Plant Ecology 145: 37–48, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

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Temporal and spatial variation of litter production in Sonoran Desertcommunities

Angelina Martınez-Yrızar, Silvia Nuñez, Haydee Miranda & Alberto BurquezInstituto de Ecolog´ıa, Universidad Nacional Aut´onoma de M´exico, Apartado Postal 1354, Hermosillo,Sonora, 83000, M´exico (E-mail: angelina@servidor.unam.mx)

Received 23 June 1998; accepted in revised form 9 March 1999

Key words:Desertscrub, Ecosystem, Litterfall, Mexico, Phenology, Spatial heterogeneity

Abstract

The seasonal pattern of litter production was analyzed in three contiguous desert communities near the southernboundaries of the Sonoran Desert. There was a large spatial variation in annual litter production mainly caused bydifferences in the composition and structure of vegetation. In the most productive site (Arroyos) annual litterfall was357 g m−2 yr−1, a figure higher than some tropical deciduous forests. Litter production was only 60 g m−2 yr−1inthe open desert in the plains (Plains) and 157 g m−2 yr−1 in the thornscrub on the slopes (Hillsides). Topographicand hydrologic features influence the composition, structure and function of the vegetation, modifying the gen-eral relationship between rainfall and productivity described for desert ecosystems. The temporal pattern of litterproduction showed marked seasonality with two main periods of heavy litterfall: one after the summer rains fromSeptember to November (autumn litter production) and another after the winter rains from March to May (springlitter production). In the open desert areas, spring litter production was significantly higher than the autumn pulse,while in the slopes, the autumn production was the most important. The Arroyos site produced similar litterfallamounts during the two dry seasons. The species composition defined the season of maximum leaf-fall. In thePlains, the vigorous winter growth of ephemeral and perennial plants made up most of the litter production, whilein the Hillsides, most perennials remained dormant throughout the winter-spring period and a significant peak oflitterfall occurred only after the summer growth. This difference in growth between seasons was less pronouncedin the Arroyos. The timing of maximum production of reproductive and woody litter also differed from site to site.

Introduction

Litter production is the main pathway of abovegroundnutrient return to the soil (West 1979; Vitousek 1984;Schlesinger 1997). Average litterfall in desert ecosys-tems is generally low, however, it varies widely be-tween localities, between sites within the same region,and over years at the same sites (Noy-Meir 1973,1985). This is mainly related to the highly variabledesert precipitation in space and time. Thus, desertproductivity in particular years and places may exhibitvalues within the range of more productive ecosys-tems such as tropical deciduous forests. In this studywe examined the effect of spatial and temporal het-erogeneity in the production of litter of desertscrubcommunities in central Sonora, Mexico. It is part of

a broader project in which the composition, struc-ture and function of Sonoran Desert communities weremonitored from 1992 to 1996 (Búrquez & Quintana1994; Núñez 1998; Búrquez et al. in press a). Theaim of the general project was to determine the poolsand fluxes of carbon under the unpredictable physicalenvironment of the desert as a baseline to explain thelong-term behavior of the ecosystem.

The Sonoran Desert has a long history of scien-tific studies that have shown its great complexity andbiodiversity (Shreve 1951; Shreve & Wiggins 1964;Wiggins 1980; Brown 1982; Crosswhite & Crosswhite1982; Turner et al. 1995; Búrquez et al. in press b).However, studies regarding ecosystem function arescant and have mainly been conducted in northern lo-calities where the physical environment is harsher than

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the more tropical southern Sonoran Desert (Halvor-son & Patten 1975; Patten 1978; Rundel et al. 1982;Sharifi et al. 1982). In its southern boundaries, theSonoran Desert merges with the thornscrub and thetropical deciduous forest. This feature makes this areamuch richer in tropical perennial elements, suggestinghigher primary productivity rates than in the northernlocalities (Maya & Arriaga 1996; Búrquez et al. inpress b).

The influence of topography and intermittentstreamways on the composition and structure of thevegetation (A. Búrquez et al. unpublished data) alsosuggests the existence of significant differences in an-nual productivity among nearby sites under the sameclimate. The objective of the present study was to ana-lyze the pattern of litter production in three adjacentand structurally different Sonoran Desert communi-ties. The relative contribution of litter components andof individual species to the annual litterfall on eachcommunity were also determined.

Materials and methods

Study area

The study area is located 4 km south of Hermosillo,Sonora, Mexico (29◦01′ N, 110◦57′ W, 250 m) ina former biological reserve. The reserve is part of alarge extension of non-transformed desert that drainsinto the lower Sonora river valley. It has a land usehistory of light cattle grazing and little wood extrac-tion. Because of its higher location, the study arearetained most of its biological integrity with smallcity influence until the abrogation of the decree andtransformation of the land into suburban housing in1996. Land use of the study area and other reservesin Sonora, is described by Búrquez & Martínez-Yrízar(1997).

The warm and dry climate is a Desert Steppe (Köp-pen BW type, García 1973). The pattern of rainfallis bimodal with convective rains in summer (x =231 mm, July to September) that often produce violentthunderstorms, and heavy, irregularly distributed rainsof short duration. Winter rains (x = 67 mm, Decem-ber to February) are usually gentle and more homoge-neously distributed in space, but highly unpredictablein time. Average annual precipitation was 347 mm(1966–1996) with considerable variation from year toyear. Minimum and maximum annual rainfall were186 mm (1972) and 598 mm (1983), respectively.

Mean annual temperature was 24.5◦C, with Januarythe coldest month (16.4◦C) and July the warmest(32.2◦C).

The regional topography is typical of the buriedranges, with extensive piedmont alluvial plains,granitic Mesozoic hills, numerous Tertiary extrusivelow-ranges, and isolated sedimentary Paleozoic rocks(Rodríguez-Castañeda 1981). The elevation varies be-tween 220 and 600 m.

The soils are Aridisols mainly derived from gran-ite decomposition. Clay content is high in the plains.Sandy soils predominate in the arroyos, while theslopes are characterized by granitic outcrops. The veg-etation of the plains and arroyos belongs to the Plainsof Sonora subdivision of the Sonoran Desert proposedby Shreve (1951), and the hillsides with northernand eastern aspect have Foothills thornscrub (Búrquezet al. in press b).

Site characteristics

Three study sites were selected according to the com-position and structure of the vegetation, topographyand presence of streamways (A. Búrquez et al. un-published data): Plains, Hillsides and Arroyos. Soilswere slightly alkaline (pH range 7.3–7.8) but differedin chemical composition (Table 1).

The Plains site has the open vegetation typicalof the Plains of Sonora subdivision which occurs onlevel areas. This is the least diverse and most sparsevegetation type in the area. Isolated trees ofOlneyatesotaA. Gray andCercidium microphyllum(Torr.)Rose & Johnston that reach more than 5 m tall arethe distinctive species of this habitat along with lowshrubs ofEncelia farinosaA. Gray andJatropha car-diophylla(Torr.) Muell. Arg. The trees create ‘islands’of greater biological diversity and higher soil fertil-ity than the average surrounding desert (Búrquez &Quintana 1994).

The Hillsides site is representative of the thorn-scrub which mainly occurs on the slopes. It is locatedon a north-facing 30◦ slope. Here the vegetation ismore tropical in character, more diverse, and moreuniform in cover than in the plains. Dominant speciesincludeCroton sonoraeTorr., Jatropha cordata(C.G.Ortega) Muell. Arg. andMimosa distachyaCav.

The Arroyos site is typical of the so-called xerori-parian vegetation of small, dry, sandy arroyo beds. Itcomprises the most diverse and dense vegetation in thearea. Dominant species areO. tesota, Coursetia glan-dulosaA. Gray, Eysenhardtia orthocarpa(A. Gray)

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Table 1. Chemical and physical characteristics of the soils in the Arroyos (n = 3), Hillsides (n = 5) and Plains (n = 4) site in theSonoran Desert, south of Hermosillo, Sonora, Mexico. Numbers are means and standard error in parenthesis.

Site pH Mg+ Na+ K+ Ca++ C N P Sand Silt Clay

(1:2.5) (mg kg−1) (total g kg−1) (g kg−1)

Arroyos 7.8 12.3 6.3 10.8 30.8 11.4 1.0 0.9 710 50 230

(0.4) (1.9) (0.2) (0.9) (1.5) (1.5) (0.4) (0.4) (0.6) (0.6) (1.3)

Hillsides 7.4 10.2 6.8 9.2 22.5 15.8 1.1 0.6 720 70 210

(0.2) (1.6) (0.3) (1.1) (3.3) (2.7) (0.1) (0.02) (1.1) (1.2) (0.8)

Plains 7.3 53.3 6.8 7.0 127.1 4.6 0.3 0.5 740 60 200

(0.3) (28.1) (0.5) (1.4) (0.07) (0.01) (0.01) (0.01) (0.8) (2.2) (1.7)

Figure 1. Daily precipitation (mm) and temporal variation of totallitterfall and litter components (g m−2 d−1 ± 1 SEM) for the periodMay 16, 1992 to May 16, 1993, in the Sonoran Desert south of Her-mosillo, Sonora, Mexico. (–2–) Arroyos, (–N–) Hillsides and (–◦–)Plains sites. Total rainfall during the study period was 546 mm.

S. Watson andM. distachya. Many tree species reachtheir highest density here, and the largest richness anddiversity of vines and herbaceous species are foundin this habitat (Molina-Freaner & Tinoco-Ojanguren1997).

Field methods

A 1-ha research area was established on each studysite. Litter was collected using polyvinyl chloride(PVC) cylindric traps (10.5 cm diameter, 10 cmheight) with a 1.4 mm aluminum mesh bottom. Thistrap design is highly efficient in collecting above-ground litter in drylands (Búrquez et al. in press a).The traps (Arroyos and Hillsidesn = 40; Plainsn = 39) were systematically located along four100×10 m parallel transects and remained fixed to theground by aluminum pins. Traps were emptied at 26–38 days and the material for each individual specieswas sorted into five plant components: leaves (leafblades, leaflets, petioles and rachises), flowers, seeds,fruits, and fine woody material (small pieces of barkand branches<1 cm in diameter). Animal debris andfaeces were also segregated. All samples were oven-dried to constant weight at 80◦C and weighed to thenearest milligram. The nomenclature of species fol-lows Shreve & Wiggins (1964), Turner et al. (1995)and Martin et al. (1998).

Data analysis

The minimum number of litter traps needed for arepresentative sample on each site, was determinedempirically by plotting the running mean against theannual total litterfall by trap. The point where theoscillation between successive mean values was lessthan 10%, was taken as the minimum number oftraps required (Goldsmith & Harrison 1976). Sincethis point was reached at 28–30 traps, the 40 trapsprovided a conservative estimate of litterfall. As thesampling units were not randomly distributed, a jack-knife technique (Sokal & Rolf 1995) was performedto obtain the mean value of the litter mass from

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20 randomly selected litter traps. This procedure wasrepeated 40 times to obtain a frequency distribution ofmeans that was compared with the actual distributionof the systematic sampling. No significant differenceswere found between both spatial arrangements (t-testPlains= 0.17, Arroyos= 0.23, Hillsides= 1.31; allvaluesP � 0.05).

We used ANOVA to test for site and temporaleffect on the total amount of litterfall and on its in-dividual components. To check for homogeneity ofvariances a Barlett test was first applied on the log10-transformed data. The accumulated monthly values bytrap, for the period 16 May 1992 to 16 May 1993,were used as an estimate of the annual litterfall bytrap. These amounts were averaged to calculate themean annual litter production value for each site withthe corresponding standard errors and coefficients ofvariation. Losses from herbivory were not includedin the litterfall estimates. All statistical analyses weremade using SPSS 6.0 for Windows (Norušis/SPSS Inc.1993).

Results

Spatial variation in annual litterfall

The ANOVA revealed significant site and time effectsin the total annual litterfall and its individual com-ponents (P � 0.05 in all cases). The Plains hadthe lowest annual total litterfall value and the Ar-royos the highest (Table 2). The Arroyos site had alsothe highest production of leaf-litter, which doubledthe amount in the Hillsides and was more than fivetimes the production in the Plains. The contributionof reproductive structures and woody material wasalso the highest in the Arroyos (Table 2). The coeffi-cients of variation (CV) of the mean annual productionwere rather high, reflecting the inherent variability inplant distribution. The CV values varied greatly de-pending on the litterfall category. Both total litterfalland leaf-fall had the lowest variation (62–95%) butthe other litter components showed a larger variationrange (63–223%).

Animal debris (mainly insects) were found in mosttraps and all sampling dates. The contribution of thiscomponent varied greatly among sites almost in thesame way as phytomass. In the Arroyos, total ani-mal debris were more than twice the amount in theHillsides (23.7 and 10.9 g m−2 yr−1, respectively),and the Plains produced only 4.1 g m−2 yr−1. In gen-eral, insect faeces made up the largest fraction of the

total animal trash (74–90%). The highest depositionoccurred from August to October, which is the periodof maximal animal activity and plant growth.

Temporal variation in litterfall

The temporal pattern of litter production was bimodal(Figure 1). As a general trend, minimum daily ratesof leaf-fall occurred during the months of highestprecipitation (July and August). At the end of thesummer rains, there was a peak of leaf-litter deposi-tion (September–November) which varied in quantityamong the sites. After this autumn litterfall event, therate of litter input declined again to reach minimumvalues from December to February during the periodof winter precipitation. A subsequent pulse of litterfallwas observed from March to May (spring leaf-litterproduction) that also varied significantly in amountdepending on the site (Figure 1).

Although the pattern of production of reproductivelitter depended on the timing of flowering and fruit-ing of the dominant species in the area, it exhibitedmarked seasonality. A maximum value was attained inthe spring. It was mainly made of seeds of the win-ter ephemerals and senescent flowers of the perennialspecies that reproduced in early spring. This maximumwas followed by a second period of high production inSeptember, mostly of fruits and seeds of the speciesflowering in summer (Figure 1).

The rate of production of fine woody litter in theArroyos and Hillsides increased during the monthsof heavy rains and strong winds in summer, partic-ularly from August to October. These changes wereless pronounced in the Plains (Figure 1). The produc-tion of woody and reproductive litter was very variablewithin individual sampling dates as indicated by thelargest standard errors as proportions of the mean val-ues (Figure 1). The coarse woody litter fraction wasnot measured because of the low probability of occur-rence in our design. Probably, it is at least as importantas the fine woody fraction and follows a similar trend.

Annual litterfall on a species basis

Seventy-two species of vascular plants (11 trees,26 shrubs, 26 herbs, 9 vines) in 34 families were cap-tured in the litter traps during the year of study. TheCompositae, Euphorbiaceae and Fabaceae were themost abundant families with six species each. Twentyfamilies were represented by only one species. Of thetotal number of species, 25 were common to the threesites (Table 3).

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Table 2. Total litterfall and individual litter components (g m−2 yr−1± 1 standard error) for the periodMay 16, 1992 to May 16, 1993, in three communities of the Sonoran Desert, south of Hermosillo,Sonora, Mexico.n = 40, except in the Plains site wheren = 39.%= percentage contribution ofcomponent to total annual litterfall.

Component Arroyos Hillsides Plains

Dry wt % Dry wt % Dry wt %

Leaves 202.7± 19.8 56.8 100.2± 7.3 63.6 35.2± 5.4 59.2

Reproductive 59.5± 12.6 16.6 18.7± 2.3 11.9 18.7± 2.4 31.4

Flowers 21.3± 5.1 6.0 6.7± 0.8 4.3 4.7± 1.6 8.0

Fruits 23.7± 6.8 6.6 4.4± 1.1 2.8 1.5± 0.3 2.4

Seeds 14.5± 2.3 4.0 7.5± 1.0 4.8 12.5± 1.2 21.0

Woody 94.8± 17.0 26.6 38.5± 13.6 24.5 5.6± 1.2 9.4

TOTAL 357.0± 35.9 100.0 157.4± 18.9 100.0 59.5± 7.6 100.0

Owing to its greater diversity and richness, eightspecies in the Arroyos made up 65% of the total lit-terfall while in the Plains and Hillsides, only threespecies accounted for 59 and 62% of the annual value,respectively. Dominant and co-dominant species weredifferent on each site. The legume treesO. tesotaandC. glandulosamade up 36% of the litterfall in the Ar-royos.C. sonoraea small shrub, and the treeJ. cordatamade up more than half of the litterfall in the Hill-sides, while the shrubE. farinosaalone was clearlythe dominant in the Plains (Table 3).

Litter production of the species grouped by growthform indicated that shrubs made up most of the totallitterfall on each site (46–55%) while the trees con-tributed a lower fraction (24–34%). The herbaceousplants, mostly desert ephemerals, were very impor-tant in the Plains (23%), but contributed little in theArroyos and Hillsides (8 and 2%, respectively). Theproportion of litterfall from the vines also differedamong the study sites (Table 3).

Litterfall seasonality of selected species

From the eight most important species in the Arroyos,J. cardiophylla a shrub with short-lived branches,and the vineMerremia palmeri(S. Watson) Hallier,showed a single pulse of leaf-fall from August toNovember and remained leafless for several months(January to June; Figure 2). In contrast,C. glandu-losa, M. distachyaandE. orthocarpahad two periodsof maximum leaf-fall (after the summer and winterrains, respectively) and remained leafless for only ashort time at the end of each of the two dry seasons.Shrubs ofE. farinosaalso shed their leaves twice ayear but the spring was the period of highest leaf-fall.

Two important evergreen tree species that shed leavesthroughout the year and showed a marked increaseof leaf-fall during the spring werePhaulothamnusspinescensA. Gray andO. tesota(Figure 2).

The woody litter fraction of the selected speciesin the Arroyos was produced almost continuously, butmaximum values were attained during the summerrains (Figure 2). The seasonal shedding of repro-ductive litter also varied greatly among the species.The most marked temporal changes were observed inM. palmeri, E. farinosaandO. tesota(Figure 2).

In the Hillsides, the temporal pattern of litterfall ofthe four most important species showed a main periodof leaf-fall from September to December. Maximumvalues were attained at different months depending onthe species (Figure 3). For example,J. cordatareacheda leaf-fall peak in October, whileC. glandulosaandC. sonoraeone and two months later, respectively.SinceJ. cordataproduces leaves once a year in sum-mer, it had a single annual pulse of leaf-fall andremained leafless through the winter and spring. Theother species produce leaves twice a year, thus show-ing a bimodal pattern of leaf-fall (Figure 3). Woodylitter was also captured throughout the year, but dif-ferent species peaked at different times. The periodsof maximum production of reproductive litter werespring and late-summer (Figure 3).

In the Plains, leaf-fall patterns of the most impor-tant species (Figure 4) were comparable to those de-scribed for the species in the other two sites.O. tesotahad a continuous turnover of leaves with a peak in thespring. E. farinosashowed marked leaf-fall season-ality, similar to the bimodal pattern observed in theArroyos.C. microphyllum, a tree species with photo-

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Table 3. Relative contribution (%) of the species to the total litter production from May 16, 1992 to May 16, 1993, in the Arroyos, Hillsidesand Plains sites in the Sonoran Desert, south of Hermosillo, Sonora, Mexico. GF= growth form: T= tree, S= shrub, H= herb, V= vine.

No. Fam1 GF Species Arroyos Hillsides Plains

(%)

1 MLV S Abutilon incanum(Link) Sweet 0.23 1.21 –

2 MIM S Acacia constrictaBenth. – – 0.01

3 MIM S Acacia occidentalisRose 2.28 0.01 –

4 MIM T Acacia willardianaRose – 2.46 –

5 EUP S Acalypha californicaBenth. 0.02 – –

6 AMA H Amaranthus fimbriatus(Torr.) Benth. ex S. Watson – – 0.10

7 POA H Aristida adscensionisL. 0.02 – 3.04

8 POA H Aristida ternipesCav. 0.11 – 0.38

9 STR H Ayenia filiformisS. Watson – 0.05 –

10 NYC H Boerhavia coccineaMill. – – 0.04

11 POA H Bouteloua barbataLag. 0.02 – 1.72

12 CMP S Brickellia coulteriA. Gray 2.65 0.43 –

13 BRS T Bursera fagaroides(H.B.K.) Engl. var. elongata McVaugh & Rzedowski – 1.11 –

14 BRS T Bursera laxifloraS. Watson 0.06 1.72 0.71

15 BRS T Bursera microphyllaA. Gray – 2.56 –

16 CAE T Caesalpinia palmeriS. Watson 2.74 – –

17 MLP V Callaeum macroptera(DC.) D.M. Jonhson 0.17 – –

18 SAP V Cardiospermum corindumL. 1.99 1.17 1.19

19 ACA H Carlowrightia arizonicaA. Gray 0.02 0.25 0.06

20 POA H Cathestecum erectumVasey & Hack. 0.15 1.71 8.01

21 ULM S Celtis pallidaTorr. 1.32 – –

22 CAE T Cercidium microphyllum(Torr.) Rose & Johnston 0.07 1.11 14.10

23 NYC S Commicarpus scandens(L.) Standley 2.99 0.02 0.03

24 FAB S Coursetia glandulosaA. Gray 13.93 9.94 <0.01

25 EUP S Croton sonoraeTorr. – 28.61 –

26 BOR H Cryptantha angustifolia(Torr.) Greene 0.02 0.11 0.24

27 BOR H Cryptantha pterocarya(Torr.) Greene – 0.08 –

28 FAB H Dalea mollisBenth. <0.01 <0.01 0.82

29 UMB H Daucus pusillusMichx. <0.01 1.75 <0.01

30 BRA H Descurainia pinnata(Walt.) Britton 0.04 0.04 0.01

31 CMP H Dyssodia concinna(A. Gray) Robinson 0.01 0.01 0.18

32 CMP S Encelia farinosaA. Gray ex Torr. 5.81 0.29 35.68

33 HYD H Eucripta chrysanthemifolia(Benth.) Greene 0.02 0.07 –

34 EUP H Euphorbia erianthaBenth. – – 0.53

35 EUP H Euphorbia polycarpaBenth. – – 1.72

36 FAB T Eysenhardtia orthocarpa(A. Gray) S. Watson 2.49 1.20 0.01

37 CMP H Filago arizonicaA. Gray – 0.14 –

38 ZYG T Guaiacum coulteriA. Gray 1.52 – 0.03

39 LAB S Hyptis emoryiTorr. 0.33 – –

40 CUC V Ibervillea sonorae(S. Watson) Greene 0.07 – –

41,42 MLP V Janusia californicaBenth. &J. gracilis A. Gray 0.94 1.99 1.48

43 MLP V Janusia linearisWiggins 1.07 0.18 0.51

44 EUP S Jatropha cardiophylla(Torr.) Muell. Arg. 4.45 – 2.52

45 EUP T Jatropha cordata(C.G. Ortega) Muell. Arg. 0.06 23.91 –

46 ACH S Justicia californica(Benth.) D. N. Gibs. – 1.26 –

47 RHM S Karwinskia parvifoliaRose – – 0.04

48 KRM S Krameria erectaWilld. – – 0.03

49 VRB S Lantana horridaH.B.K. 0.39 – –

50 PLM H Linanthus bigelovii(A. Gray) Greene <0.01 <0.0 –

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Table 3. Continued.

No. Fam1 GF Species Arroyos Hillsides Plains

(%)

51 SCR H Linaria texanaScheele – 0.01 –

52 CRY H Loeflingia squarrosaNutt. – – 0.01

53 SOL S Lycium berlandieriDunal 0.37 1.30 1.38

54 SOL S Lycium exsertumA. Gray 1.69 – –

55 FAB H Marina parryi (Torr. & A. Gray) Barneby <0.01 – 1.22

56 ASC V Marsdenia edulisS. Watson 0.01 – –

57 CUC V Merremia palmeri(S. Watson) Hallier 3.94 0.64 –

58 MIM S Mimosa distachyaCav. var. laxiflora (Benth.) Barneby 4.92 6.10 6.12

59 FAB V Nissolia schottii(Torr.) A. Gray 0.55 1.36 –

60 FAB T Olneya tesotaA. Gray 21.93 <0.01 9.01

61 CAC S Opuntia thurberiEngelm. 2.32 – 0.02

62 CAC S Opuntia versicolorEngelm. ex J.M. Coulter 1.48 0.02 –

63,64 CAC S Opuntiaspp.2 1.08 <0.01 –

65 BOR H Pectocarya recurvataI.M. Johnston 0.07 0.04 0.76

66 POA H Pennisetum ciliare(L.) Link 0.43 0.21 0.02

67 CMP H Perityle californicaBenth. 1.09 1.72 0.19

68 HYD H Phacelia crenulataTorr. ex S. Watson 0.01 0.02 –

69 PHT S Phaulothamnus spinescensA. Gray 8.04 0.09 0.28

70 MIM T Prosopis velutinaWooton 0.18 – 0.03

71 RUB S Randia obcordataS. Watson 0.82 0.02 <0.01

72 CMP S Trixis californicaKell. 0.02 – –

Unidentified material 4.81 3.75 4.17

Unidentified grasses 0.27 1.42 3.59

Trees 29.05 34.07 23.89

Shrubs 55.14 49.30 46.11

Herbs 2.28 7.63 22.64

Vines 8.74 5.34 3.18

1Family acronyms after Weber (1982).2Opuntiaspp. includeO. thurberi, O. versicolor, O. fulgidaEngelm. andO. leptocaulisDC.

synthetic trunks, had a prolonged period of leaf-fallfrom November to May with a maximum value inMarch. It also had the largest production of repro-ductive litter (mainly flowers) along withE. farinosa(seeds) in April and May (Figure 4).

Contribution of litterfall components by species

Leaf-litter was the most important component ofthe total litterfall, but its relative contribution variedgreatly among the species. While in the Arroyos, 80%of the litter of P. spinescensand M. distachyawascomposed of leaves, leaf-litter ofJ. cardiophyllaandC. glandulosaaccounted only for about 50% (Fig-ure 2). Similar trends were observed in the other twostudy sites (Figure 3, 4).

The percentage of woody litter also differed amongthe species on each site. In the Arroyos, the vine

M. palmerimade up only 3%, whileJ. cardiophylla,a shrub with short-lived branches, had half of its litterin this category (Figure 2). In the Hillsides,J. cordata,a tree with papery bark, had the highest fraction ofwoody litter whileC. glandulosathe lowest (46 and13%, respectively; Figure 3). The species in the Plainshad in general a small relative contribution of woodyelements (5–11%; Figure 4).

The fraction of flowers, fruits and seeds of the to-tal litterfall produced by each species was also veryvariable. In the Arroyos, the taxa with the highestfraction of fruits wasM. palmeri(20%), seedsE. fari-nosa(18%) and flowersO. tesota(16%; Figure 2). Inthe Plains,E. farinosawas also the species with thehighest percentage of seeds (23%), and the spectacu-lar spring blooming ofC. microphyllumaccounted for34% of its annual litterfall (Figure 4). In the Hillsides,

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Figure 2. Temporal variation of litterfall components (g m−2 d−1 ± 1 SEM) of selected species in the Arroyos site, for the period May 16,1992 to May 16, 1993, in the Sonoran Desert south of Hermosillo, Sonora, Mexico. (–2–) leaf-litter, (–•–) fine woody litter, (–4–) seeds,(–�–) flowers, and (–◦–) fruits. The bars show the relative contribution (%) of each component to the total annual litterfall produced by eachspecies.

only 1–2% of the annual litterfall of each species wascomposed of seeds (Figure 3).

Discussion

Spatial and temporal variation in litterfall

Annual litter production varied greatly among our ad-jacent sites. These differences show the extent of thevariation in productivity found within the desert overrelatively short distances. The xeroriparian habitats,which support the most productive vegetation, are dis-tributed in a spatially complex array within the leastproductive plains. This array is largely determined by

the pattern of runoff in the area. Also, the effect oftopography on the structure of vegetation influencesthe phytomass and productivity on the slopes (Búrquezet al. 1992). Therefore, it is essential to considerthe extent of the spatial heterogeneity to characterizethe productivity of a given area. By using a digitalelevation model and aerial photography (from Insti-tuto Nacional de Geografía, Estadística e Informática,México; resolution 3′′) was possible to evaluate thecoverage of each of the three habitats over thousandsof hectares around our study sites. A gross estimateindicated that about 80% of the area was level terrain.This in turn was partitioned into 45% ‘pure plains’and 35% xeroriparian environments or arroyos. The

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Figure 3. Temporal variation of litterfall components (g m−2 d−1 ± 1 SEM) of selected species in the Hillsides site, for the period May 16,1992 to May 16, 1993, in the Sonoran Desert south of Hermosillo, Sonora, Mexico. (–2–) leaf-litter, (–•–) fine woody litter, (–4–) seeds,(–�–) flowers, and (–◦–) fruits. The bars show the relative contribution (%) of each component to the total annual litterfall produced by eachspecies.

Table 4. Total litter production (g m−2 yr−1) in various desert and tropical dry forest ecosystems.

Ecosystem type Total litterfall Sampling period Reference

and site (g m−2 yr−1) (years)

SONORAN DESERT

Baja California Sur, Mexico 120a 2.8 Maya & Arriaga (1996)

Sonora, Mexico 1 Present study

Arroyos 357

Hillsides 157

Plains 60

MOJAVE DESERT

Nevada, USA 19–53b 2 Strojan et al. (1979)

MONTE

Mendoza, Argentina 1 Martinez-C. & Dalmasso (1992)

Larrea cuneifolia 471

Larrea divaricata 202

TROPICAL DRY FOREST

Ibadan, Nigeria 560 1 Madge (1965)

Calabozo, Venezuela 820 1 Medina & Zelwer (1972)

Indian Church, Belize 1261 1 Lambert et al. (1980)

Guanica, Puerto Rico 288a 2.3 Lugo et al. (1978)

Sobral, CE, Brazil 290–311b 3 Schacht et al. (1989)

Quintana Roo, Mexico 502–767b 4 Whigham et al. (1990)

Jalisco, Mexico 5 Martınez-Y. & Sarukhan (1990)

Valley 613–700b

Hill 363–434b

Watershed I 10 Martınez-Yrızar et al. (1996)

Upper Plot 329a

Middle Plot 319a

Lower Plot 421a

aMean based on the number of years of the study period.bMinimum and maximum annual values during the study period.

46

Figure 4. Temporal variation of litterfall decomponents (g m−2 d−1

± 1 SEM) of selected species in the Plains site, for the periodMay 16, 1992 to May 16, 1993, in the Sonoran Desert south ofHermosillo, Sonora, Mexico. (–2–) leaf-litter, (–•–) fine woodylitter, (–4–) seeds, (–�–) flowers, and (–◦–) fruits. The bars showthe relative contribution (%) of each component to the total annuallitterfall produced by each species.

remaining 20% was assigned to the steep terrain of thehillsides. Given this spatial arrangement, the weightedmean large-scale productivity value for the region was183.2 g m−2 yr−1.

The temporal variation of litterfall was signifi-cantly seasonal as most of the species shed theirfoliage at the onset of the dry periods. The increasedamount of litterfall in the spring is an indication ofthe favorable effect of the winter precipitation on thegrowth of some perennials and winter ephemerals.Highest phytomass and density of the winter ephemer-als in these communities are attained during El Niñoyears when above average precipitation is experiencedin the region (Martínez-Yrízar et al. 1995; Polis et al.1997). This plant growth response has been well doc-umented in the Sonoran Desert (Patten 1978; Felger inpress; Venable & Pake in press, Búrquez et al. in pressa). The fall of reproductive litter in the traps was morevariable in time because of the inherent variation of

flowering and fruiting between species as observed byGuevara de Lampe et al. (1992) in tropical semi-aridvegetation of northeast Venezuela.

Not surprisingly, herbivory was conspicuous in thesummer when insects were abundant and the amountof insect debris in the traps was higher. During thisseason, non-senescent leaf fragments were commonlyfound in the litter traps, suggesting increased insectactivity especially in the denser Arroyos site. It is alsorecognized that significant losses of flowers and fruitsby herbivory may have occurred in the canopy, andthat a large proportion of the seed production may havebeen taken by granivores, as reported to occur in aridand semi-arid deserts (Inouye et al. 1980; Noy-Meir1985). A more detailed study of leaf herbivory in thestudy area is currently under investigation (R. Díaz-Calva & A. Búrquez unpublished data).

Species contribution to total litterfall

The dominant perennial species on each site werealso the most important to the annual litterfall. There-fore, dominance typifies the general pattern of litterproduction described for each study site. These pat-terns, however, may not necessarily show the seasonalchanges of the less abundant species or those thathave exceptional growth response only during yearsof favorable precipitation.

Although some species of similar phenology werepresent in more than one site (i.e.,C. glandulosa,E. farinosaandO. tesota), they differed in contribu-tion to the annual total litterfall on each site. Thisis related to structural differences between commu-nities and underlines the importance of conductingmultispatial comparative studies.

Since considerable yearly variation on primaryproductivity is a distinctive feature of desert ecosys-tems (Noy-Meir 1985; Ludwig 1986), long-term re-search is of major priority to understand the extent ofthe temporal variation of ecosystem functioning andits relationship with environmental factors (Stohlgren1995). The analysis of litterfall, surface litter and de-composition data from continuous measurements dur-ing the 4-year period of study in our Sonoran Desertcommunities is yet to be completed.

Comparisons between arid and semi-aridcommunities

Litterfall and productivity of deserts span a large rangeof variation, from negligible production in extremely

47

dry conditions to relatively high values in mesic habi-tats (Whittaker & Likens 1973; West 1979; Ludwig1986). Compared to the values in arid and semi-aridcommunities (Table 4), litterfall in the Arroyos andHillsides was higher than the production of a SonoranDesert site in Baja California Sur, Mexico (Maya &Arriaga 1996). However, our Plains site had a muchlower annual litterfall. This illustrates the extent ofthe spatial variation that may be found between siteswithin a given region. The very low litter productionin the Mojave Desert (Strojan et al. 1979) is prob-ably caused by the low winter temperatures (as lowas−15 ◦C) coupled with a hot, dry summer that im-poses a severe growth limitation to large perennials.Litter production in a Monte community dominated byLarrea divaricatais similar to our Sonoran Foothillsthornscrub (Martínez-Carretero & Dalmasso 1992).Contrastingly, the production of aLarrea cuneifoliacommunity, also in the Monte, is above the value ofour most productive Arroyos site. The difference inproductivity betweenLarreacommunities was not ex-plained only by the effect of elevation or soil type,but also by a complex interaction of soil moisture,temperature, species composition and phenology.

Compared to tropical dry forests, which also ex-perience strong seasonality, annual litter productionwas lower in our Hillsides and Plains desert sites (Ta-ble 4). By contrast, total litterfall in the desert Arroyoswas higher than the long-term average of some sitesin the tropical deciduous forest of Chamela, Mex-ico (Martínez-Yrízar et al. 1996). Also, the Arroyosshowed a higher litterfall than the forests of Guánica,Puerto Rico and Sobral, Brazil (Lugo et al. 1978;Schacht et al. 1989). These results indicate that al-though the lowest phytomass and litterfall are usuallyfound in desert ecosystems, the productivity undercertain conditions (mainly in xeroriparian habitats)can attain values that fall within the range reportedfor tropical deciduous forests, tropical and temper-ate grasslands, and tropical thornscrubs (Meentemeyeret al. 1982; Noy-Meir 1985; Vogt et al. 1986; Ludwig1987; Martínez-Yrízar et al. in press). Our sites in theSonoran Desert highlight the complexity of neighbor-ing communities with strong differences in structureand functioning under the same macroclimatic de-terminants. The Arroyos and Plains sites reflect twoextremes of a continuum of water catchment and soilfeatures that need to be evaluated in more detail.

Acknowledgements

We thank A. Bracamontes, R. Díaz, A. Esquer,A. Gómez, I. Granillo, A. Quijada, M. A. Quintanaand M. Villegas for their assistance. E. Solís for thechemical analysis of soils. We also thank R. Felger andC. Martínez del Río and three anonymous referees forcritical comments. D. Retes and S. Ocaña gave permis-sion to work in the area. Climatic data were providedby Comisión Nacional del Agua, Hermosillo, México.This project was supported by Consejo Nacional deCiencia y Tecnología, Ref. 0080-N9106 and DirecciónGeneral de Asuntos del Personal Académico, UNAM,Ref. PAPIIT IN212894.

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