Patterns and processes in a seasonally flooded tropical plain: the Apure Llanos, Venezuela

Patterns and processes in a seasonally ¯oodedtropical plain: the Apure Llanos, VenezuelaGuillermo Sarmiento* and Marcela Pinillos ICAE, Universidad de los Andes, NuÂcleo

la Hechicera, MeÂrida 5101, Venezuela

Abstract

Aim The factors and processes determining the ecology and distribution of ecosystemsin the Apure lowlands, Venezuela, are critically discussed.

Location The Middle Apure region, an alluvial Quaternary plain built by the Araucaand Apure rivers, occupies a subsidence area caused by faulting of the geologicalbasement during the last and most active phases of the Andean uplift.

Methods To assess the determinants of the hydrological and ecological variability inthis region, we used remote imagery and ®eld surveys.

Results The hydrological and ecological variability responds to a set of interconnectedprocesses of different nature: geological, climatic and paleoclimatic and hydrographic,which together have conditioned the geomorphological history of the regionallandscape. This evolution, in turn, has guided the soil forming processes and thefunctional behaviour of the different types of savanna, forest and wetland ecosystems.The continuously subsiding trend of the Llanos that accompanied the Andean Orogeny,together with the Quaternary climatic oscillations, settled the framework in whichsuccessive depositional and erosional events took place. Five Quaternary depositionalevents, gave rise to the actual landscape pattern. The youngest, Q0a is the actual ¯oodplain, Q0b is the Holocene ¯ood plain, Q1 is from Upper Pleistocene, Q2 from MiddlePleistocene, while Q3 the oldest sedimentary material in this area, is of Lower Pleistoceneage. Within each sedimentary unit, we analysed the pattern of land forms, soils andnatural ecosystems. The Q0 and Q1 land units show a characteristic unstable drainagesystem, with frequent changes in the course of rivers and their af¯uents and dif¯uents.Soil genesis mainly correlates with the contrasting characteristics of the wet and drytropical climate, and with the water regime of each land form on each sedimentary unit.So, a sequence from entisols, through inceptisols and al®sols, to ultisols, correlates withthe sequence of land units from Q0 to Q3. Different vegetation types predominate oneach land form/soil unit. Gallery forest (evergreen and semi-evergreen) exclusively occurson Q0a entisols, semi-deciduous forest characterizes Q0b and Q1 inceptisols, semi-seasonal savanna is almost entirely restricted to Q0 and Q1 units, hyperseasonal savannawidely dominates on Q2 and Q3 al®sols and ultisols, while seasonal savanna just occurson the well-drained Q1 and Q2 al®sols, and on dunes.

Main conclusions The genesis and dynamics of the various land forms and soilsthroughout the Quaternary, provide the key to their particular hydrology, anddetermines the kind of plant formation able to occupy each habitat, thus explainingthe complex pattern of quite different ecosystems found in the region.

Keywords

Wetlands, water regime, soil genesis, landscape evolution, tropical savannas, tropicalforests, Venezuelan Llanos.

*Correspondence: Blanco Encalada 2614, Buenos Aires 1428, Argentina. E-mail: [email protected]

Journal of Biogeography, 28, 985±996

Ó 2001 Blackwell Science Ltd

INTRODUCTION

The Colombo-Venezuelan Orinoco Llanos, a half millionsquare kilometres region predominantly covered by tropicalsavanna, constitutes an active economic frontier whereannual crops and cattle raising have been rapidly expandingfor the last 30 or 40 years (Silva & Moreno, 1993;Sarmiento, 2000). Within this large Quaternary plain, theupland, well-drained soils, formerly occupied by seasonalsavannas, have been increasingly converted into ®eld cropsand improved pastures; whereas the lowlands, sufferingvarious degrees of seasonal ¯ooding, have remained almostexclusively devoted to an extensive livestock system thatalmost entirely depends on the herbage resources of thenatural savannas.

The most widespread seasonally ¯ooded area of theVenezuelan Llanos occurs in Apure state, and it continuesacross the Colombian frontier in the Arauca and Casanareplains. In the central part of the Apure lowlands (Fig. 1),between the formerly forested lands to the west, and theyoung inner delta in the northeast, bordering the Orinoco,the Venezuelan government promoted, and in most casesbuilt, a system of low, earth dykes, aimed to control theinundations and to improve the livestock-based economy.The terrains encircled by dykes and river levees were called`moÂdulos' (modules), as they operate both as hydrologicaland as grazing units. In spite of their ecological impact andimportant economic role, neither the functioning of thesemodules nor their effects on the distribution and thefunctioning of the natural ecosystems under modulatedconditions, are fully understood.

An international cooperation research project was estab-lished in 1997, as a part of the European Union InternationalCooperation with Developing Countries (INCO DC) pro-gram (European Commission, 1997), on the `Ecological basesfor the sustainable management of tropical ¯ooded ecosys-tems: case studies in the Llanos, Venezuela, and in thePantanal, Brazil'. The project was centred in the region of theApure Llanos and took the NhecolaÃndia area of the BrazilianPantanal (AdaÂmoli, 1999), as a second study case to becompared with the Apure. In the context of this researchproject, the objective of this preliminary study is to provide anoverall picture of the environmental conditions of the MiddleApure area. We want to integrate the dynamic processes ofvarious kinds: geological, climatic and palaeoclimatic, geo-morphological and pedological, into a single conceptualframework, in order to demonstrate their part in thepatterning and functioning of its ecosystems. Our ultimategoal is to achieve a more coherent view of the dynamics of thevarious natural ecosystems and their potential use.

To achieve these goals we started reviewing the pertinentliterature on the physical environment in the area, inparticular geology, geomorphology and soils. Afterwards,three types of remote sensing imagery were visually ana-lysed: the two existing air photographic missions, oneLandsat TM image, and three ASL radar images. Finally,in several ®eld missions, land forms, soils and vegetationwere sampled all along the area, in different seasons, duringthree consecutive years. Although these materials weremainly used in another aspects of the INCO DC project,the ®eld experience and the interpretation of remote imagerygreatly helped to understand the land form patterns and theecological processes determining the distribution of regionalecosystems.

The Middle Apure covers about 30,000 km2 (Fig. 1). It ismainly occupied by ¯ooded savanna ecosystems of the twotypes recognized in the literature: hyperseasonal and semi-seasonal savannas (Sarmiento, 1984). Forests also coverimportant extensions, as do various types of semi-permanentwetlands, permanent swamps and lagoons. The whole areais very ¯at, with a general SW±NE slope in the order of0.2%. Human in¯uence has been limited to livestockintroduction two centuries ago, and to road and dykeconstruction from the 1960s onwards.

THE OVERALL ENVIRONMENTAL SETTING

Geological framework

In order to understand the crucial processes that cause theoutstanding features of the Llanos landscapes, speci®callythose related to the study area, we adopted a wide spatio-temporal overview, embracing the last part of the geologicalhistory of the northern Andes and of their adjacent sub-Andean subsidence zone. The Colombo-Venezuelan Llanosare a part of the huge subsidence system that, in concordancewith the Andean uplift, runs all along the eastern, inwardborder of the mountain chains, and extends to the craton(Guiana Shield), that together with the Brazilian Shield,

Figure 1 Situation of Apure State within Venezuela, and delimita-tion of its major landscapes. (1) Forested piedmont; (2) Q0 ¯oodplain, mainly under evergreen and semi-deciduous forests; (3) MiddleApure plains, a mosaic of Q0, Q1, Q2, and Q3 land units, mostlycovered by ¯ooded savannas; (4) aeolian plains, on Q2 and Q3 landunits, with seasonal savannas; (5) high plains, on Q4 land units,dominated by seasonal savannas; (6) inner delta ¯ood plains, on Q0,a mosaic of gallery forests, semi-seasonal savannas and swamps. Thestudy area belongs to unit 3. Sites mentioned in text: a. El Nula,b. Elorza, c. Bruzual, d. Mantecal, e. Hato El frõÂo, f. El SamaÂn,g. San Fernando. Adapted from ECOSA (1980).

Ó Blackwell Science Ltd 2001, Journal of Biogeography, 28, 985±996

986 G. Sarmiento and M. Pinillos

make up the core of the South American continent. TheLlanos del Beni, in northeastern Bolivia, between the Andesand the Brazilian Shield, seem to be the mirror image in thesouthern hemisphere of the geological, hydrological andecological conditions of the Orinoco Llanos in northernSouth America (Hanagarth, 1993).

The sub-Andean subsidence zone, the Llanos geosynclinein Venezuela (Feo Codecido, 1972), has been ®lled up withalluvial materials that accumulated in thick continentaldeposits from the Miocene to the Quaternary (Fig. 2). Thesematerials overlie folded and faulted Cretaceous and Eocenerocks of marine origin, that in turn cover the undifferenti-ated Palaeozoic-Precambrian basement. Towards the end ofthe Eocene, one of the strong pulses of the Andean Orogenyuplifted the Sierra Nevada, leading to an active erosionperiod in the plains. Later, the Upper Tertiary and Quater-nary sediments piled up in the basin, forming a mightysequence that, near to the piedmont, attains more than3000 m in thickness (Gonzalez de Juana et al., 1980).

A system of faults, parallel or perpendicular to the Andes,re¯ects the last strong uplift events, one in the Eocene, theother in Mio-Pliocene times, when the ®nal, vertical uplifttook place. The fractures in the basement below theunconsolidated sedimentary materials left evident surfacetraces. The drainage system in particular acquired a patterndirectly imposed by these faults (Fig. 3).

The faulting of the basement gave rise to slightly upliftedhorsts and depressed grabens. In spite of Quaternaryprocesses of ®lling and erosion, which tended to erase thesetectonic imprints, the faults roughly delimit the Llanos major

subregional units (Sarmiento, 1983). The upraised blocksremained as the actual high plains, the Venezuelan mesasand the Colombian altillanuras, well above the youngeralluvial plains. Besides the actual ¯ood plains (Q0a), ®vesuccessive Quaternary depositional units have been recog-nized in the northern Andes and their surrounding plains, Q4

being the oldest (Lower Pleistocene), and Q0 the youngest(Holocene) (Tricart & Millies-Lacroix, 1962; ECOSA,1980). The southern Apure high plain is a Q4 surface(Fig. 1, landscape 5), while both Q2 and Q3 materialsoutcrop in the aeolian plains (landscape 4). The sunk blockscorrespond to the depressed lowlands, or alluvial over¯owplains (FAO, 1964), where ¯ooded savannas and swampspredominate (Fig. 1, landscapes 2, 3 and 6). Landscapes 2 and6, the youngest in the Apure, are Holocene ¯ood plains (Q0),while our study area, the Middle Apure (landscape 3), themost heterogeneous, shows surface materials from Q0 to Q3.

The study area represents one of the most noticeablesubsidence areas in the whole Colombo-Venezuelan Llanos.It is located in the central and northern parts of theVenezuelan Apure state, and consists of alluvial plains builtby the Arauca and the Apure rivers from the MiddlePleistocene to the Holocene (Q3±Q0). Several geologicalformations provided the materials that were later depositedtherein by these two rivers. Cretaceous sandstones arealmost the only rock type outcropping on the eastern ¯anksof the Colombian Cordillera Oriental in the Sierra Nevadadel Cocuy, where the sources of the Arauca river are found(GonzaÂ lez et al., 1988). In Venezuela, the slopes of the SierraNevada de MeÂrida overlooking the Llanos, are the sources ofthe Uribante and Caparo rivers, that later converge formingthe Apure. They are somewhat more heterogeneous from alitho-stratigraphic viewpont, as Cretaceous and Jurassicsandstones, Palaeozoic micaschists and Precambrian migma-tites and gneisses outcrop across the mountain slopes. But asin the Cordillera Oriental, there is a wide predominance ofsandstones and other related coarse-textured sedimentaryrocks (Bellizzia, 1976).

Figure 2 A NW±SE geological pro®le from the Andes, across theVenezuelan Llanos, to the Guiana Shield. The Venezuelan mapshows the position of the transect. The depth of post-Eocenesediments and the main faults are shown. Apure State extendsbetween the Apure and the Orinoco rivers. Its major landscapes areindicated: (1) Middle Apure plains, (2) aeolian plains, (3) highplains. Adapted from Feo Codecido (1972).

Figure 3 The major fault system in Apure State and its in¯uence onthe drainage. The arrows indicate the orientation of the drainagesystem in each block. Adapted from ECOSA (1980).


Patterns and processes in the Apure lowlands 987

The exclusive presence of sandstones in the upper basinexplains the nature of the Arauca sediments. In fact, theymostly are quartzitic silts, originated from the erosion of thesandstones, and the subsequent abrasion of the sand duringtheir long transport from the Andes to the Middle Apureregion. In contrast, the Apure river alluvial materials aregranulometrically and mineralogically more heterogeneous.However, it should be noticed that most of the study areahas been ®lled with the Arauca alluvial materials, and only inthe northernmost part of the region does the Apure river¯ow over its own sediments.

Summarizing, three important ecological consequencesderive from the geological history of these plains. First, thedrainage pattern, that approximately re¯ects the LateTertiary and Pleistocene tectonic events. Secondly, thedifferentiation of large landscape units according to theupward and downward movement of the blocks resultingfrom those tectonic events. Thirdly, the nature of the surfacealluvial materials, composed of poor quartzitic sands andsilts, almost entirely derived from Mesozoic sandstones.

Climatic factors and palaeoclimatic events

Quaternary climatic oscillations seem to be one of the mostrelevant factors in¯uencing nature, speed and extent of thevarious processes involved in the genesis of the landscape.The available palynological evidence clearly points outhow frequently climate and vegetation changed in thetropical South AmeÂrica lowlands throughout the Quaternary(Wijmstra & van der Hammen, 1966; van der Hammen,1974, 1984) During the moist periods, tropical rainforestspredominated, while under somewhat drier and moreseasonal conditions, the savannas rapidly became thedominant regional ecosystems. In the Llanos in particular,a pollen diagram from `Laguna de Agua Sucia' (Fig. 4), inthe southernmost part of the Colombian Llanos, in an areanowadays dominated by savannas and gallery forests, showsthat c. 5000 years ago forests and savanna elements aboutequally shared the local landscape. Later, forest elementsbecame dominant and, at c. 2000±2200 years ago, a savannawoodland replaced the forest, to be thereafter slowlyreplaced by a more open savanna vegetation (van derHammen, 1974).

In concordance with the climatic cycles that characterizethe last 2 Myr, successive morphogenetic phases did occur.In some of them, active erosive processes worked on themountain slopes and the river beds, and the eroded materialswere subsequently transported to and, whenever the ¯ow ofthe streams decreased, accumulated in the ¯ood plains. Inother phases, corresponding to more stable climatic condi-tions, the processes of river dissection and sedimentation inthe plains slowed down. The more energetic phases havebeen called `rhexistatic', and the more calm ones `biostatic'(Zinck, 1981; Hanagarth, 1993). All through the Quater-nary, the genesis of the relief, both in the Andes and in theLlanos, was deeply in¯uenced by the occurrence of thesekinds of climatic and morphogenetic ¯uctuations, withrhexistatic phases roughly coincident with the dry glacial

periods, and biostatic phases coincident with the humidinterglacials.

Superimposed on the complex array of land units and landforms produced by the successive climatic phases and theircorresponding geomorphogenetic processes, the modernclimate, marking the start of the Holocene period, thatbegan to prevail about 14,000 years ago, determined thebroad characteristics of each type of habitat and theconditions of existence of the various ecosystems. So,nowadays, the tropical savannas predominate under atropical wet and dry climate, where a considerable rainfallseasonality is coupled with high mean temperatures and ahigh atmospheric evaporative demand (Nix, 1983).

In the Apure plains, the mean annual precipitation,estimated from a 26-year record at several meteorologicalstations, shows a slight but clear east±west gradient. Thus,rainfall is 1300 mm in San Fernando de Apure, and2760 mm in El Nula, 500 km westwards, showing a sharpincrease directly related to the proximity of the Andeanchains. The interannual rainfall variability also is quiteremarkable. Mean monthly precipitation, their standardvariations and absolute ranges, in two localities in the Apureplains, are shown in Fig. 5. The rainfall regime corresponds

Figure 4 Pollen diagram from `Laguna de Agua Sucia' (ColombianLlanos). The core covers about 6000 years, the last part of theHolocene, and shows the alternation of open savanna, savannawoodland and forest. From van Der Hammen (1974).



to the typical one-peaked `llanos pattern', highly seasonaland with its maximum at the middle of the year. This patternmainly obeys two different climatic phenomena: ®rst, thepresence of local convective systems fed by an increase insolar radiation during the passage of the IntertropicalConvergence Zone (ITCZ) over this latitude. Secondly, amesoscale climatic system determined by the heat differencebetween the north and south hemisphere during the australwinter, that activates the southeast trade winds. When thesewinds, blowing from the high pressure zone on the Tropic ofCapricorn towards the ITCZ, hit the Andean chains, the airmass rises in the troposphere, adiabatically cool, anddischarge their humidity as rainfall. Consequently, theseevents are responsible for the peak in precipitation in June orJuly, during the maximum of the austral winter (Snow,1976).

As in all tropical regions, the temperature regime is verystable throughout the year, with mean values oscillatingaround 26 °C, with the rainy areas tending to show thelowest mean temperatures (ECOSA, 1980). The difference inmonthly mean temperatures between the warmest and

coldest months are never higher than 3 °C. In contrast,daily temperature cycles are remarkable, ranging from 7 to8 °C during the rainy season, to 12 °C and even more,during the dry season. Evaporation also shows importantseasonal differences, positively correlated with the annualprogression of air temperatures (Fig. 5). It reaches thehighest values, in the order of 8 mm day±1, during the drymonths.

In summary, there have been two kinds of climaticin¯uences on the pattern and processes of natural ecosys-tems. One is indirect, as a consequence of climatic ¯uctu-ations over the distribution of ecosystems. The other, moredirect, affects the seasonal behaviour of ecosystems, deeplydependent on rainfall pattern and its variability, on the highevaporative demands, and on the constantly high airtemperatures (Sarmiento et al., 1985).

Hydrological processes and drainage patterns

The study area is located south of the Apure river, and is partof the 161,000 km2 that conform to the watershed of this big

Figure 5 Monthly and annual precipitation and pan-evaporation in two localities of the Middle Apure. Both localities show a clear one-peakedrainfall pattern, with almost negligible precipitation during the 4 month long dry season. Notice how the highest evaporation occur at theend of the dry season. Standard errors and ranges indicate the great interannual variability in rainfall, and to a lesser degree, in evaporation.



Venezuelan river, one of the major tributaries of theOrinoco. Most of the streams in this sector are dif¯uentsand/or abandoned arms of the Arauca river. For instance, theGuaritico river originates as a dif¯uent of the Arauca in thewestern sector of the study area, while the Macanillal creekarises as a dif¯uent of the Caicara river, which in turn is adif¯uent arm of the Arauca. From this point of view, thelandscape genesis relates to the hydrographical dynamics ofthe Arauca river from the Middle Pleistocene (Q2) to theHolocene. Moreover, we consider that the Apure and theArauca watersheds constitute a single hydrographic unit, atleast in this sector, as their alluvial plains originated frommultiple depositional events through the system of channelsthat interconnect both rivers.

In general, all stream¯ows exhibit very wide seasonalvariations because of the seasonal rainfall regime in theplains and in the catchment areas. In fact, both the slopes ofthe Colombian Cordillera Oriental, as well as those of theVenezuelan Sierra Nevada de MeÂrida overlooking theLlanos, show very contrasting rainy and dry seasons. Fromthe point of view of water resources management, such awide annual variation in the discharge of rivers produces ahigh probability of ¯ooding during the rainy season.

The two rivers originating in the Andes (Arauca andApure), sharply differ in terms of discharge, over¯owfrequency, bed shape and sediment charge, from thenumerous creeks that arise in the plains. Finally, one of themost characteristic features in the behaviour of all streams, istheir low competence, both because of the slight regionalslope and the dam effect exerted by each major river overtheir af¯uents (inner delta situation) (ECOSA, 1980; Zinck,1981). Therefore, sediments tend to accumulate on thestream bed, leading to their uprise above the surroundinglevel. As a consequence, the water courses become quiteunstable and often change, by breaking their natural levees.The resulting drainage system is more like a network ofaf¯uents and dif¯uents than like an ordered pattern oftributaries of various orders (Fig. 6).

The origin of the land forms

As we have already discussed, the genesis of the landscapewas linked with deposition and erosion processes clearlycorrelated with the Quaternary climatic oscillations. Thusfor instance, the dunes, typical wind-induced land forms,originated during the driest periods of the Quaternary, andtheir orientation re¯ects the direction of the dry season tradewinds (NE±SW). These aeolian land forms were laterstrongly eroded and frequently fossilized by deposition ofyounger alluvia, every time the regional climate switchedfrom a dry desert climate to a dry-and-wet savanna climate.Four or ®ve such arid cycles may have occurred during thelast glacial periods (Tricart, 1974; Khobzi, 1981).

In the Middle Apure, a ®rst distinction of landscape unitsis based on the different deposition periods, from Q3 to Q0.The corresponding alluvia either still remain on the surfaceor underlie younger depositions. Generally, these sedimen-tary landscapes have been later dissected by the drainage

system or were buried under new ¯uvial deposits. The ®rstcase occurs when the surface is higher than the stream baselevel, as in the Q3 and Q2 surfaces. The second processoccurs in the more depressed areas, where the sedimentarysurface lies below the level of the stream channels. We haveto take into account the long-term morphoclimatic regime(rhexistatic or biostatic), under which the genesis of thelandscape has occurred, as it determines the frequency andthe extension of the river over¯ooding, and of course, of thenew depositions (Coplanarh, 1975; Hanagarth, 1993).

When a river changes its course abandoning its former¯ood plain, the original ¯uvial land forms start to erode andtheir materials redeposited, in a process that may beconsidered as a slow landscape homogenization and level-ling. In this way, the various processes of formation anddismantlement of the alluvial forms occur simultaneously,but in different land units, determining the active dynamicsand the geomorphological complexity of the landscape(Fig. 7).

Finally, we would like to remark that, as a consequence ofthe biostatic character of the climate prevailing during theHolocene, the energy of the rivers has decreased, in such a

Figure 6 Sector of the 1 : 250,000 topographic map of the MiddleApure plains. It can be seen the intrincate drainage networkcharacteristic of this landscape, and the occurrence of many largelagoons. Gallery forest accompanies rivers, permanent creeks andsome seasonal creeks. The Apure river appears at the top. Sheet NB19-2 ELORZA, DireccioÂn de CartografõÂa Nacional, Venezuela.



way that the most recent sediments (Q0a, or actual and Q0b,or early Holocene), just form narrow strips bordering theactual river channels. These river shore complexes (ECOSA,1980), mainly occur on the convex shores of the streams,and show a very irregular and broken relief of juxtaposed,predominantly sandy, alluvial forms. However, there alsoexist other areas of predominantly Holocenic deposition,possibly subsidence zones, where the Q0 land forms are morewidely segregated from each other (Coplanarh, 1975).

Soils and soil forming processes

In the study area, the soil features are clearly linked with theposition along the topographical gradient and with thecharacteristics of the corresponding land forms. From this

point of view, the levees, the highest position in the gradient,have sandy soils, while in the over¯ow mantles of inter-mediate topographical positions, loamy to moderately light-textured soils occur, and in the over¯ow and decantationcuvettes, ®ne to very ®ne textures predominate (Schargel &GonzaÂ lez, 1972). Soil evolution times superimpose thisgranulometric gradient, differentiating the soils accordingto the degree of pro®le development. So, entisols andinceptisols predominate on the Q0 and Q1 units, whileal®sols and vertisols dominate on the Q2 land forms(Table 1). Even so, the early development of al®sols onwell-drained sites of Q1 units is possible, because of thestrong climatic seasonality, the constantly high tempera-tures, and the free percolation, which accelerate clayilluviation and the chemical degradation of primaryminerals. Moreover, buried ultisols often appear near tothe surface (when thin Q2 layers cover Q3 materials), withcharacteristic plinthitic aggregations, deep red coloursand very slow hydraulic conductivities, which notoriouslyin¯uence the water budget of the overlying soil (MalagoÂn &Ochoa, 1980).

The soils in the study area are highly desaturated(Schargel & GonzaÂ lez, 1972; MalagoÂn & Ochoa, 1980;MogolloÂn & Comerma, 1994), probably because of both theintensity of leaching and the poverty in cations of theparental alluvial materials. Their low cation exchangecapacity (Table 1), as a result of the high proportionof 1 : 1 kaolinitic clays, also favours leaching. Even ifdesaturation generally occurs, soils may differ in theirrelative fertility. Thus for instance, the soils developed oversediments deposited by the Andean rivers are richer thanthose deposited by the local streams. The clear trend towardsthe transformation of clay minerals from the 2 : 1 to the1 : 1 type favours nutrient losses, acidi®es the soil, increasesthe amount of changeable aluminium, and promotes theimmobilization of phosphorus. In this way, in terms ofchemical fertility, the oldest soils always are the poorest.

The organic matter content of soils in the study area israther low, generally less than 1.5% (Schargel & GonzaÂ lez,1972; ECOSA, 1980). It seems that the soil carbon contentof savanna ecosystems is positively correlated with theirprimary productivity (Montgomery & Askew, 1983). Forestand swamp soils tend to have the highest organic carboncontent (Table 1), while seasonal savannas generally showthe lowest values. In any case, the low soil organic mattercontent of savanna ecosystems could also be attributed tofrequent ®res and to the high below-ground decompositionrates, which in turn are a consequence of the constantly highsoil temperatures, higher than in forest or in swamp, thatpromote carbon mineralization.

Finally, the formation of argillic horizons, because of theaccelerated clay illuviation caused by the strongly seasonalclimate, characterizes al®sols and ultisols (ECOSA, 1980;MalagoÂn & Ochoa, 1980). The argillic horizons are oftenassociated with ferric concretions, mainly between layers ofsharply contrasted hydraulic properties, which eventuallylead to the differentiation of plinthitic horizons (hardpan)with almost null saturated hydraulic conductivity. The soil

Figure 7 Above: Air photograph from the Middle Apure plains(Hato El FrõÂo). Approximate scale 1 : 50,000. The photographcorresponds to the dry season, during the rainy season almost theentire area is ¯ooded. Below: Delimitation of land units and landforms. The creek in the northwest corner is the `Guaritico' and thewide creek in the southeast is the `Macanillal'. In this area Q0b isabsent. (A) Actual river shore complex under semi-evergreen galleryforest; (B) mosaic of small deltaic arm levees and over¯ow mantles,slightly to moderately ¯ooded, mostly under hyperseasonalsavannas; (C) over¯ow and decantation cuvettes, moderately todeeply ¯ooded, under semi-seasonal savannas and swamps; (D) non-¯ooded high levees, under seasonal savannas, with small patchesof semi-deciduous forest; (E) low levees and over¯ow mantles,slightly to deeply ¯ooded, under hyperseasonal and semi-seasonalsavannas; (F) over¯ow cuvettes, deeply to permanently ¯ooded,under semi-seasonal savannas and swamps.



hydromorphism, initially promoted by slow surface drainageand by the sharp rainfall seasonality, then becomes moreevident, and determines the appearance of perched water-tables near to the surface, during the rainy season (MalagoÂn,1995).

In summary, the morphogenetical dynamics of the Apureplains (deposition±erosion±burying of sedimentary layers),results in land forms where different materials have beensuperimposed along geomorphological time. This phenom-enon has led in many sites to a succession of buried soils,some of them near to the surface, distinguished by theirdifferent mineralogical and hydraulic properties, degree ofdevelopment and texture. As a consequence, some of thehorizons in the soil pro®le have not been formed during thecurrent pedogenesis, but inherited from a former one. Thesepolycyclic and polygenic features make the spatial analysisof the different soil units in the study area and the predictionof their properties from the morphology of the landscape,much more complex.

ECOLOGY OF THE REGIONAL ECOSYSTEMS

In the plains of the Apure region, as a result of the spatio-temporal complexity of their syngenetic processes, a widevariety of ecosystems occur. These range from permanentswamps, where the primary productivity is mainly of aquaticmacrophytes, to tropical savannas, where the grass layer isdominant, to tropical forests, with a closed canopy ofevergreen and semi-deciduous trees. In this section, wediscuss some distinctive features of these ecosystems andtheir relationships with the overall environmental setting.

Rainforests

As a consequence of the sharp seasonality in rainfall,physionomically and phenologically the rainforests of thestudy area mostly correspond to the tropical semi-deciduousforest, although the evergreen type also occurs. According toour ®eld observations, three forest types are recognized, eachlocated over a particular land unit.

The Q0a gallery forestThis ecosystem characteristically appears on the ¯ood plainof rivers and creeks, over the entisols developed on the newlydeposited alluvia of the river shore complex. As a conse-quence of the already mentioned surface roughness of the¯ood plains, the gallery forest either becomes deeply(bottomlands) or shallowly (higher topographic positions)¯ooded by rivers and streams, during the rainy season. This¯ood gradient is more evident in the wider ¯ood plains,where the alluvial land forms are better de®ned than in thenarrower strips. The Q0a gallery forest appears as almostcontinuous and interconnected bands along the drainagesystem. It can be considered as a vaÂ rzea or seasonal swampforest, well adapted to an extended inundation period(Pires & Prance, 1985; Sarmiento, 1992). The degree ofevergreeness of these forests is related to the annual cycle ofwater availability. Thus, the large permanent rivers maintainT

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an evergreen forest gallery, while the smaller, more seasonalstreams, show a semi-evergreen gallery forest, where leaf fallmostly occurs during the dry season.

The semi-deciduous forest on Q0b and Q1 land unitsA tropical semi-deciduous forest borders abandoned riverarms and ancient river courses, where the ¯uvial morpho-genesis is not active anymore. It appears as discontinuousbands, or even as isolated forest patches, covering thehighest topographic positions (levees). It can be consideredas a terra ®rme rainforest (Pires & Prance, 1985), sensitiveto ¯ooding but needing, under this contrasting climate,a permanent water-table within reach of the roots. It seemsthat this ecosystem is suffering a gradual and slow disinteg-ration process, as the land form, where it occurs, ages andthe soil water supply becomes entirely dependent on rainfall.In addition, the former forest inceptisols show signs ofevolution towards al®sols by developing an argillic horizon.We might predict that on sandier and deeper levees, wherethe volume of water available to deep-rooted trees through-out the dry season is larger and the argillic horizonformation is slower, the forest will persist the longest,forming islands in a rather continuous matrix of savannaecosystems.

The forest strips along the damsEven if these forest strips are hardly visible at the scale ofour analysis, as they appear over the raised sides of thedykes and roads built in the area during the last two orthree decades, they may be important in the overallregional vegetation dynamics. According to our ®eldobservations, these forest strips seem to represent earlysuccessional stages of the above mentioned terra ®rmeforest type, being mainly dominated by pioneer shrub andtree species. They contribute to the stabilization of theroadsides, but apparently do not proceed further awayfrom this particular habitat.

In general, we assume that there are two relevantconstraints for the establishment of forests in the plains ofthe Middle Apure region: nutrient availability and watersupply throughout the year. In addition, seasonal ¯oodingmay represent a third severe ecological constraint thatrestricts the terra ®rme type of forest to the highest andbest drained land forms of the Q0 and Q1 land units. Theremaining units, with heavy soils, will be quickly colonizedby ¯ooded savannas and permanent swamps. In the bot-tomlands of the Q0 landscapes, the transition from thevaÂ rzea type forest to seasonal herbaceous swamps and otherwetland ecosystems, may result from seasonal changes in thesoil water content, given that when their clay soils dry out,the water potential sharply decrease, favouring the compet-itive ability of herbaceous plants.

Parallelly, the nutrient constraint may explain the restric-ted localization of the semi-deciduous forest to young,relatively richer soils (entisols and inceptisols), given the fastprocesses of leaching, acidi®cation and base desaturationthat impoverish the soil in a relatively short pedogenetictime. However, for the Q0 and Q1 forests, this transition

from inceptisols to al®sols may be slower, because of theoccurrence of rather impermeable clay layers near tothe surface. In these cases, even if leaching does occur, theeffective soil volume for water storage becomes greatlyrestricted, leading to an oversaturation during the rainyseason and to strong water de®cits during the dry season.Both situations represent severe constraints to this non-¯ooded type of forest ecosystem.

On the other hand, taking into account that the sandier asoil is, the less active the clay illuviation process, we suggestthat the change from entisols towards al®sols will also beslower in the sandier units, even if the leaching losses arefaster under these conditions. We think that, even in extremedystrophic conditions, as long as the soil's effective volumefor water storage and root penetration is large enough, theforest will change from semi-deciduous towards evergreensclerophyllous forms, as is the case with the cerrado and thecerradaÃ o in the Brazilian Pantanal area (Dubs, 1992). Ourhypothesis about spatial and temporal forest dynamics in thearea is summarized in Fig. 8.

Tropical savannas

Seasonal savannas occur on the relatively high topographicalpositions. According to its original de®nition (Sarmiento,1984), the seasonal savanna ecosystem has a 6±9-monthperiod of soil water availability (in the top soil, for grasses,and/or in deeper subsoil layers for trees), followed by a3±6-month dry period where soil water, at least in theuppermost horizons exploited by the grasses, decreasesbelow the permanent wilting point (PWP).

Under the wet-and-dry tropical climate of the VenezuelanLlanos, this soil water regime is only possible on well-drained sites, without an impermeable layer (clay or ironhardpan), and where the water-table is deep or absent. Inour area such conditions do not occur in any land formdeveloped on Q0±Q3 units, except on the relict dunes. All theother high positions have either an oscillating water-tableapproaching the surface during the rainy season, or aperched water-table over a soil hardpan. For this reason, theMiddle Apure Llanos essentially appear as an area ofwetlands consisting of ¯ooded forests, wet savannas andswamps.

The driest type of seasonal savanna, a tree savanna, onlyoccurs in the study area on the old, ®xed dunes. Anothertype of dry, wooded, seasonal savanna, appears on hightopographical positions on Q3, particularly where theerosion, after having swept away the topsoil of the formerultisol, leaves the hardened illuvial horizon exposed, oftenplinthitic and with an incipient ferric induration. Whenfragmented, this material is not an impermeable layer anymore and acts as a new parent material for soil evolution.However, true plinthitic cuirasses only appear on theQ4 surfaces, southwards from the Middle Apure area.

The Q1 and Q2 river levees maintain a somewhat moist typeof seasonal savanna grassland. In fact, this savanna showssome features transitional to an hyperseasonal system. Thistransitional condition is mainly because of its occurrence on



al®sols, where a perched water-table appears over the argillichorizon, at intermittent periods during the rainy season.

The hyperseasonal savanna, with a highly contrastedannual soil water regime where soil water switches in a fewdays from a dry situation to water saturation, characterizessilty materials (over¯ow mantles). There the soil ®lls upquickly at the start of the rainy season, remains near tosaturation during this whole season, being even intermit-tently waterlogged for some weeks, and dries off againshortly after the rains end. In the Middle Apure, thesesavannas extend almost uninterrupted over the widespreadQ2 and Q3 over¯ow mantles. They are pure grasslands, greenduring the rainy season and quite dry after the end of rains.Normally, they remain waterlogged with 5±10 cm of waterfor short periods. A peculiar trait of all these savannas is thebroken microrelief, caused by closely packed earthwormmounds whose upper part emerges from the shallow,discontinuous water sheet.

In the third ecological type of savanna, the semi-seasonalsavannas, the soil never dries below the PWP, but doesremain saturated and ¯ooded for several consecutivemonths. This system appears on shallow soils, easilywater®lled, and in cuvettes where in®ltration is so slow thata free water sheet rapidly accumulates. These ecosystems,popularly known as esteros, are the most productiveherbaceous formations in the region, with the additionaladvantage that their green biomass persists throughout thedry season, when they constitute quite palatable herbages.It is important to notice that this type of savanna only occurson Q0 and Q1 units because in the older ones the moremature drainage system has drained the bottomlands.

As depicted in Fig. 9, these three ecological types ofsavanna ecosystems, set apart by the interplay of threedifferent soil water conditions during the year, the three axesof our diagram, represent the extreme situations. Most of theinner space inside the triangle corresponds to intermediatesituations, that probably exist in the ®eld, but have yet beenneither identi®ed, nor de®ned on the basis of their ¯oristiccomposition and function.

CONCLUSIONS

Within the broad geological framework, de®ned by theAndean uplift and its consequences on the newly formedsub-Andean geosyncline, the gradual construction of theactual landscapes during the last half of the Quaternary

Figure 9 A model of the distribution of savanna ecosystems in theecological space determined by soil water de®cit, water availabilityand waterlogging. Seasonal savannas occur towards the upper leftborder, on the right border is the ®eld of existence of semi-seasonalsavannas, and towards the centre and the bottom the hyperseasonalsavannas occur. The white parts of the triangle correspond to otherkind of ecosystems. The pattern of grey follows the primaryproductivity trend in tropical savanna ecosystems, from the lightestgrey corresponding to the lowest productivity values, to the darkestgrey, corresponding to the highest values.

Figure 8 Idealized transect across thegeochronological land units, showingthe distribution of the major vegetationtypes as in¯uenced by the level of¯ooding and the degree of soilevolution.



forms the basis for understanding the ecology of the ApureLlanos.

The study area stands as a depressed part of the subsidingLlanos, that originated as the surface expression of astructural graben in the deep Pre-Cretaceous basement,produced during the Andean orogeny. During the uplift ofthe neighbouring Andean chains, a system of faults fracturedthe plains, leaving unambiguous traces on the drainagesystem. Thus, the course of the Apure river follows thissystem of faults, while the Arauca river gradually shiftedfrom a fault-induced SW±NE direction, when it ¯owed intoa now abandoned Apure river bed, to its actual, eastwarddirection, directly ¯owing into the Orinoco.

This wide wandering of the principal rivers left its imprintson the landscape, guiding the successive alluvial depositionstowards a pattern concordant with the southward divagationof the river. An old Q3 surface was ®rst dissected, then ®lledwith Q2 alluvia, and later dissected again and partially ®lledwith Q1 materials. Meanwhile, several desert climatic eventstook place, remodelling the ¯uvial landscapes and redistrib-uting the river sands to form extensive ®elds of dunes, andsilts forming a thin, widespread loessic cover.

The sharp rainfall seasonality together with the high airand soil temperatures, led to a rapid soil evolution. In such away, entisols are just restricted to the most recent land units(Q0), an incipient pro®le differentiation characterizes incep-tisols on Q1 materials, more advanced stages in the pedo-genesis (al®sols) prevail on Q2 units, whereas ultisols appearon the Q3 surfaces. In any chronological unit, the accumu-lation of clays in cuvettes promoted the evolution towardsvertisols, while on the dunes, the soil became ®xed at aquartzipsamment stage, i.e. a young, sandy soil almostentirely composed of quartz grains.

The distribution of the major ecosystems closely follows thesoils and geomorphological setting. In each isochronous landunit (Q0±Q3), land forms, soil and vegetation evolved togetheras an integrated functional and dynamic unit: the ecosystem.Seasonal savanna ecosystems characterize the coarse soils onthe highest topographical positions of the Q1 units, providingthe occurrence of a hardpan or of an impervious clay layerdoes not impede free water percolation through the pro®le.Thus, seasonal savannas do not occur when Q1 cuvettes havebeen fossilized by younger, coarser deposits, or on Q2 and Q3

units, as all these soils show an impervious claypan. Theexception are the dunes, developed on Q2 and Q3 units, withfree draining, sandy soils (quartzipsamments).

The hyperseasonal savanna appears on the Q1, coveringnarrow over¯ow mantles. It becomes the most widespreadecosystem on the ill-drained Q2 and Q3 units, where itoccupies almost any landform, except the lowest topograph-ical positions. The semi-seasonal savannas, already appear-ing in over¯ow and decantation cuvettes of Q0b, extend overthe depressed, ill-drained units of Q1. But they are mostlyabsent in Q2 and Q3, where they are replaced by a typeof highly contrasted, heavily waterlogged, hyperseasonalsavanna. This is because, in the relatively higher and oldersurfaces, the drainage system has attained a greater develop-ment, draining all bottomlands.

Finally, permanent swamps and lagoons occupy themostly undrained, everwet, bottomlands, whose frequencyseems to be inversely correlated with the age of the landunits. Along the Q0 ¯ood plains, evergreen to semi-evergreengallery forests appear in any position, except those perma-nently ¯ooded, while semi-deciduous forests appear as arelict, post-climax formation, on the disaggregating Q0b toQ1 levees of already scarcely functional rivers.

In this way, the genesis and dynamics of the various landforms and soils, throughout the Quaternary, collectivelyoutlined the actual landscape, providing the key to under-stand the distribution and the major ecological trends of theregional ecosystems. This is possible, ®rstly, because inyoung sedimentary basins the dynamics of deposition acts asthe overwhelming genetic factor in determining land forms,soils and vegetation. In this sense, if the ecosystem concepthas to keep its integrated holistic meaning, it must includenot just the soils and the biota, but also the land forms andtheir genesis. Otherwise it would be almost impossible tounderstand the genesis and to predict the future trends ofchange in these ecosystems.

A second factor that has allowed the elucidation of thepart played by various processes in determining presentecosystem patterns, is the observation of the effects of recent(only two centuries), slight human in¯uence. This has juststarted to leave their feetprints on the ecosystems. However,the medium and long-term consequences of recent land useintensi®cation based on the building of the dam system stillremains to be fully clari®ed.

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BIOSKETCHES

Guillermo Sarmiento is Professor of Ecology in theUniversity of Los Andes, Venezuela. Dr Sarmiento hasworked on tropical ecology for 35 years. His mainresearch interest is on tropical ecosystems, particularly inaspects of their functioning, dynamics and biogeography.Dr Sarmiento published a book on The Ecology ofNeotropical Savannas (Harvard University Press), andhas just published the electronic version of another bookon The Transformation of Latin American Ecosystems.

Marcela Pinillos obtained her Bachelor degree at theColombian National University (1995) and her MasterDegree on Tropical Ecology (1998), at the University ofLos Andes, Venezuela. Her main interest is in modellingof tropical ecosystems.



Documents

Patterns and processes in a seasonally flooded tropical plain: the Apure Llanos, Venezuela