floristic and structural patterns along a chronosequence...

Pores~~dogy

Management Forest Ecology and Management 95 (1997) 161-171

Floristic and structural patterns along a chronosequence of secondary forest succession in Argentinean subtropical montane

forests

H.R. Grau a3b, * , M.F. Arturi a*c, A.D. Brown a, P.G. Acefiolaza d a Laboratorio de Investigaciones Ecolbgicas de las Yungas, Vniversidad National de Tucumrin, Casilla de Correo 34, (4107). Yerba Buena,

Tucum&, Argentina b Department of Geography, University of Colorado, Campus Box 260, Boulder, CO 80309, USA

’ Laboratorio de Sistemas Ecol6gicos y Ambientales, Facultad de Ciencias Agrarias y Forestales, Vniversidad National de I.JJ Plata, Diagonal 113 No. 469, (1900), LA Plata, Argentina

’ Facultad de Ciencias Naturales, Universidad National de Tucum&n, Miguel Lillo 205, (4000), Tucumcin. Argentina

Accepted 19 December 1996

Abstract

We studied forest structure and composition along a chronosequence of secondary forest succession in Northwest Argentina’s montane forests (‘Yungas’) at 27”S, between 700 and 900 m. Early herbaceous stages, forested stages of 11, 25, 45, and 50 years after abandonment, and old-growth forests were surveyed. Secondary forests included stands that originated in abandoned herbaceous crops and in abandoned fruit orchards. Basal area and species composition differed between 50-year-old secondary forests and old-growth forests. In contrast, tree density and species diversity were similar in the 50-year-old and in the old-growth forests. The previous use (herbaceous crops or fruit orchards) was an important influence on secondary forest composition. Whereas stands originating in herbaceous fields were dominated by wind-dispersed native species such as Heliocarpus popayanensis, Tecoma starts, Parapiptadenia excelsa, and Tipuana tipu, fruit-orchard-originated stands were dominated by animal-dispersed species. Among the animal-dispersed species, the exotic tree Morus alba was the most abundant, and its abundance in secondary forests seems to slow the succession toward old-growth forest composition. Overall, after accounting for differences attributable to pre-abandonment conditions, secondary forest succession showed a trend toward compositional convergence, with the rate of succession apparently regulated by the demography of long-lived pioneer species. 0 1997 Elsevier Science B.V.

Keywords: Long-lived pioneers; Invasive plants: Forest structure; Seed dispersal; Yungas

1. Introduction

* Corresponding author at: Laboratorio de Investigaeiones Ecologicas de las Yungas, Universidad National de Tucumfk, Casilla de Correo 34, (4107). Yerba Buena, Tucumkr, Argentina. Fax: 54-81-254468; e-mail: [email protected].

A large percentage of neotropical forests can be considered secondary forests where structural and floristic characteristics depend not only on environ- mental and biogeographical factors but also on the

0378-l 127/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO378-1127(97)00010-8

162 H.R. Grau rt al./ Forest Ecology and Management 95 (19971 161-171

history since a disturbance event (Budowski, 1965). There is now a general consensus that secondary forests are important resources both for economic and ecological purposes (Brown and Lugo, 1990; Corlett, 1995; Riswan and Hartani, 1995). However. basic information on patterns and underlying processes is lacking or restricted to particular geo- graphic areas. In South-American subtropical forests, for example, even basic descriptive studies are scarce.

Information necessary to understand and manage plant succession can be classified into two major categories. One central question relates to the mechanisms that lead to successional development. The importance of pre-abandonment conditions has been documented in several forest successions on abandoned agriculture fields. In particular. different studies have shown that a more complex structure of the pre-existing vegetation tends to increase the early input of animal-dispersed seeds, which in most cases tends to accelerate the succession (MC Donnell and Stiles, 1983; Finegan, 1984; Debussche et al., 1985; Guevara et al., 1986). However, little is known about the time during which such initial effects last (Fine- gan, 1996). Both for practical and for theoretical reasons. it is important to know if succession leads towards a compositional convergence despite different initial compositions (Glenn-Lewin and van der Maarel, 1992).

The second central question is the rate of succession. Rate of successional development is baseline information necessary for timber management in secondary forests (Finegan, 1992). The rate of recovery of plant species diversity after disturbance is basic information for conservation purposes (Brown and Lugo, 1990). Also, changes in structure associated with floristic changes influence the quality of the forests as wildlife habitat (Richards, 1983) and influence the role of forests in the global carbon budget (Lug0 and Brown, 1992). Since invasion by exotic species is usually favored by disturbance (Hobbs and Huenneke, 1992), it is also important to assess the effects of such invasions on the direction and rate of succession.

In this paper we describe a chronosequence of secondary succession in the southern end of the neotropical montane forests. The main goal of this work is to infer rates of recovery of different structural and floristic parameters of forests originating in

abandoned crop fields. Pre-abandonment condition (herbaceous crops and fruit orchards) is the facus of a comparison aimed at understanding the influence of crop structure and exotic plant invasion on secondary succession.

2. Methods

2.1. Study area

This study was conducted in the Iuwer montane forests of the east slope of the San Javier range (27”S), TucumLn, Argentina. Most of the plots were located in the Parque Biofbgico Sierra de San Javier, a protected area owned by the National University of Tucumin. This biogeographic area is considered the southern end of the neotropical montane forests. or ‘Yungas’ (Cabrera and Willink, 19801.

Mean annual temperature is 18°C. Annual rainfall is about 1400 mm. Most precipitation occurs in summer, under a monsoonal regime (Bianchi, 198 I). Frost occurs typically between June and August. An absolute minimum temperature of - 7°C is expected to occur once every 50 years, and of -5°C once every 10 years (Torres Bruchman, 1977l.~Soils usually show an AC profile (Hapludol) with a pH between 5.5 and 6.5 (Zucardi et al., 1968). Details on forest structure and composition in the area can be found in Moyano and Movia (1989) and Grau and Brown (1997). Species nomenclature follows Leg- name (1982) and Killeen et al. (19931 for native species, and Dimitri (1977) for exotic species.

A large percentage of the area was deforested during the first half of this century, and planted with annual crops and fruit orchards, mostly citrus. As a consequence of soil fertility loss, lands were abandoned and progressivety revegetated. The inclusion of the area on the reserve since 1976 enhanced forest recovery processes. Consequently, the present landscape is a mosaic of different successional stages.

2.2. Data collection

Forest structure was studied in six l-ha plots. Plots fl, f2, f3, and f4 were secondary forestsPlots fl and f3 were forests of approximately 11 to 12 years and 45 to 50 years, respectively, and~both plots

H.R. Grau et al./ Forest Ecology and Management 95 (1997) 161-l 71

originated in abandoned herbaceous crops and were located in relatively flat areas (slope < 15%). Plot f4 had the same age and pre-abandonment use as f3, but it was located on a steeper slope (40 to 55% with SSE aspect). Plot f2 was a 20- to 25-year-old forest, which originated in an abandoned citrus orchard. The time of abandonment of all plots was estimated from aerial photographs, taken in 1941 and 1973, and from interviews with local inhabitants. Plots f6 and f5 were considered old-growth or ‘mature’ stands, located on south- and north-oriented slopes, respectively. All the plots are conserved as permanent plots with all trees tagged since June-September 1991.

Each plot was subdivided into 25 20 X 20-m quadrats. Variables measured were DBH (diameter at 1.35 m) and height of all individuals larger than 10 cm of DBH. Tree height was visually estimated, always by the same person, and periodically compared with trigonometric measures using a clinome- ter. The two methods always differed less than 20%. In each plot, two categories of saplings were surveyed. ‘Small saplings’ (more than 50 cm height and

f2 f3 f4 f5

lOoor g 60.0 z w.0 1 .- b 40.0 ‘ii 30.0 s p 20,o E 10.0

s a,0

23 2

1.5

S-4 1 I .E 0.5 ii 0

1 f5 f5 f4

f3

a 4.5 0 -1

1,5

-2 1

163

fl

f2

d -0,s 0 035 1 I.5 2 2.6 CA axis 1

Fig. 2. Distribution of the forest plots in the two first axes of

correspondence analysis (ter Braak, 1987). Plots fl-f4 are secondary forests of different ages. Plots f5 and f6 are mature forests.

Plot f2 is originated in abandonded citrus orchards whereas f 1, f2, and f4 are originated in herbaceous fields.

less than 3 cm DBH) were counted in 16 20 X 4-m randomly located transects per plot. ‘Large saplings’ (3-10 cm DBH) were counted in five randomly located 20 X 20-m quadrats per plot.

Small saplings were also surveyed in three recently abandoned areas, following a similar spatial

z 40.0 5 350 E 30.0

- 25.0 $ 20,o $ 15,0

= 10.0 3 5.0

a 0.0 ti

f5

I[

10-12 2425 45-W 45-50 mature mature

Age W

1012 20-25 4550 45-54 mature mature

Age (yr)

Fig. 1. Forest structure parameters along the chronosequence. fl-f4 are successional forests of different ages, f5 and f6 are mature forests.

Plots are ordered according to the first axis of CA.

164 H.R. Grau et al. /Forest Ecology and Management 95 (1997) 161-l 71

sampling design. Plot el was a 3-year-old abandoned field of herbaceous crop (sugar cane harvested and burned) on a 8-10% slope. Plot e2 was a citrus orchard abandoned about 3 to 4 years previous to the survey, also on a less than 15% slope. Plot e3 was an area of denuded soil caused by road construction. Due to its steep slope (40 to 60%), e3 was assumed to be similar to the abandoned field where plot f4 had originated.

2.3. Data analysis

Parameters calculated were density (trees ha-’ 1, basal area (m2 ha-‘), h’ (Shannon-Weaver’s diversity index), species richness of trees > 10 cm DBH, maximum height index (average height of the three tallest trees in each 20 X 20-m quadrat; Holiihidge, 1978), and complexity index (H&ridge, 1978). Forested plots and tree species were ordered using

Table 1

Composition of trees larger than 10 cm DBH along the chronosequence of forested plots (fl -f6)

R Species fl t2 f-3 f4 f5 f6 D

M) Density I trees ha - ‘) I

2 3 4

5 6 7

8 9

10

II 12 13

14 15

16 17

18 19

20 21 22

23 24 25

26

Tecoma starts

Heliocarpus popayanensis Morus alba (e) Jacaranda mimosijolia

Juglans australis Vassobia breviflora Cedrela lilloi

Rapanea laetevirens Solarium riparium (u) Bohemeria caudata

Cupania vemalis (u) Tipuana tipu

Parapiptadenia excelsa Allophyllus edulis (u) Urera caracasana (u)

Blepharocalix salictyolius Phoebe porphyria

Ruprechtia laxifora Piper tucumanum (u)

Terminalia trifrora Myrcianthes pungens (u) Tabebuia avellanedae

Pisonia ambigua Stenocalix unijZora (u) Ruprechtia apetala

Chrysophyllum marginatum Xylosma pubescens Enterolobium contortisiliquum

Per-sea americana (e) Ligustrum lucidus (e) Fagara coca Psidium guajaba (e) Prunus persica (e) Acacia aroma Citrus aurantium (e) Scutia buxifolia Carica quercifolia

536 16 91 28 0 0

28 32 29 0 0 0 6 172 8 15 0 0 8 1 1 6 0 0

0 0 56 0 0 2 0 0 10 1 1 0 0 3 31 10 I 3 0 21 6 17 I 5 0 33 14 4 4 s 6 6 15 1 I 4

0 13 0 0 0 2 0 3 I 15 1 5 5 25 22 136 21 I? 0 2 10 24 11 17 0 1 7 0 9 :9 0 0 16 13 1 7 3 20 10 32 II 28 0 0 2 3 0 7

0 0 3 5 25 148

0 0 5 41 22 51 0 0 0 0 11 20 0 0 0 1 0 12

0 0 0 1 38 13 0 0 0 1 34 12 0 0 0 0 19 4

0 0 0 0 2 12 0 0 0 0 0 I 0 6 0 0 0 0 0 2 0 0 0 0 0 1 1 0 0 0 0 1 0 0 0 0 0 4 0 0 0 0 0 2 0 0 0 0 4 0 0 0 0 0 0 1 0 2 0 I 0 0 0 I 0 0 0 0 0 0 0 2

w

w

a

w

a

a

w

;:

i:

w

a

tv

u

3

a

a

il

w

a

w

a

vv

a

a

w

a

a

a

a

a

a

a

a

a

a

a

a

H.R. Grau et al/Forest Ecology and Management 95 (1997) 161-171 165

Table 1 (continued)

R Species fl f2 f3 f4 f5 f6 D

(BJ Basal area cm2 ha - ‘) 1

2 3

4 5 6

7 8 9

10 11

12 13

14 15

16 17 18

19 20 21

22 23 24

25 26

Tecoma starts Heliocarpus popayanensis Mows alba (e)

Jacaranda mimosifolia Juglans australis Vassobia breotflora

Cedrela lilloi Rapanea laeteuirens Solarium riparium (u)

Bohemeria caudata Cupania vernalis (u)

Tipuana tipu Parapiptadenia excelsa

Allophyllus edulis (u) Urera caracasana (u) Blepharocalix salicifolius

Phoebe porphyria Ruprechtia laxiflora

Piper tucumanum (u) Terminalia trifora Myrcianthes pungens (u)

Tabebuia auellanedae Pisonia ambigua Stenocalix uniflorus (u)

Ruprechtia apetala Chrysophyllum marginatum

Xylosma pubescens Enterolobium contortisiliquum

Persea americana (e) Ligustrum lucidus (e) Fagara coca

Psidium guajaba (e) Prunus persica (e) Acacia aroma

Citrus aurantium (e) Scutia buxifolia

8.58 1.93 0.09

0.15 0 0

0 0 0

0.06 0

0 0.09

0 0 0

0.04 0 0

0 0

0 0 0

0 0

0 0

0 0 0

0 0 0.7

0 0

Carica quercifolia 0

1.51 1.29 0.71 0 0 w

3.22 0.41 0 0 0 w

7.77 0.6 0.95 0 0 a

0.05 0.06 0.18 0 0 w

0 2.5 0 0 0.12 a 0 0.08 0.01 0.01 0 a

0.06 1.4 0.3 0 0.51 w

0.58 0.39 0.94 0.05 0.87 a 0.13 0.15 0.13 0.08 0.16 a

0.05 0.09 0.01 0.04 0.4 w 0.15 0 0 0 0.5 1 a

0.56 1.15 1.47 0.5 3.56 w

0.38 1.35 8.46 5.67 3.2 w

0.02 0.15 0.31 0.32 0.4 a

0.03 0.06 0.04 0.1 0.35 a 0 0.13 0.14 0 1.2 a

0.54 0.6 1.7 3.8 12.1 a 0 0.02 0.04 0 0.43 w 0 0.03 0.2 3.6 0.49 a

0 0.04 0.97 4.57 4.96 w 0 0 0 0.6 1.13 a 0 0 0.02 0.77 0 w

0 0 0.04 2.47 2.28 a 0 0 0.01 0.65 0.19 w

0 0 0 0.8 0.3 w

0 0 0 0.7 015 a 0 0 0 0 0.03 a

0.46 0 0 0 0 a

0.11 0 0 0 0 a 0.34 0.1 0 0 0 a 0.01 0 0 0 0 a

0.85 0 0 0 0 a 0.26 0 0 0 0 a 0 0 0 0 0 a

0.01 0 0.03 0 0.02 a 0 0 0.01 0 0 a 0 0 0 2 0.15 a

R, rank in the first axis of CA. Only species with more than ten individuals were classified. (e) exotic species; (u) understory species.

D, dispersal mode: w, wind; a, animal.

correspondence analysis (CA; ter Braak, 1987). Sapling and tree composition of the different plots were compared using a matrix of Czekanowski’s similarity coefficients based on relative density (Kent and Coker, 1992). For this analysis, canopy composition was arbitrarily defined as the relative density of

trees larger than 10 cm DBH in fl, and of trees larger than 30 cm DBH in the older forests (f2, f3, f4, f5, f6). Such a similarity matrix, and size distribu- tions of the most abundant species in the forested plots, were used to describe successional trends in species composition.

166 H.R. Grau et al. /Forest Ecology and Managemenr 95 (I9971 I6I- 171

3. Results

3. I. Forest structure

As expected, stem density showed a decreasing trend through the chronosequence, from ca 600 trees ha-’ in plot fl to less than 350 trees ha-’ in plot f3 and older plots. Basal area increased through the chronosequence, and the values in the oldest successional plots were still much lower than in the old- growth plots (Fig. 1). Canopy height showed an increasing pattern along the chronosequence. We

found small differences in canopy height between the 45-50-year-old plots and the old-growth plots. Species richness, diversity, and complexity also showed an increasing trend, but the older secondary forests (f3 and f4) had values similar to the old- growth ones (Fig. 1).

3.2. Forest composition

The first and second axis of the CA accounted for 4 1% and 23% of the variation, respectively. Whereas the first axis was clearly related to the successional

Table 2

Density (individuals ha- ’ ) of small (A) and large (B) saplings along the cbronosequence. Plots e 1, e2, and e3 are recently abandoned areas, wheras fl-f6 are forested plots of different successional ages (see text for details). Plots e2 and f2 were originated in abandoned citrus

orchards. Plots f5 and f6 are old-growth stands. See text for more details on the characteristics of the plots

Species fl f-2 f3 f4 f5 f6 el e2 e3

(A) Small saplings (between 50 cm height and 3 cm DBH)

Tecoma starts 0 0 0 0 0 0 13225 Heliocarpus popayanensis 0 0 0 0 0 0 225 Morus alba (e) 63 0 87 50 0 0 212 Jacaranda mimosifolia 13 0 13 0 0 0 125 Juglans australis 0 0 0 13 0 0 0 Vassobia bretiflora 0 0 37 0 0 200 0 Cedrela lilloi 300 13 6200 50 2s 0 0 Rapanea laeteoirens 750 500 5250 3900 100 212 0 Solarium riparium 13 288 38 113 63 SO 0 Bohemeria caudata 162 288 925 1063 25 13 38 Cupania rentalis 325 1200 550 3325 687 1150 0 Tipuana tipu 0 0 0 0 0 0 317 Parapiptadenia excelsa 0 0 0 0 0 0 0 Allophyllus edulis 1288 750 1300 1700 1988 512 0 Urera caracasana 0 0 0 75 88 200 17 Blepharocalix salicifolius 137 50 437 237 100 15 0 Phoebe porphyria 463 loo 325 1013 733 100 0 Ruprechtia laxtjlora 0 0 25 25 25 25 0 Piper tucumanum 1113 188 3413 4388 1113 1.50 0 Terminalia triflora 13 0 257 163 25 25 0 Myrcianthes pungens 0 0 0 0 875 438 0 Stenocalix unifrorus 125 2s 1213 5963 16513 3538 0 Chrysophyllum marginatum 0 0 25 13 337 88 0 Prunus persica (e) 38 0 13 0 0 0 0 Citrus aurantium (e) 75 100 125 0 0 (, 0 Tessaria integrifolia 0 0 0 0 0 0 0 Xylosma pubescens 0 13 0 25 50 113 0 Trema micrantha 0 0 0 0 0 0 0 Randia spinosa 0 0 0 25 0 0 0 Anadenanthera macroarpa 0 0 0 0 0 0 0 Citrus limon (e) 0 0 0 13 13 0 0 Prunus tucumanensis 0 0 13 38 0 0 0 Podocarpus parlatorei 0 0 0 13 0 0 0 Total 4878 3515 20246 22205 22760 6889 13843

444 205 777 21.5

1167 0 0 1120

u 25 0 0 0 0

0 5s 1847 25

56 25 II (I 0 2010 0 689

0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0

0 150 0 0 0 0

Ii 0 i.: IS0 !i 0 0 235 0 0 0 25 0 0 0 0 0 0

429 1 4929

Table 2 (continued)


Species fl t2 t-3 f4 f5 f6 el e2 e3

(31 Big saplings (3-!0 cm DBHI

Tecoma stans Morus alba (e) Jacaranda mbnosifolia

Juglans australis Vassobia breutflora Cedrela lilloi

Rapanea laetevirens Solanum riparium Bohemeria caudata

Cupania uernalis Parapiptadenia excelsa

Allophyllus edulis Urera caracasana Biepharoealix saiicifolius

Phoebe porphyria Ruprechtia laxflora

Piper tucumanum Tenninalia triflora Myrcianthes pungens

Pisonia ambigua Stenocalix untflorus

Ruprechtia apetala Chtysophyllum marginatum Psidium guajaba (e)

Prunus persica (e) Citrus aurantium (e) Total

0 0 25 0 0 0 13 88 0 0 0 0 0 0 0 25 0 0

13 0 175 13 0 0 0 0 13 0 0 0 0 0 137 38 38 0

25 75 125 13 25 13 13 125 0 13 88 0

162 137 175 0 0 25 88 100 0 0 0 38

0 13 0 63 0 0 175 63 50 212 25 25

0 0 13 63 487 475 0 25 350 237 13 13

88 88 0 88 13 38 25 0 0 137 0 0 25 63 175 325 175 250

0 0 287 450 13 0 0 0 0 13 13 13 0 0 0 25 25 38 0 0 88 0 850 38 0 0 0 0 50 0 0 0 0 0 38 0

50 0 0 0 0 0 13 15 25 0 0 0 13 25 0 50 0 0

703 877 1638 1765 1853 966

(e) exotic species.

gradient of the chronosequence, the second axis was related to the differences in origin (herbaceous vs fruit orchards) in the early successional stages (Fig. 2). Species composition along the chronosequence is shown in Table 1. Tecoma stans (L) Juss. dominated plot fl with most individuals being smaller than 20 cm DBH (Fig. 3(a)). At this early successional stage two other pioneer species, Heliocarpus popayanensis H. B. K. and Jacaranda mimoszfolia D. Don, ranked second and third in density having larger individuals than T. stans. These three species are wind dispersed. In plot f2, T. stans and H. popayanensis ranked second and fourth in density. The exotic animal-dispersed Morus alba L. ranked first (39% of the relative density), and the native animal- dispersed Solanum riparium Pers. ranked third. Some species characteristic of the old-growth forest, such as Phoebe porphyria (Griseb) Mez., Rapanea laeteuirens Mez., and Cupania vemalis Cambess, were

more abundant in plot f2 than in both plots fl and f3 (Table l(A)).

Juglans australis Griseb and Cedrela lilloi C. DC showed the highest values of basal area in plot f3 where T. stans showed the highest tree density. In this plot Tecoma stans showed a bell-shaped size distribution, whereas J. australis and C. lilloi did not show a clear pattern of size distribution (Fig. 3(c)). Plot f4 was also dominated by a wind-dispersed species: Parapiptadenia excelsa (Griseb) Burk. Other wind-dispersed species such as Tipzazna tipu (Benth) 0. K. and Jacaranda mimosifolia were also common in plot f4. Species characteristic of the old-growth forests, such as Blepharocalix salicifolius (H. B. K.) 0. Berg., Terminalia trifora (Griseb) Lillo, Allophyllus edulis (St Hill) Radlk, and Phoebe porphyria, were already present in the smaller size classes of plots f3 and f4. Parapiptade- nia excelsa, Tipuana tipu, Morus alba, Juglans

168 H.R. Grau et al./ Forest Ecology and Management 95 (1997) 161-I 71

ausrralis, and Jacaranda mimosifolia were abundant in the older secondary forests (f2, f3, and f4), and showed an almost complete lack of saplings in the forested stages (Table 2). Such a pattern implies that they can be considered long-living pioneer species (Gomez-Pompa and Vasquez-Yanez, 1981; Swaine and Whitmore, 1988).

3.3. Sapling composition and similarity with canopy composition

The total densities of saplings were greatest in plots e 1. f3, f4, and f5, while plot f’2 (dominated by Morus alba in the overstory) had the Iowest~sapling density (Table 2).

Phoebe porphyria showed the highest values of Animal-dispersed Solarium riparium and .Morus basal area (34%) in the south-facing old-growth for- alba were the most abundant saplings in recebtly est (plot f6) with several very large trees (> 80 cm abandoned citrus orchard (plot e2>, although Tecoma DBH). Also important in this plot were Terminalia stans (wind-dispersed) was also abundant there. In tr$!ora and Piper tucumanum C. DC, an understory the abandoned herbaceous fields, the early forest species dominant in the small size classes (Fig. 3(f)). successional stages were dominated by the wind-dis- The north-facing old-growth forest (plot fS> was persed species Tecoma stans, Hehocarpus popuya- dominated by T. trzfiora, and Pisonia ambigua nensis, Tipuana tipu, and Jacaranda mimosi$&z in Heimerl in the canopy, and P. tucumanum and plot e 1, and Parapiptadenia excelsa, Jacaranda mi- Steno&ix unifzorus (L.) Kaus in the understory (Ta- mosifolia, and Tipuana tipu in plot e3 (Table 2). ble 1, Fig. 3(e)). Sapling composition of plot el showed the highest

- T&am

- T.s&ws - Mppayanensis

35i f3

- - - T.&a/w - J.dlfSZlWiS - C./Hoi

---+--- _. +-,

0 50 103 150

m8t-d (cm)

1% I f6

loo : : -----.P.~~ - T.&Y&m - P.

22; 'I

I. 0 c-- -4

0 50 im 150

Df3l-l (cm)

Fig. 3. Size (DBH) distribution of the most abundant species in each plot. Plots fl-f4 are secondary folwts of different successional ages. Plots f5 and f6 are mature forests. See text and Table 1 for details on composition and age.


Table 3 Similarity (Czekanowski coefficient) between sapling and canopy composition of the different plots. Plots el-e3 are recently abandoned areas, plots fl-f4 are secondary forests, and plots f5-f6 are old-growth forests. See text for details on the characteristics of the different

plots

Trees Small saplings

el e2 e3 fi t-2 f3 f4 f5 f6

fl (> 10 cm DBH) 0.933 0.385 0.109 0.027 0.011 0.019 0.017 0.007 0.007

f2(>30cmDBH) 0.046 0.447 0.119 0.034 0.021 0.025 0.024 0.004 0.021

D(>30cmDBH) 0.060 0.179 0.316 0.185 0.060 0.326 0.079 0.037 0.045

f4 ( > 30 cm DBH) 0.039 0.075 0.351 0.188 0.096 0.078 0.122 0.051 0.068

f5 ( > 30 cm DBH) 0.320 0.042 0.201 0.101 0.046 0.042 0.080 0.082 0.047

f6 ( > 30 cm DBH) 0.015 0.013 0.212 0.215 0.0% 0.132 0.133 0.133 0.163

Bold numbers indicate relatively high values (see text).

similarity with the canopy composition of plot fl, plot e2 with plot f2, and plot e3 with plot f4 (Table 3).

In the forested plots, sapling composition was dominated by shade-tolerant species such as the understory species Stenocalix uniflorus, Allophyllus edulis, and Piper tucumanum. Canopy species characteristic of the old-growth forests, Phoebe porphyria, Blepharocalix salicifolius, Rapanea laetevirens, and Cupania uemalis, were also relatively abundant as saplings in the forested stages (Table 2). Plot f3 showed close similarity between its sapling and canopy composition. All the other plots had sapling compositions that were more similar to old- growth canopy composition (plot f6). Among the forested stages, plot e2 had a sapling composition least similar to old-growth forests (Table 3).

4. Discussion

4.1. Rate of successional change through the chronosequence

This chronosequence suggests that some structural parameters such as species diversity and richness and stem density reach values very similar to the values for old-growth forest within 50 years. Such a pattern is similar to the observed patterns in other neotropical secondary successions &.hlaniaga et al., 1988; Kappelle et al., 1995; Finegan, 1996). The compara- tively high species richness in the oldest secondary forests is due to the coexistence of light-demanding

species established early in the succession and shade-tolerant species already established at this successional stage (Connell, 1978; Denslow, 1985). For example in the 50-year-old plots co-occurred early successional species such as T. stuns, P. excelsa, J. mimosifolia, and T. tipu; and species characteristics of the mature forests such as P. porphyria, B. salicifolius, T. triflora, P. tucumanum, and S. uni-

fl or-us. On the other hand, basal area, which can be

associated with biomass accumulation, and presence of very large trees was much lower in the oldest secondary forests than in the old-growth forest. Canopy height, which can be associated with habitat structure, also appears to differ between the oldest secondary and the old-growth forests, in concordance with lower latitude tropical forests (Terbourgh and Petren, 1991). However, our visual estimation of tree height is probably not accurate enough to assess the degree of significance of the relatively small differences between old-secondary forests and mature forests.

4.2. Compositional convergence and the role of pre- abandonment conditions

Two main features of floristic patterns emerge from this study. First, structure of the vegetation previous to the abandonment plays a key role in determining tree species composition of the early successional stages. The perches provided by the citrus orchards favored the early input of animal-dispersed seeds as has been shown in other ecosystems

170 H.R. Grau et al. / Forest Eco1og.v and Management 95 (19971 161-I 71

(MC Donnell and Stiles, 1983; Debussche et al., 1985; Guevara et al., 1986). However, in this case, such a factor favored the invasion by the exotic tree Murus alba, and the Torus-dominated forest had low sapling density and relatively poor similarity between its saplings and the old-growth forest composition. Based on these observations, we hypothe- size that such invasion could slow or even divert the succession.

Second, despite differences in the initial floristic composition, forest composition seems to follow a trend toward a compositional convergence in the long term. Convergence was supported by the com- paratively close similarity between saplings in the secondary forests and canopy composition in the old-growth forests, and also by the ordination of the plots along the first axis of the CA, according to the chronosequence. These results are consistent with those of the most detailed studies both in temperate and tropical forests (Purata, 1986; Christiansen and Peet, 1987; Terbourgh et al., 1996).

In summary, initial floristic composition (Egler, 1954) plays an important role in forest composition for several decades, and it is largely dependent on structural characteristics of the pre-abandonment vegetation. However, forest composition eventually tends to converge to a general ‘climax’ composition acting as a distant force (Pickett and MC Donnell, 1989). The demographic characteristics of the long- lived pioneer species (Gomez-Pompa and Vasquez- Yanez, 1981; Swaine and Whitmore, 19881, such as Parapiptadenia excelsa, Tipuana tipu, Juglans australis, Jacaranda mimosifolia, and the exotic Morus alba, play a key role in regulating the rate of the successional change.

5. Concludiug remarks

The space-for-time substitution involved by chronosequence studies implies assumptions that may not be valid (Pickett, 1989). The most important one is that successional time is more important than site conditions in conditioning forest structure and composition. Our study in subtropical Argentina montane forests supports such an assumption, and these find- ings are important for generating hypotheses that can

be tested by subsequent remeasurement of permanent plots, and by experimental studies.

Succession can be understood as the combined effect of factors that govern site availability, species availability. and differential species performance (Pickett et al., 1987). Our results show that species availability, given by the existing species pool (in-. eluding exotic invaders) and the dispersal mesha- nisms mediated by crop structure, plays an important role in conditioning the initial floristic composition. Quantification of site availability, in this case given by the patterns of land use, would help in translating the results of these studies into a landscape spatial scale. To enlarge the scope of these studies in the temporal domain, research should focus on the rela- tionship among the differential performance of the species dominating the initial floristic composition, with particular emphasis in long-living pioneers.

Acknowledgements

This work was supported by the CONICET, Ar- gentina; the Parque Biologic0 Sierra de San Javier; and the Research Council of the National University of Tucuman Brian Finegan, Julie Densiow, Thomas Veblen, Mark Miller, Lori Daniels, Sally Suznowitz and three anonymous reviewers provided very help- ful comments on different drafts.

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