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ECOTROPICOS 18(1):21-29. 2005Sociedad Venezolana de Ecologa

GEOMORPHOLOGY, SOIL TEXTURE AND TREE DENSITY IN A SEASONALSAVANNA IN EASTERN VENEZUELA

GEOMORFOLOGIA, TEXTURA DEL SUELO Y DENSIDAD DE LEOSAS EN UNASABANA ESTACIONAL EN EL ORIENTE DE VENEZUELA

Susan Smith1 , Juan F. Silva2 and Mario R. Farias2

1Instituto Forestal Latinoamericano, Va Chorros de Milla. Ncleo Forestal, Apartado 36.Mrida, Venezuela. E-mail: ssmith3_9@hotmail.com.

2Instituto de Ciencias Ambientales y Ecolgicas (ICAE). Facultad de Ciencias.

Universidad de Los Andes. Mrida (5101), Venezuela

ABSTRACT

We studied the relationship between geomorphology, tree density, and soil texture in a seasonal savanna from eastern Venezuela.Six landforms were identified and mapped. Using a regular design, we sampled soils at depths of 0-20 and 20-40 cm, anddetermined textural fractions. Alternatively, using a stratified procedure, we measured tree densities by counting and identifyingall woody individuals disregarding their size in 5x5 m plots. Contrary to our expectations, results showed that total treedensity was positively linked to sand content that assures better drainage but lower water retention capacity. Moreover, treedensity was also linked to the more stable landforms, suggesting that surface stability is more influential on tree populationgrowth than water availability in the topsoil. Although most species are present in all landforms, the results also showed thatthree species (Curatella americana, Byrsonima coccolobifolia and Casearia sylvestris) were associated to the more stabledepositional surfaces with sandy soils and other three (Byrsonima crassifolia, Roupala complicata and Bowdichia virgilioides)were associated to surfaces undergoing active morphogenesis with clayish top soil.

Key Words: Geomorphology, heterogeneity, seasonal savanna, soil, texture, tree density, Venezuela

RESUMEN

Estudiamos la geomorfologa, textura del suelo y densidad de leosas en una sabana estacional del oriente de Venezuela.Identificamos y mapeamos seis formas de relieve. Usamos un diseo regular para muestrear los suelos a 0-20 y 20-40 cm deprofundidad y luego determinar textura. Paralelamente, en un muestreo estratificado en formas de relieve, identificamos ycontamos todos los individuos leosos sin importar tamao en parcelas de 5x5 m. Los resultados muestran que la densidad totalde leosas est positivamente relacionada con el contenido de arena, que favorece el drenaje pero no la capacidad de retencinde agua. Adems, la densidad de leosas aparece relacionada a las formas de relieve ms estables, sugiriendo que la estabilidadde las superficies ejerce ms influencia que la disponibilidad de agua en los horizontes superiores sobre el crecimiento arbreo.Aunque la mayora de las especies estn presentes en todos los relieves, tres especies (Curatella americana, Byrsonimacoccolobifolia and Casearia sylvestris) aparecen asociadas a las formas ms estables con suelos arenosos y otras tres (Byrsonimacrassifolia, Roupala complicata and Bowdichia virgilioides) aparecen asociadas a superficies con morfognesis activas ytexturas superficiales arcillosas.

Palabras clave: Geomorfologa, heterogeneidad, sabanas estacionales, suelos, textura, densidad arbrea, Venezuela

Ver artculo con figuras a color en: http://ecotropicos.saber.ula.ve

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GEOMORPHOLOGY, SOILS AND SAVANNA TREE DENSITY

INTRODUCTION

Variations in the physiognomy of Neotropicalsavannas seem to be largely determined by wateravailability and fire frequency at several scales(Cole 1982, Medina and Silva 1990). In regionssuch as the Orinoco Llanos that experience annualfires we would expect open savannas topredominate and tree cover to be of lesserimportance. However, we find areas with variabletree density reaching as much as 4000 trees ha-1(Sarmiento 1983). These variations also take placeat the site scale with patches of wooded savannain mosaic with more open savanna and grassland,and are attributed to topographic and edaphicheterogeneity. Moreover, this heterogeneity is theresult of geomorphological processes shaping thelandscape (COPLANARH 1974a, 1974b).

Rainfall in savannas is seasonally distributedwith almost all rains concentrated in a six-monthperiod. During the rest of the year soils runprogressively dry until there is no more wateravailable for plant growth. Savanna trees, however,grow actively during the dry season, proving to besomehow independent of rainfall seasonality(Goldstein and Sarmiento 1987, Goldstein et al.1990). Nevertheless, tree seedlings seem toexperience drought even during the short lapseswith no rain in the wet season, and this has aneffect upon tree growth and survival (Hoffman1996). Provided that in the long run rainfall is evenlydistributed at the local scale, we would expectdifferences in yearly water availability to dependon the capacity of soils to store water. In turn, thesedifferences affect the probabilities for treeindividuals to grow and get established.

A series of studies have documented anddescribed the spatial heterogeneity of the landscapein the Orinoco Llanos, emphasizing the relationshipsbetween land forms, soils and vegetation within thesame climatic region (Silva and Sarmiento 1976,Silva et al. 1971). Together with other evidence(Cole 1982), this suggests that savannaphysiognomy is determined by a hierarchical systemcomprising landform-soilwater availability. At thesite scale, water available for plant growth dependson the capacity of the soil to store water, and this isdetermined by soil texture and structure (Dodd andLauenroth 1997, Grassi 1998, Sala et al. 1992, Salaet al. 1997, Saxton et al. 1986, Singh et al. 1998).In turn, soil texture and structure are the result ofthe pedogenetic development at a landform scale

(Tricart 1964).There is a scarcity of studies relating

geomorphology, soil textures and tree densities atthe site scale. Furthermore, we do not know howthe different savanna tree species behave in thiscontext. As a contribution on this direction, westudied the relationships between landform, soiltexture and tree density at a site scale in an areasubmitted to the same annual fire regime. Moreover,we compared the behavior of the different treespecies present in the area.

METHODS

The study area is a high, dissected plateau ofearly Quaternary sediments geologically referredas Mesa Formation. Here, hydric erosion has beenthe most important geomorphogenic process(COPLANARH 1974a, 1974b), originating alandscape with erosive surfaces such as the Mesaand its relicts, with steep cliffs in the borders anddepositional landforms such as colluvial valleys andcones (Figure 1). The area is located in southeastAnzotegui State, Venezuela (939N 6334W), inan area known as Mesa de Guanipa, north of theOrinoco River (Figure 2a). Soils are Entisols,Oxisols and Ultisols deep, with good drainage, pH4.5 to 5.5, generally low water holding capacityand very poor in nutrients. Mean annual rainfall is1240 mm; mean annual temperature is 25.5 C.Land use is currently restricted to very extensivecattle ranching, burnt annually during the dry seasonfrom December to April, although not all areas areburnt in the same fire event. Predominantvegetation is open savanna with the grassesTrachypogon plumosus (Humb. & Bonpl. exWilld) Nees, as the dominant species in the upperterrain and Andropogon selloanus (Hack.) Hackand Axonopus canescens (Nees ex Trin.) Pilg.,in the lower terrain. The most important treespecies are Curatella americana L., Byrsonimacrassifolia (L.) Kunth, and Bowdichiavirgilioides Kunth.

We made a photo-interpretation of a pair ofaerial photographs, scale 1:25.000 (Mission 040198,1977). We identified and delimited all landformsaccording to the Systematic Method (Buzai andSanchez 1998) and selected a study area of 288 hawhich represented the geomorphologicalheterogeneity found. The whole study area wasreticulated with a sampling point every 150 m for atotal of 156. At each point, landform was identified

23ECOTROPICOS 18(1):21-29. 2005

SMITH, SILVA AND FARIAS

and soil samples were taken at 0-20 and 20-40 cmdepth. This systematic procedure allowed us toassess independently the textural properties of thedifferent landforms. In the laboratory, the differenttextural fractions were determined using theBouyucos method (Day 1965). Then, the resultsfor each soil depth were grouped by landform. Sand,silt and clay content were compared using Kruskal-Wallis and Mann-Whitney non-parametric tests.

Vegetation was sampled in each landformusing a stratified random procedure. A total of 159plots (5x5 m) were used. In each plot, all treeindividuals (disregarding their size) were countedand identified by species. Results were organizedby landform, and mean density values werecalculated for total trees, and by species. Non-parametric tests (Kruskal-Wallis, Mann-Whitney)

were used to detect significant differences amonglandforms in terms of total tree density and speciesdensity.

We also used Detrended CorrespondenceAnalysis (DCA) to order vegetation plots andintegrate results from geomorphology, soils andvegetation. The information for DCA multivariateanalysis was obtained from 44 plots (10 x 15 m)distributed among different landforms. Data in eachplot included type of landform, slope, tree densityand species density according to the proceduresmentioned above. A soil sample was takenaccording to soil sampling procedure. Based ondensity data, two matrixes were built, one mainmatrix with the species data and the other with thesoil data. This was conformed to quantitative anddummy variables. Additionally, correlation analyses

Figure 1. Photograph of the landscape in the study area, showing some of the characteristic landforms. In thebackground, the mesa (M) and the cliffs (C); in the middle ground, the colluvial valley (CV) with small mounds (M);there are three small relicts of mesa (MR), and in the foreground, a colluvial cone (CC). Photo M. Farias.

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were made with the DCA data to detectrelationships between the density of each speciesand the soil texture.

Indicator Species Analysis (IV) was used todetect the species association with the landforms(Dufrne and Legendre 1997). The analysis wasmade with DCA data and the statistic significancewas obtained with a Monte Carlo test.

RESULTS

We identified six contrasting landforms: (1)Mesa, (2) Mound, (3) Mesa Relict, (4) Cliff, (5)Colluvial Cone, and (6) Colluvial Valley (Figure 2).1) Mesa, is the largest continuous and the moststable surface except at the borders where active

erosive processes are taking place; 2) Mound witha core of lateritic cuirass arise from the plateau orthe colluvial valleys as a result of differential erosion;3) Mesa Relict corresponds to slightly inclinedremnants of the Mesa found in depressions; 4)Cliff is a steep, rough terrain produced by theintense erosion of the Mesa at the borders, andrepresent the least stable surfaces; 5) ColluvialCone formed by the redeposition of sedimentsfrom the borders of the Mesa and the Mesa Relict,and conforms to slightly inclined areas; 6) theColluvial Valley is an extended surface at thelowest level, with a soft rolling relief with flatlandsand seasonal lagoons, covered with a thin layer ofsediments from the neighboring higher plateaus.Although erosion is taking place in all landforms,

Figure 2. Diagram representing the location of the area of study area in Venezuela (a); the spatial distribution of thelandforms within the study area (b); and an idealized cross-section with the relative position of each landform (c,vertical scale exaggerated). Diagonally hatched= Mesa; vertically hatched= Mesa Relict; black= Cliff; stippled=Colluvial Cone; crossed hatched= Mound; white= Colluvial Valley; horizontally dashed= seasonal lagoon.

GEOMORPHOLOGY, SOILS AND SAVANNA TREE DENSITY

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these differ by origin and relative stability, whichdepends on the balance between morphogenesisand pedogenesis. Surfaces where pedogenesis ispredominant are more stable than surfaces wheremorphogenesis still prevails. Mesa and ColluvialValley are depositional in origin and more stable;Cliff and Mesa Relict are erosional in origin andmore dynamic; whereas Colluvial Cone, ofdepositional origin, and Mound, of erosional originare intermediate landforms in terms of relativestability.

Landforms are significantly different in soiltexture (p

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The most abundant tree was C. americanawith a mean density of 1817 ind. ha-1, and the leastabundant was B. coccolobifolia with 139 ind.ha-1.Although the non-parametric tests revealedsignificant differences in density of individualspecies between landforms (Table 1) thesedifferences do not seem to have any meaning interms of origin and stability of the landforms.

Two species showed a significant associationwith landform: Casearia sylvestris (IV = 22.1,p< 0.001), with its highest mean density in theColluvial Valley (1836 ind.ha-1) and Byrsonimacoccolobifolia (IV = 12.3 p< 0.001) with itshighest mean density in the Mesa (733 ind.ha-1).

Density was significantly correlated to texturein three species as follows. Density of C. sylvestriswas positively correlated to sand (r= 0.41, p

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Figure 5. Results of the correlation analysis on the DCA. Variables outside the correlation circle are significantlycorrelated to the axes (r = 0.294, p

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DISCUSSION

The relationships between landform and soiltexture found in the study area showed that soils inthe depositional and more stable landforms arericher in sand whereas the more dynamic landformsare richer in clay. Similar results have been reportedfor other areas in the Orinoco Llanos (Ponce et al.1994, Sarmiento 1990, Escobar et al. 1995,Fassbender et al, 1979). In the more stablelandforms, pedogenesis is the dominant process andthe eluviation of clay progresses withoutdisturbance. That is not the case in the moredynamic landforms originated by erosive processesthat removed the upper, sandy layers exposing theclayish layer below. Furthermore, the activemorphogenesis hinders the pedogenetic processesto take place (Elizalde and Jaimes 1989).

Total tree density is high and variessignificantly between landforms. Although the non-parametric statistics did not show that differenceswere related to the land dynamics described, theDCA showed a clear link between tree density andthe more stable landforms. By the same token,although the direct correlation analysis of total treedensity and soil texture was not statisticallysignificant, the multivariate analysis showedlandforms and tree density were associated to percent of sand. Direct, simple correlations betweenvegetation variables and physical determinants maybe difficult to find since there are many explanatoryvariables and they are not independent. In thiscontext, the results of the multivariate analysis areof greater importance. There are alternativeexplanations to the lack of significance in the directcorrelation analyses between soil texture and treedensity. It may be that the change in speciesdominance occurring along the texture-geomorphological gradient introduces acompensatory effect. In addition, the fact that soilsampling and vegetation sampling were conductedindependently may have foiled any directcorrelation.

The multivariate and the correlation analysescoincide to show differences in the behavior ofindividual species. Within the range of texturesfound, three species (R. complicata, B. crassifoliaand B. virgiliodes) are more abundant on theheavier soils whereas the other three (B.coccolobifolia, C. americana and C. sylvestris)favour sandy soils.

The above mentioned species are widely

distributed in the Neotropical savannas. C.americana, B. crassifolia and B. virgilioides areprobably the most common tree species in thenorthern savannas of the Neotropics from CentralAmerica and the Caribbean to the northern Amazonregion (Sarmiento 1983), but they are also commonin the savannas of the Brazilian Shield (Ratter andDargie 1992). However, their spatial distribution atlarge or local scale remains unexplained.

In these well drained savannas, higher claycontent is related to higher water retention capacity;therefore we expected the clayish soils to havehigher tree density. Our results showed the opposite.Tree density was positively associated to sandcontents, but also to more stable surfaces. Too activemorphogenesis is likely to hinder tree populationgrowth whereas more stable surfaces allows forhigher tree density despite lower water retentioncapacity in the top soil. In addition to this trade-off,drainage may be more influential than waterretention capacity on savanna tree growth. Othervariables such as soil depth, soil structure, coarsefraction, etc., were not measured. A modeling effortto explain changes in tree density as a result ofvariations in water available for plant growth shouldindeed include those variables.

ACKNOWLEDGEMENTSThis study was funded by FONACIT

(Venezuela) Grant # 98003404 and by IAI GrantCRN #040. Thanks to P. Alvizu, D. Thielen, J.Smith, and N. Smith for assistance in data analysis.We are grateful to A. Azocar, G. Silva, and K.Kaylor, as well as to two anonymous reviewersfor comments and critiques to earlier versions ofthe manuscript.

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SMITH, SILVA AND FARIAS

ECOTROPICOS 18(1):21-29. 2005

Recibido 14 de enero 2005; revisado 18 de julio 2005;aceptado 05 de septiembre 2005.