic, dominant species was only found in La Mancha. It is associated with Psidiurn guajava and Bidens pilosa and forms clumps, 2 - 3 m tall, in stabilized areas.

51. Verbesinapersicifolia type. (3 bcl; 18 rel). This type is frequently accompanied by Macroptilium atropurpureurn, Porophyllurn nummularium and Randia laetevirens. In La Trocha this type was commonly associated with Cissus sicyoides and Passiflora sp. It forms clumps of dense thicket 1 - 2 m tall, in stabilized areas.

52. Coccoloba barbadensis type. (6 bcl; 82 rel). The characteristic dominant species belongs to the Polygonaceae. It forms a 2 - 3 m high thicket with Randia laetevirens, Verbesina persicifolia, Cissus sicyoides, Psychotria spp. and Burnelia retusa. It occurs intermingled with the R. laetevirens type. C. barbadensis is a species also found in the deciduous tropical forests of the area. It grows on gentle slopes and plateaus of stabilized areas and on stabilized foredunes.

53. Diphysa robinioides type. (5 bcl; 56 rel). This type is only found in La Mancha and Rancho El Quijote, with Randia letevi- rens, Amphilopium paniculatum, Acacia macracantha and Bursera simaruba. Iresine celosia is very common in Rancho E1 Quijote as an undergrowth herb. These species form large patches of high, species rich thickets; at places they appear on

165

the border of the deciduous tropical forest bordering the dunes. They can be 3 - 8 m tall, especially in the last mentioned areas.

54. Chrysobalanus icaco type. (4 bcl; 24 rel). This member of the Rosaceae is the dominant characteristic shrub of the type. The accompanying species vary very much between the four areas. It is found only in the north and in one site in the south of Veracruz (La Trocha). Among the frequent species found in the type are: Citharexylurn berlandieri, Cissus sicyoides, Psy- chotria erythrocarpa, Randia laetevirens, Firnbristylis spadicea (in Bocatoma), Dalbergia brownei (in E1 Raudal). It forms a 1 .5-2 m tall, thicket, distributed discontinuously in clumps. It grows in sheltered areas behind the foredune (Tamaulipas) or in gentle slopes and swales between low ridges.

55. Psidiurn guajava type. (2 bcl; 19 tel). This type occurs only in Barra del Tordo and Riachuelos. In both areas it is associated with Randia laetevirens. It forms high thickets, 2 - 3 m, which are often protected by the people because of the fruits. The characteristic species, a Myrtaceae, is commonly found in disturbed areas, having been left as a remnant of the former vegetation. It grows in sheltered areas behind the foredune or forms clumps in stabilized zones.

56. Hibiscuspernabuscensis type. (3 bcl; 15 rel). The characteristic species, a Malvaceae, is associated with Psychotria erythrocarpa. In Sontecomapan it was found on the beach close to the mouth of a small stream, associated with several shrub or tree species like Piper sp. and Chiococca alba. In the other site, Riachuelos, it appears close to the humid and wet areas, where the undergrowth is mainly Lippia nodiflora, Cyperus articulatus and Scirpus sp. In Tamaulipas it was found associated mainly with Conyza canadensis. The first three species are characteristic of Tamaulipas. It forms a 2 - 4 m tall tree layer; the undergrowth of shrubs and herbs differs.

57. Nectandra coriacea type. (1 bcl; 5 tel). This type is found in El Raudal, associated with Ambrosia arternisiifolia, Serjania racernosa, and less commonly with Randia laetevirens, Dalber- gia brownei and Citharexylum berlandieri. The characteristic species belongs to the Lauraceae. It forms a rather diverse thicket, with 2 - 3 m tall clumps in stabilized areas.

58. Crossopetalum uragoga-Schaefferia frutescens type. (l bcl; 6 rel). This type has many frequent species besides the two characteristic ones: Casearia nitida, Karwinskia hurnbold- tiana, Eugenia capuli, Chiococca alba, Bursera sirnaruba, Di- ospyros sp. Less frequent are Randia laetevirens, Diphysa robinioides and Arnphilopiurn paniculatum. The type is very rich in species and forms tall thickets, with virtually no undergrowth, which share many species with the deciduous tropical forest of the area.

Community types of the Yucatan Peninsula

The Yucatan Peninsula region includes 31 community types, 5 of which are based on one basic cluster, 10 on two and 16 by 3 - 8 basic clusters. They will be described in the following sections: beach, embryo dune and foredune, humid and wet slacks, sheltered zone, sites with a humid climate, or where fresh water is available, and dry dunes. Many of the communities are similar t o the Gulf

166

types described above. There are no blowouts and active dunes in the Peninsula. The rare occurrence of strong winds (with the exception of some hurricanes), the generally moist winds (also the strong 'nones' in winter are moist), the coarseness of the sand, and heavy tidal drift prevent sand movement. Physiognomically and floristically the vegetation resembles the coastal vegetation of the Caribbean islands (Gleason & Cook, 1926; Sauer, 1982; Beard, 1949; Randall, 1970) as well as that of Florida (Kurz, 1942; Oosting, 1954).

1. Beach vegetation

Salt spray and wind are major environmental factors affecting the vegetation on beaches. Howev- er, there is relatively little sand movement as explained above. The relatively high moisture content of the sand is explained by the heavy summer rainfall in the humid parts of the coast of the Peninsula and the humidity of the 'nortes'. Airborne salt becomes important in the period of drought. High amounts of tidal drift material are deposited upon which the ephemeral community types thrive. The communities are monospecific, and have only one layer (Table 8; Fig. 3D-E) formed by succulent erect herbs that complete their life-history in summer time. They tolerate salt spray, high amounts of nitrogen (from the algal drift), as well as occasional

inundations with sea water. Ephemeral tide mark communities rarely act as dune builders, though their roots help to bind the sand.

59 Cakile spp. type. (3 basic clusters inc luding 29 relev6s). The character is t ic succulent herbs are C. edentula and C. lanceolata which were difficult to differentiate during field sampl ing . To avoid confus ion , the two species were considered as character iz- ing one and the same community type, since both occur in exact- ly the same envi ronment . The creeping species Ambrosia hispida and Sporobolus virginicus and the erect species Atriplex pentandra and Suaeda linearis are f requent c o m p a n i o n s wi th a low cover. They grow as i so la ted ind iv idua l s and migh t occur on the foredunes or a t the edge of the mangrove. The community is 40 cm high and single-layered. It is very common in s u m m e r on the h igh- t ide m a r k a long the no r the rn beaches of the Penin- sula.

60. Suaeda [inearis type. (3 bcl; 14 rel). The character is t ic species is a ha lophyt ic , succulent , annua l , erect herb, typical of the salt marshes of Sou thern Texas. Euphorbia buxifolia and Cakile spp. are f requent compan ions . I t can also grow at the edge of the mangroves where sal ine cond i t ions prevail. Dur ing the summer the community has a pa tchy d i s t r ibu t ion a long the northern beaches of Yucatan.

I1.a. Embryo dune vegetation

On embryo dunes of the Peninsula various types are formed by single dominant creeping herbs. The communities are formed by clumped individuals of one or two species. They can grow close to the beach communities and can also mix with the fore-

Table 8. Basic clusters and community types for the beach, embryo and foredune vegeta t ion a long the Car ibbean coast . Cakile spp. includes C, lanceolata and C. edentula. Cenchrus spp. includes C. echinatus, C. tribuloides and C. incertus. Hymenocallis spp. includes

tl. americana and H. caribaea.

BASIC CLUSTER NUMBER: 11 100 646

COMMUNITY CODE: 59

Cakile spp, 886 5uaeda lineari8 I,. A t r i p l e x pentandra 5,6 A t r i p l e x sp . . . . I p o l o e a p e a caprae 1.1 Seauvitm portutacastrum .1 . Euphorbia b u x i f o l l a 1.. Sporobolua vizlginicus 11. Canavmlia r o ~ . . , Croton Ix~ctat~s . . . Ambrosia hispida 1.1 Cenchrus app. .3, Iournmfortia gmK~halodes . . . 5vaevo|a pltmieri . . . Lycium carolinianun . . . Sorimmmaritima . . . Hymenoeallis spp . . . . Thrinax radiata . , . Ernodes ]ittoralJs . . . Coecoloba u v i f e r a . . .

1 1 1 24 491 823791 449 10

268 730 657622 295 38

60 61 62 63 64

132 2.1 .211.1 . . . . . 793 2,1 111..1 2 1 . . . , .1 4,1 . . . . . . . . . . . 1 t6 . , 1 .1 . . . . . 1. . . . .4 595 . . . . 11 . 1 . . . . 1 . .11 998898 1 5 . . . 216 ,12 111121 6 8 6 . , .2 . .11 111211 812 11 . . . . . . . . . I.. .1. 65 I .... 1 1.3..I .2... ..... 1 11141..I. 11 . . . . 11 I...1, 11, ..

• . .2. I,,21. ,I. ..

..... I..I .......

. . . . . . I .... 1 ....

. . . . . . . . 11. I ....

11 1 I 1 301 2555780 2367780 2357881

1696 53243255 44480547 38503264

65 66 67 68

. 1 . . 1 . . . . . . 1 . . . . . . . . . . . . . . . . 13.1 12 . . . . . 2 111 . . . . 1 . . . . . . . . ,..I ....... I ,.I ..... 1.1 ..... .I .......................... . . . . . 1 . . 3 . . 1 .1 . . . . . 1 . . . . . 113 1111 .1 . . . . 12 111 . . . . 1 1 , 1 3 1 . 1 . 5415 33.31122 1 3 1 , . 1 . 2 4 ,24 .112 111. . 1 . 1 . 1 1 . 1111111, 1 , 1 . 3 1 1 . . . . . . 1 . , . 1 . 1 . . . . . . . . . . 61 .1 . 5482 1 . . . . . . . 1 .4 . . . . 1 3.1 . . . . . 1131 11181511 11124432 11194.3. 1 .Z2 . . . . . . 13 11.1.111 2 1 1 1 1 . . . . . . . 99998668 . 1 .1 ,111 1 . . . 1 . 1 . 1.11 2 5 . . . 3 1 3 99788998 3 . 1 . 1 . . . 111. .1 . . . . . . 111 . . . . . 1.1 . . . . . . . . 1 1 1 . . 8 4 4 . . 3 . 3 7 1 1 4 99989889 . . . . . . . . . 11 . . . . 24 .1 . 1 . 1 . . 2 2 . . . . . . . . . . . . . . . . 5 6 1 1 . . 1 . . . 6 5 . . . . . . . . . 5111 . . . . . 31Z . 1 . . 1 . 2 . . . . . . . . . . 1 . . 1111 . . . . . 1 1 . . . 1 .

d u n e types . Sporobolus virginicus, a f r e q u e n t g rass

h a s u n d e r g r o u n d v e g e t a t i v e r e p r o d u c t i o n . I p o m o e a pes-caprae a n d Sesuvium portulacastrum a re

s t o l o n i f e r o u s h e r b s , a n d Euphorbia buxifolia h a s a

d e n s e r o o t sy s t em. A l l t o l e r a t e s a n d a c c r e t i o n a n d

sa l t spray. T h e c o m m u n i t i e s a re c a l l e d ' e n r i c h m e n t '

t y p e s b e c a u s e t h e i r spec ies m a y b e i n t e r p r e t e d as in -

d i c a t o r s o f t e m p o r a r y c h e m i c a l e n r i c h m e n t ,

o r i g i n a t i n g f r o m t i de m a r k m a t e r i a l a n d sa l t spray.

61. Ipomoea pes-caprae type. (3 bcl; 12 rel). The dominant species is a perennial creeping herb occurring in most areas of the Gulf and Caribbean regions. It is especially common on beaches with high sand accretion. On Caribbean islands it seems to be a more important dune-builder but on the Peninsula it is an uncommon outpost pioneer abundant in disturbed ruderal areas, recreational beaches and dumps close to coastal villages. Many other pioneer herbs are associated and form a 10 to 20 cm high community which may cover large areas.

62. Sesuvium portulacastrum type. (6 bcl; 69 rel). This succulent halophytic Aizoaceae is a common beach pioneer that also occurs in seashore mangroves and on limestone beaches. It is a perennial 15- 20 cm high herb which forms isolated mounds of dense vegetation with a patchy distribution along the beaches. The community is similar to the Gulf type and can be considered one and the same for the overall survey, but differs floristically through Caribbean herb species such as Euphorbia buxifolia and Suaeda linearis. Grasses like Sporobolus virginicus and Cenchrus spp. are common species. It is found on beaches where it tolerates accretion, and together with the following community type it is the best dune-builder along the shores of the Peninsula.

63. Euphorbia buxifolia-Sporobolus virginicus type. (3 bcl; 15 rel). The characteristic Euphorbia is an erect 10-15 cm high herb, and if longer it grows postrate. The dominant Sporobolus is a creeping grass common on tropical beaches. This type can occur on foredunes together with pioneer thickets when growing on uppermost small beach ridges. Suaeda linearis and Sesuvium portulacastrum are frequent. The type resembles that in the Gulf but the dominance of Euphorbia differentiates it as a Caribbean type.

64. Canavalia rosea type. (2 bcl; 8 rel). The name-giving Leguminosae occurs together with Sporobolus virginicus. In general it occurs in flat areas where foredunes are missing; here Ambrosia hispida is the main companion species. It can also be found as a climber on shrubs of Lantana involucrata in the Pithecellobium keyense type, especially in disturbed areas in southern Quintana Roo.

ll.b. Foredune vegetation

F o r e d u n e v e g e t a t i o n c o n s i s t s o f w o o d y vege ta -

t i o n g r o w i n g o n t h e u p p e r m o s t p a r t o f t h e b e a c h .

T h e f o r e d u n e is u n i n t e r r u p t e d a n d 1 - 4 m h igh .

M o n o s p e c i f i c c o m m u n i t y t y p e s a re c o m m o n l y

r e p r e s e n t e d b y f o u r m a i n s h r u b s w i t h a d e n s e c a n o -

py f o r m i n g o n e layer o f v e g e t a t i o n . B e a c h h e r b s oc-

167

c u r as b o r d e r l i n e species , excep t Euphorbia buxifolia. T h e y f o r m a 1 . 5 0 - 2 . 0 0 m h i g h be l t o f c lose

s h r u b s ( h e d g e ) w h i c h w o r k s as a w i n d b r e a k e r . Spe-

cies f o r m i n g t h e s e c o m m u n i t i e s t o l e r a t e sa l t s p r a y

a n d s a n d d e p o s i t i o n ; t h e y h a v e a b u n d a n t s u c c u l e n t

leaves. T h r e e o f t h e f o u r c o m m u n i t y t y p e s m a y oc -

c u r o n l i m e s t o n e b e a c h e s w h e r e s t a b l e soi ls a re

f o u n d . ( I n t h a t case t h e y a re a s a l t - s p r a y s c r u b in

t h e s ense o f D o i n g , 1981).

65. Euphorbia buxifolia-Croton punctatus type. (4 bcl; 91 rel). Especially in Yucatan a mixture of pioneer herbs and shrubs develop. Herbs (15-20 cm) and low shrubs (50-70 era), constitute this type. Pioneer herbs are frequent whilst Melan- thera nivea and Ambrosia hispida are common backdune herbs species. Some thicket species like Ernodea littoralis and Lantana involucrata can be present in the type. In disturbed places the parasitic Cassytha filifera is expanding. The type is similar to those with C. punctatus in the Gulf but again the presence of E. buxifofa is important to differentiate it as a Caribbean community type.

66. Tournefortia gnaphalodes type. (8 bcl; 72 rel). On some beaches this is the outpost scrub type. The dominant species is a Boraginaceae usually found at the bottom of the frontal ridge, in front of the Scaevolaplumieri or the Suriana maritima type. Pioneer herbs such as Euphorbia buxifolia, Ambrosia hispida and Sporobolus virginieus are still frequent. It constitutes a belt of 1-1.5 m height with dense vegetation. In Quintana Roo it can be the only foredune scrub type. It also occurs on limestone beaches where it forms a salt-spray scrub.

67. Scaevola plumieri type. (8 bcl; 86 rel). The dominant species, a Goodeniaceae, forms a scrub belt along the ridge. Ambro- sia hispida and Sporobolus virginieus are borderline companion species which can grow around shrubs. Probably this pattern was produced after severe cutting of the vegetation (building of roads or planting of coconuts). When humid conditions prevailed the type goes further inland and it can mix with sheltered zone types and fixed dune thickets.

68. Suriana maritima type. (8 bcl; 45 rel). Several species are frequent in this type but Suriana maritima, Simaroubaceae, is the only dominant. The type occurs on the uppermost part of the foredune where it forms a high belt of dense vegetation. The type tolerates accretion and salt spray. If erosion occurs it may be found as the outmost pioneer community type. It is present at the transition between beaches and sheltered zones where beach and embryonic herbs like Sesuvium portulacastrum and Eu- phorbia buxifolia are mixed with palm (Thrinax radiata) communities or dry thicket types (Bravaisia tubiflora). Where narrow ridges occur it mixes with seashore mangrove.

111. Vegetation o f b lowouts (absent in the Caribbe- an region)

I E Vegetation o f h u m i d and wet slacks

I n t h i s s e c t i o n c o m m u n i t y t y p e s o n a k i n d o f

' o v e r w a s h ' s a n d y f la t s , t h a t t o l e r a t e f l o o d i n g a re in -

168

Table 9. Basic clusters and community types for the vegetation of humid and wet slacks. Hymenocallis spp. include H. americana and H. caribaea. Gomphrena spp. include G. disper- se and G. flilosa.


COMMUNITY CODE:

Lyciua carolinianum Flaveria l inear ia Gossypium hirsutum Waitheria indica 5caevola pIumieri Suaeda linearis Bravaisia tubiflora Atriplex sp. 2 Maytenue phylantoides Atriplex sp. 1 Opuntia stricta Ireeine celoaia Sesuvium portulacaetrum Cenehrue app.' Melanthera nivea Ernodea littoralie Euphorbia buxifolia Gomphrena app. Lantana involucrata Agave angustifolia Lippianodi f lora Eleoeharia geaiculata C o n o c a g p u a e r e c t u a Phyloxereus vermicularis Cyperua articulatu8 Suriana maritima Thrinax radiata Sporoboius virginicus Ambrosia hispida NymenocaIIie app. Canavalia rosea

1 23 11

879 99

69 70

985 1 ) . . 4 87 1 . . 31 1 . . 1. . 1 . •1 1 . . . . . 1 . . 1 , . 2 , . . . . . 4 . . . 1. . . . 21 . . . 1 1 • . 1 . 1 111 . . 12, . . . . . 1. 1 . . . . .12 5. .11 . . 1 , . 1.

9 68 7 29

71 72

. I

21

96 •1 ,1 73 24 3 . .2 31 .1

cluded (Table 9; Fig. 3 D - E ) . In the Peninsula there are no wet or humid slacks proper as in the Gulf region, but flat humid plains f looded during the summer alongside the lagoons or close to the seashore mangroves provide a similar environment. Sand movement may still occur but water and salt spray are more important, hence highly salt- tolerant community types are present. Herbaceous communities o f these areas resemble those on the salt marshes o f North America.

A. Community types along inland mangrove ridges

They mainly occur opposite to lagoon mouths, in flat swampy areas covered with herbaceous vegetation with some scattered mangroves (Conocarpus erecta and Laguncularia racemose). Both are windward types occurring in relatively exposed areas, mainly on the coasts o f Yucatan. They are considered here only as far as they occur intermingled with the dune vegetation. This mixture o f vegetation types is very c o m m o n in the Peninsula. In Yu-

catan they grow together with Batis maritime and Salicornia bigelovii and have a high cover.

69. Lycium carolinianum type. (3 bcl; 14 tel). This type is dominated by the Solanaceae L. carolinianum, a postrate succulent 2 0 - 2 5 cm high herb. It grows on both seaward and land- ward ridges. It tolerates wind action from the north when growing on foredunes, and from the south when growing on the borderline with mangrove vegetation• Herbs as Melanthera nivea and grasses of the genus Cenchrus are common in this type. This community is absent along the Quintana Roo coasts. It tolerates high salinity values during the drought period.

70. Flaveria linearis type. (2 bcl; 30 rel). The dominant species, a Compositae, forms a herbaceous 3 0 - 5 0 cm high vegetation. The type occurs at the margins of mangrove forest where high soil salinity values may occur. In flat areas Lycium carolinianum is frequent and has high cover values. The erect herb Iresine celosia may be abundant during the rainy season. The succulent Opuntia stricta var. dilleni and the shrub Gos- sypium hirsutum are also common• The type is often found along roads, and Tribulus cistoides is a companion species. The type is abundant in Yucatan but rare in Quintana Roo.

B. Seashore mangrove community types

Where the dune or barrier island is too narrow the mangrove reaches the seashore and two special community types develop. They receive saline water at both sides, at the back from the lagoon and at the front from the sea. They occur mainly in Quin- tana Roo, but can be found in Yucatan as well, as borderline communities of the high seashore mangrove complex.

71. Lippia nodiflora-Eleocharis geniculata type. (1 bcl; 6 rel). A herb community type which occurs only in seashore mangrove forests• This is the community of the inundation zone which tolerates regular sea water inundation. This explains why salt marsh species occur as companion species. Both dominants are 1 0 - 2 0 cm high, the first is a creeping Verbenaceae, the second a Cyperaceae (which may become taller elsewhere). Three main frequent species are Ambrosia hispida, Hymenocallis caribaea, and Sesuvium portulacastrum. The latter species occurs at the margin towards the sandy beach, if present. The Lippia type is similar to the type of the dune slacks of the Gulf region but differs in companion species and here it thrives closer to the beach.

72. Conocarpus erecta type. (2 bcl; 26 tel). The dominant species (Combretaceae) has a shrubby growth and in this community type it never reaches more than 2 m. Thicket species like the palm Thrinax radiate are frequent• In the seashore mangroves of Quintana Roo Suriana maritima is a frequent species while in Yucatan Maytenus phyllantoides is a common species growing at the edge of the community; the Caesalpinia vesicaria type is also frequent in the neighbourhood of the type. The type occurs on flooded soils with saline water, between the last stabilized inland dunes and the edge of the lagoon.

V. Vegetation of the sheltered zone behind the foredunes

Immediately behind the foredunes the vegetation is formed again of herbaceous species that grow around sparsely distributed patchy shrubs. In contrast to the pioneer types the communities of sheltered zones are very rich in species. On dry coasts the Agave type can be found along the land- ward slope of the foredune, where still some sand deposition occurs (Table 10, Fig. 3D-E) . Physiog- nomically the sheltered zone is different due to the different growth forms of the community types.

73. Ambrosia hispida type. (6 bcl; 72 rel). The dominant A. hispida, a Central American Compositae, 15-25 cm high, tends to spread over large areas; it is frequent over all the Peninsula dunes. Pioneers such as Sporobolus virginicus, Canavalia rosea, Phyloxerus vermicularis and Euphorbia buxifolia represent beach communities; but inland species such as Hymenocallis caribaea, Melanthera nivea and Gomphrena dispersa are the most frequent ones. The type occurs especially around the monospecific salt-spray scrub (Scaevola plumieri).

74. Hymenocallis spp. type. (2 bcl; 12 rel). The dominant herbs (30-50 cm) are H. caribaea and tl. americana (Amaryl- lidaceae). These species are taxonomically very similar; both grow in scattered patches on the top of ridges or in flat areas behind foredunes. At Sisal the companion species Agave angustifolia abundantly mingles with the dominant species, in Quintana Roo Ambrosia hispida and Canavalia rosea are the

Table 10. Basic clusters and community types for the vegetation of the sheltered zone. Hymenocallis spp. include H. americana and H. caribaea. Gomphrena spp. include G. dispersa and G. pilosa.

BASIC CLUSTER NUMBER: I 56880

937038

COMMUNITY CODE: 74

Ambrosia hispida 899988 Thr inax r a d i a t a . . 1 2 . . Euphorbia buxirolia 1.1.11 Sporobolu8 v i r g i n i c u a . .3111 Hyuenocallis app. 1.211. Ernodea littoralis . . 3 . 11 Gomphrena app. 1 . . 2 1 . Melanthera nivea 11.111 W a l t h e r i a i n d i c a 1 . . . . . Cenchrus app. 47 . .11 Canavalia rosea I..31.

17 48

75 76

24

. .

. . 89 .1 11 11 1. .1 .2

Biden8 p i l o 6 a . . . . . . 1. Sporoboluspyrmmidatua . . . . . . . .

Sur iana mar i t ima Opu.tia ~t~i~ta ; i i i i i ~i Agave anguatifolia I ..... 2. Bravaisia tubiflora 1 ..... 2. Lippia oodiflora ...11. .I Eleocharis caribaea .... 1. ..

Lantana involiucrata ...I.. 11 Sesuvium portulacastrum ..... 3 .. Conoearpua e rec tus . . . . . . . .

169

main companion species. The type is more conspicuous during rainy summer months.

75. Bidenspilosa-Sporobolus pyramidatus type. (1 bcl; 3 rel). Bidens a Compositae and Sporobolus, a pioneer grass form a community type frequent on narrow beaches along roadsides in Campeche. Several pioneer thicket species are found here as well. The type is common on ruderal places in sheltered zones. In Yucatan it is rare, but it might become more common since new 'beach roads' are being built. It is also found on low foredunes. The type is similar to the Bidens types of the Gulf region (Veracruz-Tabasco-Campeche), but the presence of Sporobolus pyramidatus makes it slightly different producing a typical combination of the Yucatan coasts.

VI. Vegetation of fixed dunes

a. Thickets in humid areas where fresh water is available (springs in San Benito, places with heavy rainfall in Quintana Roo)

The vegetation is three-layered although the herbs are almost inconspicuous; shrubs are non- spiny and small trees and palms ( 2 - 4 m tall) form the higher layers (Table 11, fig. 3D-E) . All species forming the following types tolerate soil salinity but they better develop at lower salinity levels.

76. Chrysobalanus icaco type. (2 bcl; 17 rel). The dominant, a shrubby pantropical Rosaceae, grows in sheltered areas where fresh water is available. Frequent accompanying species are Bor- richia frutescens, Erithalis fruticosa and Ernodea littoralis. To- gether they form a dense thicket, 1 - 2 m tall, with a poor herb layer. The type occurs also in Tamaulipas but here it is more abundant.

77. Pseudophoenix sargentii type. (1 bcl; 58 rel). The dominant species is a typical Caribbean palm. Structurally it is more complex than related types: there are three layers and epiphytes are abundant. The herb layer is poor in species, with mainly Gomphrena spp. Frequent accompanying shrub species with high cover are Bravaisia tubiflora, Pithecellobium keyense and Enriquebeltrania crenatifolia and palms of similar height (1.50-2.50 m) such as Thrinax radiata and Coccothrinax readii. In this layer succulent thorny species are frequent: Acan- thocereus pentagonus, Selenicereus hondurensis and Agave angustifolia. The ephiphytes include Bromeliaceae as well as lichens. The dominant species is a rather tall (3 m) native palm growing on the ridges of the Peninsula. In El Cuyo and on limestone soils in Quintana Roo it reaches 6 m. It grows mainly in hollows that are wind-protected and moist. It is the only place outside the Caribbean islands where the type occurs (Quero, 1981).

78. Thrinax radiata type. (7 bcl; 153 rel). Thrinax is a common small 2 - 3 m high palm species. The endemic Coccothri- nax readii and the poisonous Metopium brownei are frequent accompanying shrubs. Few herbs grow under the palms shadow; they occur as borderline species. The type intermingles with other thicket types covering interior ridges. It is also common on

1 7 0

Caribbean islands and in Florida where it grows together with Coccothrinax readii and Coccoloba uvifera.

79. Coccoloba uvifera type (8 bcl, 26 rel). The dominant is a Caribbean Polygonaceae, a coastal small tree of 2 - 4 m or a high shrub of 1.5-2.0 m common inal l the coastal areas of the Peninsula. The herb layer is dominated by the creeping herb Ambrosia hispida and the lily Hymenocallis spp. Other shrubs like Pithecellobium keyense, Ernodea littoralis and the palm Thrinax radiata are frequent. The dominant species forms dense thickets mainly on the Caribbean coasts where it is supposed to be the most important dune builder acting in a similar way as the Suriana, Scaevola and Tournefortia foredune types in the Peninsula of Yucatan.

80. Ernodea littoralis type. (6 bcl; 28 rel). The characteristic species of the type, a Rubiaceae, form an 0.5-1.2 m high impenetrable thicket which covers most of the interior ridges. Am- brosia hispida occurs frequently as a borderline species. The Caribbean shrub Erithalisfruticosa is a companion in Quintana Roo. Small trees of Coccoloba uvifera and the palm Thrinax radiata are found as well.

Table 11. Basic clusters and community types for the vegetation of the non-spiny thickets located in areas where fresh water is available• Hymenocallis spp. include H. americana and H. caribaea. Gomphrena spp. include G. dispersa and G. pilosa. Ageratum spp. include A. littorale and A. maritimum.


COMMUNITY CODE:

Chrysobalanus ie~mo Pithecellobium keyense P~u(k)p#~mix M r c J ~ t i i l h r i n a x rodiata Coccothrinax readii Metopium brownei 8ravaisia tubiflora Suriana maritima C o c c o l o b s u v i f e r a A r u n d o d o n a x

Hymenocallis app. Ambrosia hispida E r n o d e a l t t t o r a l i a E r i t h s l i a fmuticoea Sporobdlus virginicus P o u t e r i a campechlana Malvaviscus arboreous Borrichia frutescens Canavalia rosea Bumelia retusa Aechmea bracteata Echites yucatanensis Euphorbia dioica 3acquinia aurantiaca Canavalia rosea $chomburgkia tibiscinia Lantana involucrata Cyperus articulatus Cynanehum schlechtendalii Gomphrena spp. Altheroantera ramosissima Capparis incana Euphorbia buxifolia Scaevola plumieri Tournefortia gnaphalodea Melanthera nivea Ageratum spp. .. Enriquebeltrania crenatifolia

1 1 1 69 3 2356669 36677889 555781 0 O

19 0 0190390 85619174 678481 3 1

77 78 79 80 f l l 82

88 . . . 1 1 . . 1 . . . . . . . . . . . . . . . . .

55 2 , 2 . . . 7 1 . . 8 . . 1 1 . . . . . . . 1 .

6 . 1 . . . . . . . . . . . . . . . . . . . . .

• 1 4 6896889 . . . 7 1 7 1 5 . . 4 . 1 . 4 2

11 4 2 1 . 6 . . 1 . . . . . . 1 . . . . . . . . 1

11 3 1 . 6 . . . . . . . . . . . . . . . . . . .

7 . . . 9 7 . . . . . . . . . . . . . . 1 • •

• 1 . . . . . . 71 1 . . . . . 1 . . . . . 11 • 1 15 1 4 1 . 1 7 . 4 69979899 2 5 . . 2 1 3 •

. . . . 11 . . . . 411 . . . . . . . . . .

.1 .... 2.. 66.4531 . . . . . 1.1 1

.1 . . 1 . . . 1 1 5 4 . 5 1 1 5 1 274623 . 4

. 1 1 1 1 . 1 . . 4 1 . . . . 1 . 7 958888 . 1

. 1 . . . . . . 1 . . 4 . . . 1 . 4 8 5 . 1 . 8 1

. . . . . . . 21 2 . . . 2 1 1 . . . . 311 . .

. . . . . . . . . . . . . . . . . . . . . . . . . 9

. . 1

11

.1

.1 z

11 41 1. .1 .1

11 . . . . .

. . . 1 . . 1

1 . . . . . .

11 . . . . .

11 . . . . .

11 . . . . .

• . 1 1 . . . 1 1 2 1 . . .

1 . . . . . .

1 . . 1 . . . .1 . . . . .

. 1 3 2 . . 1

. . . . . . . . . . . . . 1 . .

. . . 1 1 . 1 . . . 1 , 1 .

. . . 2 . . . . . . . . . 1

1 . . . . . . . . 4 . . . .

. . . . . 11 . .1 . . . .

. . . . . . . . . 3 . . . •

. . . 1 1 . 1 . . . 1 . 1 1

. . . . . . 1 . . . . 11 . 1

. . . . . . 1 . . . 1 . 1 . . . 1 2 3 . . 1 . . . . . . . . . . 1 . . . 1

11 .1 . . . . . . . . . 1 . . . 3 1 . 1 1

. , : . . . . 1 . . . . . . 1 . . . . . 1 . .

1 . . . . . 1 . . . . . . . 1 . . 3 . . . 1 . . . . . . 1 . . . . . 1 . . . . . 1 .1 . . . . . . . . 18 . . . . . . . . . . . 1 . . . . . . . . . 2 . . 1 . 1 . . . . . . 1 .

.1 . . . . . 2 . . . 1 . 1 . . . . . . 1 .

. . . . . . 1 . . . . . . . 2 . . . . 1 . 1

11 . . . . . . . . . . . . . . . . . . . .

81. Erithalisfruticosa type. (1 bcl, 3 rel). The dominant species is a Caribbean 1-1.5 m tall Rubiaceae shrub. Other species in the shrub layer are Ernodea littoralis, Thrinax radiata and Coccoloba uvifera, and in the herb layer Ambrosia hispida and the tiny climber Cynanchum schlechtendalii.

82. Pouteria campechiana type. (1 blc; 8 rel). The dominant species, a Central American Sapotaceae, is a shrub or small 1.5-2.0 m tall tree, 1.5-2.0 m high. Frequent companions are Bumelia retusa and the palm Coccothrinax readii. The herb layer is dominated by the creeping Ambrosia hispida. The type occurs in patches on the wide internal dunes of Quintana Roo. The dominant shrub is an edible plant and it is cultivated inland in rain forest clearings behind the lagoon.

b. Thickets on dry dunes (Table 12; Fig. 3 D - E )

Such thickets are dominated by succulent spiny species and in contrast to the previous thickets few palms occur. Herbs are also more abundant, especially close to the top of ridges.

83. Cordia sebestena type. (2 bcl; 8 rel). The characteristic species is a small tree 2 - 3 m high. Frequent species in the herb layer are Gomphrena spp. and Hymenocallis spp. In the shrub layer other thicket species are frequent. In Yucatan it mixes with other thickets especially the Bravaisia tubiflora and Pithecello- bium keyense types.

84. Caesalpinia vesicaria type. (2 bcl; 8 rel). The characteristic species is a shrub or small tree of 1.5-2.3 m, which mixes with many dune thicket types at the edge of the lagoon. Accom- panying species are dry Yucatan species such as Flaveria linearis and Maytenus phyllantoides, Together with the Pithecellobium community this type forms a dense impenetrable thicket which is common in the Yucatan littoral.

85. Bumelia retusa type. (2 bcl; 7 rel). The dominant Sapotaceae species forms a dense net of shrubby 0.5 - 1.2 m high vegetation. In Yucatan this type intermingles with other thicket types forming a dense scrub. In Quintana Roo it can be found as isolated shrubs surrounded by herbaceous types described in section V on sheltered zones. Herbs are frequent but only at the margins as borderline species. Thicket shrubs such as Coccoloba uvifera and Erithalisfruticosa and the palm Thrinax radiata are companions in Quintana Roo, whereas Pithecellobium keyense, Bravaisia tubiflora and some Cactaceae and Agavaceae are companions in Yucatan.

86. Bravaisia tubiflora type. (6 bcl, 162 rel). The dominant Acanthaceae, a shrub, is often surrounded by herbs such as Gomphrena spp. Succulent spiny species are common, e.g.: Acanthocereus pentagonus and Agave angustifolia. In San Benito, a high number of fresh-water springs favours the co- occurrence of the palms Thrinax radiata and Coccothrinax readii. For this reason the community is at some places enriched by rain forest species like Hippocratea celastroides, Gymnantes lucida and Manikara achras. It can also be found along the driest edges of the lagoon, but in general the type dominates the hollows between parallel ridges of the dry coasts of Yucatan.

87. Pithecellobium keyense type. (6 bcl; 182 rel). The main constituent is a common Leguminos~te shrub (1.0-2.5 m), which forms a dense thicket. The frequent companions Bumelia

retusa, Enriquebeltrania crenatifolia, Bravaisia tubiflora differ in cover from area to area• In Rio Lagartos-Las Coloradas lichens cover the branches of Pithecellobium keyense. At Sisal and Telchac Thrinax radiata is absent in the type whereas it is frequent in San Benito and Quintana Roe. In Quintana Roe the community invades abandoned coconut plantations and both Cocos nucifera and Lantana involucrata are the main companion species. Nevertheless, the natural community is more characteristic and abundant on the dry coasts of Yucatan where it forms an impenetrable thicket together with other types described in this section. Together with the Bravaisia tubiflora type this is the most common community along the Yucatan coasts.

Table 12. Basic clusters and community types for the vegetation of open, dry thickets as well as the thickets on dry dunes or hollows with moist brackish water. Hymenocallis spp. include H. americana and 1-1. caribaea. Gomphrena spp. include G. dispersa and G. pilosa. Ageratum spp. include A. maritimum and A. littorale•

BASIC CLUSTER NUMBER: 1 19 14 79 22441 134691 78 81 76 729035 224413

COMMUNITY CODE: 83 84 8 5 86 87

C o r d i 8 s e b ~ t o , a 89 . . . . . . . . . . . . . 11. £ a e s a l p i m i a ¥esicaria . . 88 . . . 2 . 1 . . . 1 . . . .

E r i t h a l i s f r u t i c o s a . . . . . 1 . . . . . . . . . . 1 .

Metopium brownei .I ..... 1.11..12... Dactyloetenium aegyptiam .• .1 ...211 ........ Tephrosia einerea . , 1 . . . I . . . . . 1 . . . . . BuBelia r e t ~ 1, . . 99 28211. 5 4 1 . . . Bravaiaim t u b i f l m r a 7, 53 . . 887877 2 . 2 . . 4

Pitheeellobium kmyenee 31 .2 .1 1 . . 3 . 1 658987 Qomphrena spp. 41 5 . . . 21.121 2241.2 Bouteloa repens . . . . . . . . . . . 1 . 1 . • . 1 Ambrosia hispida .1 47 1 . . . . . . 1.211 Ernodea l i t t c c a l i s .1 . . 15 . . . . . . . 1•111 Agzlve ar~Justifolia 1, 1 . . . 4...21 1 2 . . . 2 Csceoloba uvifera .1 . . . 1 1 1 . • 2 . . 1 . . 1 1 Thrinax radials .2 .1 24 . 2 1 1 2 . . 1 . 2 5 . Maytenuo phyllantoides . . . . . . . 341.5 . . . . . 1 Aeanthoeereus penlagonus . . 11 . . 11,111 1 1 5 . . 2 3 u s t i e i a c a r t a g i n e n s i s 1 . . . . . 1.1141 114 . ,1 Opuntia s t r i c t a 1. 1 . . . 1 . . . . 1 1 . . . . . Cynanehum sehleeblendal l i i 11 .1 . . 1 . .111 131 . .1 Lantana involuerota .I . 1 . .

£ n r i q u e b e l t r a n i a erenatlfolia . . . . . .

Hymenocallis spp. .3 . . . . I resine p a n i c u l a t a SporoboIus pyramidatus MaIvaviscus arboreus Selenicereus sp . Gossypium hirsutum Distiehlis spioata Melanthera nivea Capparis fiexuosa Cspparis i neana Ageratum spp. Commelina erecta Rhaeoma crossopetalum Aechmea bracteata Euphorbia dioica Lichens Flaveria linearis Lycium earolinianum 3acquinia aurantiaca Schomburgkia tibiscinis Caesalpinia bonduc Echites y u e a t a n e n s i s Tribulus cistoides Pseudopboenix ssrgentii

J . I... I ..... I. I... I ..... I. 3... I. 2. .. .11.1. 1. ., 11 1 . . 1 .1 . 1 1 . . .

1 . . . 1 . . . .1 . . 1 . . . 11 . . 11 . . 1 . . .

1 . .111 121121 1 . . 1 1 . 121 . .1 1 . . . . . . . . 21. . 1 . . . 2 1 . . . . 1 . 1 . . 2 . , 1 2 . . . 11 .1 .1 211 . .1 11.111 1 1 . . . 1 2 . . . . 1 1 . . . . 1 1 . . . . . 1 . . . . . 2 . . . . . 11. .11 1 1 . . . 1 1 . . 1 . 1 1 . .111 111.12 • 1 . . 1 . 1 . .111 1 . . . . . 1 . . . . 1 1 . . . 1 1 1 1 . . . 1 1111. . 1 3 1 . . . . 1 1 . . 1 . . . . 11 1111.1 1 3 . . . 1 1111.2 . . . . . 1 1 . . . . 2 . . 1 . . 1 11.121 . 1 1 . . 1 1 . . . . . 1 1 . . . 1 .1 . .11 . . 5 . . 1 1 . . . 1 . . 1 . . . 1 1 . . . . . 1 1 . . . 1 • . .12. .1 . . . .

111 11234011 14 01135278 5 6

88 89

. . . 1 . . . . .

. • . 1 . . . . .

1 1 . . . . .

1 1 . 1 . . . il 1 1 1 . 1 . . 2 11 • 1133211 1• 3 2 . 1 • . 1 1 13 • .21 . .92 . . 1 1 1 1 . 1 1 . . . 1 .14 .11 • . . 83558816 1. 1.1551 . . . . • . . 1 . 3 . . . 1

. . . . . . . 1 . • 11.14,15 15 11.15.11 15 ,91...11 I• I..21..I 21 31.2..11 ,I ........ 98 I .... 1 .... ....... 1 2. . . 1 . 2 • . . 15 1 1 . 1 . . . 1 2. 1 . 1 1 . . • 1 1. 41 . . . . 12 2. 15 . . . . . . 2 .

11.1 . . . . 2. 1 . . . . . . . 1. 1 . . . . . . 2 11 1 . . . . . . 1 21 1 1 1 . . . 1 . 11 1111. .11 .1 1 . .1 . . . . 11 . 1 . . . 11 • • . 1 . . 2 . . 1 1 . . . . . . . . 11 . . 1.1 . . . . . . . .1115.11 .2 . . . . . . . . 1. . . . . . . . 1 .3 . . . 1 . . . . . 1 11.1 . . . . . 1 • . .11 . . . . .

171

c. Open, dry vegetat ion on the top o/ ridges (Ta- ble 12; Fig. 3D-E)

These types are independent o f ground water level and as they occur on higher parts o f the ridges wind and salt spray has some effect on the vegetation. Many Cactaceae and grasses occur as companions. Isolated tiny shrubs o f Pi thece l lob ium keyense and Jacquinia aurantiaca are c o m m o n . One c o m m u n i t y is endemic and hence there are no references to similar thorny communi t i e s in other parts o f the Caribbean coasts.

88. Agave angustifolia type. (8 bcl; 100 rel). The dominant species is prominent because of its 0.5 to 1.0 m high succulent rosettes and an inflorescence up to 3 m. This species is supposed to have been cultivated many years ago when 'sisal' leaves were exported. Probably, this type is derived from planted popula- tions. It is found behind the foredune and intermingles with the Bravaisia tubiflora and Pithecellobium keyense types. Herbs like Ambrosia hispida, and the grass Bouteloua repens are also common. The succulent thorny species Opuntia stricta and Acanthocereuspentagonus are the most frequent accompanying species. Open shrubland with the Celastraceae Rhacoma cros- sopetala, occurs frequently.

89. Enriquebeltrania crenatifolia type. (2 bcl; 14 rel). The physiognomy is determined by the irregular shape of the dominant endemic Rhamnaceae shrub, which is usually 0.50 to 1.0 m high (2.5 m in E1 Cuyo). Frequent companions are the herb Justicia carthaginensis, the succulent thorny shrubs Acan- thocereus pentagonus and Agave angustifolia, while Schom- burkia tibiscinis and Cyrtopodium punctatum are frequent field layer orchids. The type is found on tops of ridges or exposed areas where it forms an 'open' thicket with a particular physiognomy. It only occurs in Yucatan and was not found on ridges in Quintana Roe.

Interpretation of the ordination results

The ordination is presented in Figs. 4 and 5 for the basic clusters o f the Gul f and in Figs. 6 and 7 for the Caribbean• The first two axes o f the Gulf ordination have eigenvalues o f 0.668 and 0.469. In Fig. 4 the c o m m u n i t y types obtained in the classifi- cat ion and regrouped according to Fig. 3 are superimposed on the basic-cluster diagram. Axis 1 reflects a gradient from the t idemark pioneers and embryo- and foredune types to the right o f the graph, through blowout- and active dune types in the central part to the grasslands, low scrubs and thickets to the left. Beach, embryo dune and foredune types are subjected to a high mobi l i ty o f the substrate, as well as wind and salt spray. Types from

172

400

@

300

200 '

I 0 0

+ ;

@

@ @

@ @¥ ¥ ¥

¥ C] ¥~ ¥ []

@

C] 0

e

¥ ¥

@

¥ ¥

A []

[].~cJ ÷

÷ ÷

@ 0

@

¥ o

¥ @ ~ ¥ ¥ ¥ @ @

¥ ~ ¥ D[3 0 0 0

¥ ~" ¥ D o o @ @ o

¥ @ @

@ © o 0 ¥ @ A A @ @ @ @

@@ @@ @ A o @@o @o

@ o @@ @ @ @ @ @(o) @@ @@ @ @ @@ @ @

@ o @ o@@@ @ @ ® @ @@ o

@ @ o o @ @ @ @ @@ @ ® o ° + o o ~+ + + + + ~+

+A o

@ @0 @ @ + +~ ~ o• o ° oo~

o o °o o

o• go•O• + o A o o

cl

o ~

A z~ o A

o o o o

Fig. 4. Ordination (DCA), of basic clusters from the Gulf region with the community types of each zone superimposed: o = beach vegetation; @ = embryo dune and foredune vegetation; A = blowout and active dune vegetation; ~ = vegetation of humid and wet slacks; o = vegetation of fixed dunes: forbs and low shrubs; + = grasslands; , = shiny thickets; ~) = non-spiny thickets.

AXIS 2

40O I

@

20o

+ • +

• O • • • ÷

O • •

• O • • G

• •

O v • O O • •

• •

o • • o

Q

o o

@ o • o

o Q o +

+ o c) • o + o ~ o c)

o ~+o • D •

D 0 0 0 o ~+%+o$

O • ~ O • o • 0 D D O 0000

• o o " o o ~ • ° + : • + ~ I o +

• D [] 4 - 0 0 0 • • • • o ODO •

o ~ O o o ° o O o • • . • - , o o

• • OoOoO o o o • O@9

• o O ° O • • 41 • O~ 0

• • ~ D O • O O o •

• O ° • • • •

• D 0

0 O •

•So

10o ~00 ~00 400 sO0 600

Fig. 5. As Fig. 4 with the geographical regions superimposed: @ = Tamaulipas; o = northern Veracruz; • = central and central- southern Veracruz; + = southern Veracruz.

173

O

A o ® []

oo o % o [] " 0 O El . ~ A A ~,~+

.e.. ,, o,~ , , ~ ~ o o o o o o 2 ~n~ A~

® r'l ~ ' A ~ c+

o o i ~ ~ ~ • • '~ ~ '~ .~

k+k~k k ~k

0 100 200 300 400 IAXlS 1

Fig. 6. Ordination (DCA) of basic clusters from the Peninsula of Yucatan with the community types of each zone superimposed: o = beach; o = embryo dune; zx = foredune vegetation; • = vegetation of humid and wet slacks; [] = vegetation of sheltered zones. * = Vegetation of fixed dunes: thickets (non-spiny and palms); • = thickets (spiny, without palms); ® = open, dry vegetation on tops of the ridges.

0

• •

o • 000 o • o • • o • •

• • • 00

• * ° * o ~ ° /~ . * 0o 0"~ %

o o o o o o

o ~ oo~ o o o%~ % QO

[3 D [ ] [ ]

[] [3

• []

• o

[]

Fig. 7. As Fig. 6 with the Caribbean samples with the geographical regions superimposed. • = basic clusters from Yucatan. o = basic clusters from Quintana Roo.

174

Table 13. Distribution patterns of the sand dune communities in the Mexican Gulf and Caribbean coasts. The community types are listed according to the code given in Tables 1 to 12 and described in the text.

Distribution patterns Community types

I. Distribution along Gulf and Caribbean coasts. Mainly beach and foredune communities which are wide-spread on tropical coasts. 5 6 7 9 12 61 62 63 64 65

II. Northern Gulf and Peninsula of Yucatan. Mainly types characterized by species of calcareous sands. 1 31 5 59 72 76

III. All along the Gulf. Mainly types found on both light quartz and dark mineral sand. 8 16 19 25 28 45 71 75 IV. Distribution throughout the Gulf coast, with the exception of the central part of the state

of Veracruz. The causes of this distribution pattern are uncertain. For some beach species the mobility of the substrate could be an obstacle to their establishment. Climatic and edaphic factors are not responsible for their absence. Only in Tamaulipas and the north of Veracruz. These types are also frequent along the coasts of Texas and a few occur on the southeastern coast of the U.S.A. In this region frosts are common during winter, which may limit the distribution of many tropical plants and favour the presence of temperate species. There is probably a correlation with the calcareous element in the sand. Only in Central Veracruz. Dominant species of the communities of this pattern are found inland in other types of vegetation. The topographical diversity of the sand dune systems of these regions allow the establishment and persistence of these species. Central and southern part of Veracruz. The pattern is similar to VI, but it includes more types with grasses. Only in Tabasco and Campeche. It includes particular types for the region, which in many cases are a mixture of Gulf and Caribbean species. Two species, Bidens pilosa and Passiflora foetida become dominant and associate with several other species forming particular types for this area (Castillo, 1984). The region can be intermediate between Gulf and Caribbean. Peninsula of Yucatan (the Caribbean region). Mainly foredune types characteristic of calcareous soils. Communities on the western part have some Gulf species. The northern part has the driest communities and the eastern side is the most closely related to the Caribbean islands and Florida. Only in Yucatan. This pattern includes the communities more tolerant to saline conditions, and inhabiting the driest coasts. Only in Yucatan and Quintana Roo. This pattern is similar to IX especially regarding dune thicket communities. Palm types are dominant. Only in Quintana Roo. Types found on calcareous soils but receiving higher precipitation than in the rest of the Peninsula.

V.

VI.

VII.

VIII.

IX.

34 15 21 30 57

13 17 41 42 47 56

11 24 34 35 51

10 18 23 29 32 33 44 52

Not included

66 67 68 73 80 87 88 X.

60 69 70 77 86 89 XI.

74 78 79 83 84 85 XII.

81 82

blowouts and active dunes are influenced by sand mobility, but they do not receive much salt spray and have high values of pH and a lower amount of nutrients. Axis 2 reflects a gradient from humid and wet habitats (and low salinity) in the lower right part of the graph to low scrubs and thickets on dry areas, in the upper left half of the graph.

If we superimpose the geographic regions to which the basic clusters belong on the same graph (Fig. 5), axis 2 appears to reflect a latitudinal gradient as well. In the upper part of the graph types found in Tamaulipas are situated. They are followed by types from the north of Veracruz, from the south of Veracruz, and finally in the bottom left appear types belonging to central Veracruz. Axis 2 also seems to reflect a gradient in dune structure.

Simple beach ridge systems are found in Tamauli- l~S as well as to the north and south of Veracruz; complex systems of active and fixed dunes, rich in habitats, are found in the central and south-central areas of Veracruz.

In Figs. 6 and 7 the ordination of the Caribbean material is presented. Axes 1 and 2 have eigenvalues of 0.636 and 0.354. The community types described are superimposed on the ordination graph (Fig. 6) obtained for the Caribbean material. Axis 1 shows a gradient from pioneer types to fixed dune thickets. To the right of the graph the exposed beaches appear with higher sand accretion, pH and salinity. Here, we find the one-layer beach types with annuals, the perennial herbs forming embryonic dunes and the shrubby communities of the foredune zone.

175

In the middle of the graph the communities of sheltered zones and humid habitats are localized and to the left, all the thicket community types of fixed dunes.

Gradual changes of soil humidity and/or salinity are expressed by Axis 2. In the upper part of the graph community types tolerating high salinity and flooded soils appear; in the middle, types of the drier zones where spiny thickets dominate the vegetation physiognomy. In the bottom of the diagram types on soils receiving heavy rainfall or growing around fresh water springs where palms dominate physiognomically are found. Axis 2 also reflects distance to the sea; types occurring in the middle are situated between the beaches and the lagoon. Two community types of humid zones are separat- ed; in the top of the graph appear those far away from the sea (inland mangrove) whilst seashore types are represented in the bottom.

Fig. 7, with the geographical regions superimposed, shows a clear geographical variation along axis 2, separating Yucatan from Quintana Roo types. A diagonal line of variation reflects the dis- tributional gradient of community types along the Gulf and Caribbean: to the upper right we find communities shared with the Gulf region, whilst the exclusive Caribbean community types appear towards the lower left. In the middle of the graph

the communities shared with the intermediate zone of Tabasco and Campeche are concentrated.

Distribution of the community types

There is a wide variation in the distribution of the 90 community types described. Some have a very wide distribution while other ones have a very restricted occurrence. The distribution of all community types was analyzed and a comparison was made with the zone of Tabasco and Campeche, as studied by Castillo (1984). In comparing those types we considered species composition, frequency and cover-abundance values. Some community types do not correspond entirely because of the occurrence of regional accompanying species, but on the whole many types are similar because they are formed by the same dominant species with similar frequency and cover values, occurring in the same environment as well. Twelve patterns can be distin- guished. They are listed in Table 13 together with the numbers of the community types they include. Fig. 8 presents the 12 types in their relation to geographical position, temperature and precipitation, as well as sand type.

LOCALITIES

TEMPERATURE

R A I N F A L L

SOIL

OtS'n~BUTIOt~ pA l - tERNS:

I

II

I l l

IV

V

V I

V I I

VI I I

I X

X

X I

XI I

CENTRAL SOUTH-CENTRAL SOUTH TABASCO YUCATAN ~UINTANA nO0

ve~cmJz vlmAc:mJz vl [hAcnuz VERACRUZ I

24 °C 24 "C 26 "C 2e "C 26 "C 26 "C 2S "C

I I - - - - 700,,1330 mm I 13~10 mm 2QeO~ 3 8 0 G ~ ; 1 8 G 0 ~ I 1 5 8 0 m i 6 9 0 ~ J 10C0-1500 ~

. . . . . . . I . . . . . . . . . . . . . . . . . . . . . 0 . . . . . . . . . i CALCAREOUS QUART;[ QUARTZ QUARTZ ; QUARTZ CALCAREOUS CALCAREOUS

Fig. 8. Geographic distribution patterns of community types along the Gulf and Caribbean coasts. Explanation of distribution patterns (Roman numerals) are found in Table 13.

176

Discussion and conclusions

Methods

The classification of the basic relev6 data was performed with program TABORD using similarity ratio and a 1 to 9 cover-abundance score. In this way local clusters were formed which were characterized by the dominance of one or a few species. This is considered realistic in view of the character of the dune vegetation, and it agrees with similar treatments in other dune areas, e.g. Hobbs & Grace (1981) and Van der Maarel et al. (1985). The next step was a regional analysis for each of the regions Gulf and Caribbean starting with a synoptic table based on 'basic clusters' (composite relev6s) which summarized data for each of the basic clusters. In these tables each area was represented with 7 to 18 basic clusters. This gives at the same time an indica- tion of the community diversity per area. The synoptic species values per cluster were based on both frequency and average cover abundance. The resulting second-order clusters were again characterized by dominant species and described as community types. This is again considered realistic because few species characterize each of the community types which are thus also physiognomically determined.

The extreme environmental conditions of the coastal sand dune system (sand movement, salt spray, low water-holding capacity of sand, high sand temperatures, scarcity of several nutrients, etc.) do not allow many species to grow. Species richness increases towards the more stabilized inner parts of dunes, where these harsh conditions are ameliorated. In selecting the characterizing, i.e. dominant or abundant differential species for each type, the original relev6s on which the basic clusters are based were consulted, mainly to check the accompanying species and their local frequency, as well as the physical environment of each relev6. The synoptic tables allowed further analysis by means of ordination analysis. Cluster ordination has been successfully used in other dune areas (Van der Maarel et al., 1985). In our case Detrended Cor- respondence Analysis proved to be the most effi- cient method. One potential drawback of this method, its sensitivity for single outlier stands, did not occur in our case because such possible outlier stands had already been 'neutralized' in the basic clusters.

The analysis of the vegetation of Tabasco and Campeche (Castillo, 1984) used the programs TWlNSPAN (Hill, 1979b) and FLEXCLUS (Van Tongeren, in prep.). The latter uses similarity ratio as a resemblance measure and offers the same pos- sibilities as TABORD, but it enables a complete clustering of relev6s before relocation and is able to treat much larger data sets. The final clusters in this other study were thus based directly on the original relev6s. The community types obtained for the intermediate region (Tabasco, Campeche) were compared both with the original relev6s forming our basic clusters and the final second order of clusters in the Gulf and Caribbean material respectively. In this way it was possible to compare the community types from both studies regarding similarity and differences in characteristic and accompanying species. Because of the clear pattern obtained it was not considered necessary to treat the three sets of final clusters any further, though this would be possible and worthwhile if similar datasets of other areas would be available.

One special floristic problem should be mentioned. The lack of regional and local floras com- plicated identification of the material. Most of the areas had not been thoroughly sampled floristically prior to this work. Disagreements on species identification with other authors are hard to resolve as in many cases herbarium material has not been deposited in Mexico. Further taxonomic studies of groups and increased collections will help to solve these problems.

Phytogeographical aspects of community types

The results show that the littoral of the Gulf of Mexico and the Caribbean can be divided into several sections according to their vegetation (Fig. 8). The main division occurs between the Gulf and the Caribbean sections. The major environmental factor connected with this separation is the type of sand: predominantly quartz in the Gulf and calcareous in the Caribbean. Similarly, different species groups on siliceous and on calcareous sand were reported by Randall (1970) for the Barbados coast. As there is a clear one-species dominance in many of the community types described, the distribution area of the species and the type often coincide.

The Gulf region

The Gulf vegetation can be subdivided into two sections, the northern (Tamaulipas and north of Veracruz) and central-southern part (the rest of Veracruz and Tabasco). This is due to the presence of calcareous material in the northern part of the study area (Moreno-Casasola, 1986; Garcia, 1985; Puig, 1976).

Few species are shared with the eastern coast of the United States (Moreno-Casasola, 1985). One of the most important dune building grasses in the dunes of the eastern and Gulf coasts of the United States, Uniola paniculata, plays a minor role in the Gulf being only abundant in Tamaulipas. The liter- ature does not allow a comparison on the community level but a few other species are shared with the southern USA: Fimbristylis castanea, Hydrocotyle bonariensis, Croton punctatus, Panicum amarum, Cakile edentula, Cenchrus tribuloides, Borrichia frutescens, Fimbristylis spadicea, Sporobolus virginicus and Ipomoea stolonifera also appear along the southern east coast (Bellis, 1980; Stalter, 1973, 1974). In this region of the country there are occasional frosts which probably limit the distribution of many tropical species (Moreno-Casasola, 1985).

Two species, and consequently also the community types they characterize, show disjunct ranges including Florida and the Gulf coast: Okenia hypogaea and Oenothera drumrnondii. There is no obvious reason for their absence elsewhere in the area (Sauer, 1967).

The sand dune vegetation of the Mexican Gulf coast shows many similarities with the south of Texas. We could not include the northern part of Tamaulipas in this survey but the studies of Dahl & Goen (1977), Gonz~ilez-Medrano (1972), Judd et al. (1977), Puig (1976) and Sauer (1967) in the north of Tamaulipas and Texas, and the studies of the latter two authors and of Poggie (1962) in the south of Tamaulipas and north of Veracruz, show that the northernmost areas we studied are geomorphologically and floristically very similar to those in northern Tamaulipas.

The same chain of barrier islands runs along the southern part of Texas and Tamaulipas. Among the more important common species are Sesuvium portulacastrum, Sporobolus virginicus, Oenothera drummondiL Fimbristylis castanea, Andropogon scoparius, Euphorbia ammanioides, Bacopa mon-

177

nieri, Erigeron myrionactis, Hydrocotyle bonariensis, Ipomoea stolonifera, Ipomoea pes-caprae (Dahl & Goen, 1977; Garcia, 1984; Gonz~ilez- Medrano, 1972; Judd et al., 1977; Sauer, 1967). Most of the common species are beach pioneers and species of humid and wet slacks. The coastal thicket in Tamaulipas, however, is richer in subtrop- ical and tropical elements not found in the thickets at more northern latitudes.

The rest of the littoral vegetation of Veracruz and part of Tabasco is geographically different. It has many dominant species, mainly in the thickets, not found in other nearby coastal areas but common in inland deciduous and semideciduous tropical forest and secondary stages, e.g. Randia laetevirens, Coc- coloba barbadensis, Psychotria erythrocarpa, Citharexylum berlandieri, Verbesina persicifolia, Acacia macracantha, Acacia farnesiana, Nectandra coriacea, Psidium guajava, Amphilopium paniculaturn, Cissus sicyoides. Sauer (1967) mentioned that the coastal dune flora of the Gulf region included primarily inland species with a restricted coastal distribution range. Only about one third of the species he found in the dunes occurred in more than two sites. This agrees with our find- ings, especially in complex dune systems like La Mancha and Rancho El Capricho. Both sites have very extensive areas completely covered by vegetation, and the thickets are diverse with numerous elements from tropical semideciduous forests and grasslands.

Other elements are primarily coastal but their distribution is restricted to the Gulf section of the littoral: Palafoxia lindeniL Chamaecrista chamaecristoides. Both are considered as endemic to the region (Sauer, 1967).

The Caribbean region

The total calcium content of the sand explains the distribution range of several community types. Floristic similarities of the littoral vegetation of the Yucatan Peninsula with that of Florida and the West Indies may also be explained by this factor.

The coasts of the Yucatan Peninsula can be subdivided climatically into the northern semi-dry coasts of Yucatan and the eastern humid coasts of Quintana Roo (Espejel, 1986).

Many of the pioneer community types are found on both the Gulf and the Caribbean beaches, for

178

instance the communities of Sesuvium portulacastrum, Sporobolus virginicus and Ipomoea pes- caprae. Some salt marsh species from the northern Gulf area are important elements of Yucatan beach communities, e.g. Suaeda linearis and Amaranthus greggii.

The northern Yucatan dune communities have many companion species that are endemic to the Yucatan Peninsula, for instance Enriquebeltrania crenatifolia. Also the thicket types of this semi-dry area may have endemic companions, for instance Agave sisalana, Coccothrinax readii and Mammil- laria gaumeri.

The major communities occurring on the dunes of humid Quintana Roo are the thicket types formed by Caribbean species which are shared with the West Indies, Antilles and Florida: Euphorbia buxifolia, Hymenocallis spp., Coccoloba uvifera, Ernodea littoralis, Pithecellobium keyense, Bume- lia retusa, Cordia sebestena, Caesalpinia vesicaria, Erithalis fruticosa, Thrinax radiata and Pseu- dophoenix sargentii (Asprey & Robbins, 1953; Beard, 1949; Borgesen, 1909; Davis, 1942; Espejel, 1986b; Howard, 1973; Kurz, 1942; Loveless, 1960; Quero, 1981; Randall, 1970; Samek, 1973; Sauer, 1982; Steers et al., 1940).

In the northern and southeastern sites of the Peninsula where the backdune vegetation consists of tropical rain forest, rather more complex communities of dune thickets occur. Examples are the Pouteria campechiana and Bravaisia tubiflora types, the latter with many tropical forest companions such as Manilkara achras, Hippocratea celastroides, Schomburgkia tibiscinis and Capparis incana.

Environmental relationships

The local diversity of community types is directly related to dune physiography. Complex dune systems include a larger variety of environments than simple beach ridge systems. The former may in] clude wet and humid slacks, mobile, semimobile and fixed dunes forming a complex mosaic in which different successional stages can be represented. Sheltered areas allow establishment and persistence of numerous inland species, con- tributing to the richness of the dune flora.

Wide beaches, with areas close to the water table, also increase the richness of the sand dune flora.

This applies also to the slacks between the dunes. Many of the species occurring in these habitats are restricted to the central and southcentral part of Veracruz (and occur also in nearby Riachuelos which is rich in aquatic and humid environments) e.g. Fimbristylis castanea, Eleocharis cellulosa, Erigeron myrionactis and Cyperus articulatus.

Even within the extreme coastal environment species richness may vary directly with micro- environmental heterogeneity. In Cabo Rojo, Ver- acruz, Poggie (1962) found that the greater variation in topography within the pioneer habitat of the northern section created more micro-environments than the uniform topography of the southern area. Sauer (1975) found a very impoverished flora on the Pacific coasts of Costa Rica, which he attribut- ed mainly to the lack of storms and onshore winds; the storm beach zone is virtually absent. This re- lates to a similar situation in Sontecomapan and Monte Pio, where inland species, such as Randia laetevirens and Coccoloba barbadensis form a thicket on the frontal slope of the foredune.

Shifting sand also favours the presence of particular species, both on the beach and in blowouts. Strand and dune building species differ with the geographical regions. There is a shift in growth form of these species from temperate areas where grasses are the main builders to tropical areas where shrubby species predominate (Moreno-Casasola, 1985).

Community types oa Tabasco and Campeche coasts show a gradual change from quartz to calcareous soils. Several types described by Castillo (1984) have dominant species frequently found in disturbed areas, i.e. Bidens pilosa and Passiflora holoserica, but many of the accompanying species reflect a change from the Gulf to the Caribbean flora. In this sense this region can be considered as intermediate. This is exemplified by the distribution of the Randia laetevirens type; this species is found throughout the coast up to Campeche. It is absent in Yucatan and Quintana Roo. In the Gulf region it accompanies Coccoloba barbadensis. In Tabasco this species does not occur and Coccoloba uvifera is present instead.

The sand dune system of the Yucatan Peninsula is a beach ridge plain where complex dunes as characteristic for Central Veracruz are absent. However, as Sauer (1967) and Miranda (1959) pointed out, its vegetation is floristically the most complex and distinctive of the Gulf-Caribbean re-

gion. The simplicity of the Peninsula is due to its recent origin and the absence of rivers. In this re- spect the Peninsula is similar to the West Indies. To explain the relative floristical richness, we have to take into account that these areas have little sand accretion, and thus rapid plant colonization is possible (Davis, 1980). This increases the possibility that other species become established. Consequent- ly, the beaches of the Peninsula share both mainland (Central American and southern Mexican) elements and typical Caribbean species. Barrier islands with their mangroves also contribute to the variation in vegetation (Flores, 1984; Lundell, 1934; Quero, 1980, 1981; Rzedowski, 1978; Espejel, 1986).

The coastal flora of the eastern littoral of Mexico is very rich. This is a reflection of the general richness of the mainland flora. Mexico constitutes the limit between two biogeographical kingdoms, the Nearctic and the Neotropical. Two principal floristic assemblages come together and are able to co-exist because of the variety of environments. The isola- tion of extensive areas of the arid zones have increased the number of endemics (Rzedowski, 1978). Geomorphologically and climatically it is also a complex territory. All these factors have their effect on the coastal flora.

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Accepted 15.1.1986

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Appendix

Survey of dune areas sampled (see Fig. 1). Number of releves (tel) in an area, and number of basic clusters (bcl) obtained for each area (see section on Methods). References are given for detailed description of areas. I. Southern half o f the state o f Tamaulipas (Garcia 1984).

The sand is composed mainly-of light coloured quartz (tan quartz, Sauer, 1967) with calcareous elements; the pH is between 7.6 and 8.5. The north of Tamaulipas was not sampled during this study. The same type of barrier beaches described for the south are found in the northern part; they are a continuation of the barrier island system of the northern Gulf of Mexico (Dahl & Goen, 1977; Judd et al., 1977; Sauer, 1967).

1. Barra del Tordo, south of the mouth of Rio Carrizal. Narrow dune strip, 3 m high, 50 m wide ending in a mangrove of Conocarpus erecta. Top of ridge crossed by a road. (168 rel, 15 bcl)

2. Barra Chavarria (south), part of Isla Mata Grande, in front of the mouth of the Rio Berberena, east of the Laguna de San Andres. Narrow beach (10 m wide), followed by two belts of dunes (ca. 100 m wide) and ending in the mangroves of the lagoon. (93 rel, 10 hcl).

3. Barra Chavarria (north), situated east of the Laguna de San Andres, on Isla Poza Rica, close to the mouth of the Rio Tigre. Three parallel ridges 1 to 1.5 m high, ending in the mangroves. (106 tel, 8 bcl)

4. Bocatoma (north and south), east of Laguna Punta Real, almost in front of the village Lomas del Real. Bar with a narrow beach and belt of low dunes. Frontal ridge being eroded by the sea. Mangroves with Conocarpus erecta along the lagoon. Big harbour under contruction. (336 rel, 27 bcl) II. Low beach-ridges in northern Veracruz. (Moreno-Casasola et al., mscr.)

5. Riaehuelos south of the town of Tecolutla, across the river. Narrow beach and wide series of very low ridges; generally high water table with semiaquatic habitats, wet slacks and higher, dry areas, extending inland from a few hundred meters to more than 3 kin. During the rainy season the Rio Tecolutla may rise considerably. With onshore winds inundations occur. The area has been used for extensive grazing. A year ago a settlement was established and now some areas are being cleared for agriculture. Sometimes the vegetation is burned during the dry season, some fires have escaped, devastating a great area of the natural vegetation. The beach is narrow and covered with much wood and plant debris brought in by the sea. Sand is tan quartz with a pH of 7.5. (279 rel, 24 bcl)

6. El Raudal, 8 km south of the town of Nautla. Narrow beach with a series of low ridges, not more than 300 m wide, bordered to the west by a small lake. Part of the area is occasionally used for grazing. Beach very similar to that at Riachuelos. (151 rel, 12 bcl)

7. E! Faro, 2 km north of Faro el Morro. Narrow beach and a series of low ridges, bordered by the highway. The sand is also tan quartz with abundant stones and pebbles. (52 rel, 9 bcl) III. Complex sand dune systems o f central Veracruz, Width of beaches varies (10- 500 m); width of dune area is 600- 2 000 m Height of dunes, 10-25 m. Dark mineral sand. (Garcia 1982; Moreno-Casasola et al., 1982; Moreno-Casasola et al., mscr).

8. La Mancha, semimobile and fixed sand dune system with a narrow beach on the north edge of the Laguna de La Mancha. Situated between headlands formed by big fossil dunes, bordered by cultivated fields and an area of semideciduous tropical forest. Patches here are situated along the more stabilized parts of the system, pH ranging from 8.4-8.8. It has become a nature reserve; human and cattle disturbance diminished over the last five years. (336 rel, 33 bcl)

9. Rancho El Quijote, Extensive, mobile sand dune system south of the Laguna de La Mancha. Narrow beach, pH of 8.3 in the beach area to 7.7 in the thickets. (259 rel, 16 bcl)

10. Pason Dofia Juana south of Rio Juan Angel. Mobile dune system, wide beach with areas in which the water table is close to the surface. Stabilized area to the west of the system, frequent grazing; in the beach and mobile areas less grazing. (295 rel, 20 bcl)

11. Playa Chalchihueca, 2 km north of the tax-bridge of the town of Antigua Veracruz. Beach with artificial ridge of Casuarina equisetifolia. Extensive wet slacks, some almost semiaquatic (with Typha spp.). Sand surface raises towards mobile dunes, pH of 7.4 (beach) and 7.6 (thickets); slacks have a higher pH, 8.6. (189 tel, 15 bcl)

12. Rincon del Pirata, mobile system north of Rio de la Antigua, south of La Antigua Veracruz. Wide beach and series of dunes. (163 rel, 12 bcl) IV. Stabilized systems in the central-south area o f Veracruz. Narrow beaches and wide series of dunes of different heights. Dark mineral sand. (Moreno-Casasola, et al., mscr.).

13. Playa Azul, 30 km south of Veracruz. Narrow beach and system of fixed dunes ca 25 m high eroded by the sea. pH of 6.6. (113 tel, 14 bcl)

14. Las Gaviotas, 8 km north of the village El Arbolillo. Narrow beach with undulated plain in which beach ridges cannot be distin- guished anymore, pH is 6.9. (180 rel, 17 bcl)

15. Rancho El Capricho, 8 km north of Alvarado. Fixed dunes with natural grasslands used for grazing. Grazing areas are rotated and enclosed for varying periods of time to allow regeneration. Narrow beach being eroded. (145 rel, 14 bcl)

16. Playa La qh'ocha, northeast of the town of Alvarado, semimobile and fixed dune system, wide undulating beach, pH of 7.5. (238, rel, 19 bcl) V. Beach-ridge systems il~ the south o f Veracruz. Narrow beaches, mineral sand (Moreno-Casasola et al., mscr).

17. Monte Pio, 35 km east of Catemaco. Narrow beach, 4 - 6 m high coastal ridge, pH values are 7.5. (73 rel, 7 bcl) 18. Sontecomapan, south of Laguna de Sontecomapan. Two areas were sampled: north of the mouth of the lagoon a narrow beach

and ridge 6 - 8 m; scrub cleared for grazing. Small narrow beach (La Playita) south of the mouth, bordered by a tall thicket which ends abruptly at the wall of a cliff. (130 rel, 11 bcl) VI. Beach-ridge systems o f the states o f Tabasco and Campeche

Thirteen areas along this coast have been sampled (Castillo, 1984). The classification they obtained is compared with that of the other

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areas. Heavily perturbated narrow beaches and low ridges, pH values were mainly in the range of 7.0-7.9 in Tabasco and 8.0-8.5 in Campeche. VII. Beach-ridges of the state of Yucatan. Barrier islands with long narrow beaches and low wide stabilized beach-ridges generally bordered by mangrove; calcareous sand with broken coarse shells and algae depositions. Beach pH 9.5, close to mangroves 8.3. (Espejel, 1984; Rodriguez, 1984).

19. Celestun - Sisal 50 km stretch of coast in north-eastern part of the Peninsula. Few limestone outcrops. Internal fixed sand dunes may mingle with mangrove system. (295 rel, 19 bcl)

20. San Benito, north of Progreso harbour. Stretch 5 km long and 500 m wide; beach and internal ridges 1-2 m high. Several fresh water springs. (181 rel, 10 bcl)

21. Telehae, 10 km east of San Benito. Narrow beach-ridges and low internal dunes (ca. 1 m high). (Rodriguez, 1984) (275 rel, 15 bcl) 22. Dzilam de Bravo. Patches of short, narrow beaches between Telchac Puerto and Dzilam de Bravo mingled with salt mines and

coconut plantations. Further east mangroves dominate. Perturbated and similar to Campeche beaches. (13 rel, 7 bcl) 23. Rio Lagartos-Las Coloradas, long narrow beaches. Stretch ca. 45 km long and 500 m wide of beach-ridges. Internal dunes are

4 - 5 m high. ending in the mangrove. Part of a natural reserve. Close to town Las Coloradas; beaches and sand dunes destroyed by salt mining activities. (246 rel, 12 bcl)

24. El Cuyo eastern corner of the Peninsula, stretch 40 km long and 800 m wide. Narrow low beaches, internal dunes 4 m high limit- ing with mangroves or at the base of the lagoon with tropical deciduous forest. Part of a nature reserve. (64 rel, 5 bcl) VIII. Beach-ridge systems of the state of Quiniana Roo. Barrier islands and dunes, narrow beaches and beach-ridges. Fine calcareous white coral sand. Some limestone rocks can be found as outcrops along beaches. Seashore mangroves occur close to capes; coconut crops frequently substitute natural vegetation. Northern parts alterated by touristical activities, pH of beaches is 9.0, in thickets 8.4. (Espejel, 1983; 1985).

25. Punts Blanca, north of Cancun, facing Contoy Island. 20 km long, narrow beach-ridge, low internal ridges ending in mangroves. (12 rel, 8 bcl)

26. Puerto Morelos 630 km south of Cancun. 10 km of littoral, from Punta Brava to Punta Maroma, small open bays and narrow beach-ridges. (54 rel, 11 bcl)

27. Capitan Lafitte, in front of Cozumel Island, south of Puerto Morelos. Narrow ridge system (20-80 m wide). (30 rel, 10 bcl) 28. Sian Ka'an Nature Reserve with 180 km of littoral, south of Tulum maya ruins. Undulated internal dune system 10-200 m wide.

(251 rel, 22 bcl)

classification and ordination of coastal sand dune vegetation along the gulf and caribbean sea of...

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