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Journal of South American Earth Sciences 46 (2013) 161e169

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Journal of South American Earth Sciences

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Spatial distribution of tropical wetlands in Central Brazil asinfluenced by geological and geomorphological settings

Alfredo Borges De-Campos a,*, Diego Antônio Botelho de Cedro a,Francisco Leonardo Tejerina-Garro b, Maximiliano Bayer a, Gabriel Tenaglia Carneiro a

aUniversidade Federal de Goiás-IESA-LABOGEF, Campus II, 74001-970 Goiânia-GO, Brazilb Pontifícia Universidade Católica de Goiás-Centro de Biologia Aquática, Campus II, 74605010, Av. Engler s/n Jardim Mariliza, Goiânia-GO, Brazil

a r t i c l e i n f o

Article history:Received 8 February 2011Accepted 13 December 2011

Keywords:Tropical wetlandsVeredaCampoVárzeaMata galeriaSeepage

* Corresponding author. Tel.: þ55 62 35211095x20E-mail addresses: alfredo_de_campos@yahoo.com

gmail.com (D.A.B. de Cedro), garro@pucgoias.edmaxibayer@yahoo.com.ar (M. Bayer), gabrieltenaglia@

0895-9811/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.jsames.2011.12.001

a b s t r a c t

Tropical wetlands in Central Brazil are located in savanna areas and are made up of more terrestrial-typewetlands e campo limpo úmido (grassland-type savanna), campo sujo úmido (shrub-type savanna), matagaleria (riverine forest); as well as more aquatic-type wetlands e vereda (valley-side marsh with palmgroves of Mauritia flexuosa), várzea (backswamp), lake, and river. They are regulated by a seasonalclimatic regime characterized by a wet-rainy season from October to March and then followed by a dryseason. Underground water is abundant and rivers frequently overflow during the rainy season. Many ofthese wetlands which are protected by law are significant regulators of water quality. In order to predicttropical wetland function and draw up environmental management policies, it is important to under-stand how abiotic and biotic factors influence tropical wetland origin and spatial distribution. In thislarge-scale study we set out to investigate the influence that geological and geormorphological settings,i.e. geological substrates and geomorphological units, have on the spatial distribution of tropicalwetlands in Central Brazil. Two watersheds, the Caiapó and Piracanjuba, were selected in order to carryout the study. They present different types of rock and unconsolidated surface materials. Planationsurface, escarpment and steep slope, gentle slope, and aggradation are the dominant geomorphologicalunits in the watersheds. Principal component analysis was conducted in order to determine the influenceof the selected abiotic variables on the spatial distribution of tropical wetlands. The study showed thatthe presence of sedimentary and low-grade metamorphic rocks and planation surface strongly influencethe spatial distribution of the more terrestrial-type wetlands. Geological and geomorphological settingsexplain about 60% of the variability in the spatial distribution of these wetland types. No clear effect ofthe abiotic variables selected was observed on the more aquatic-type wetlands. An association betweenstratified layers or planar geological surfaces, groundwater discharge zones, and slope breaks is sug-gested to explain the influence of the geological and geomorphological settings on the wetlands underreview. The study demonstrated the importance of considering abiotic factors, not usually included inclassification schemes, to further understand the spatial distribution of tropical wetlands.

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1. Introduction

The Tropical wetlands of Central Brazil are located in the Cer-rado Biome, a savanna area of about 2 million km2 occupyingalmost a quarter of the country’s total territory. These wetlands areimportant as a source of drinking water. They also provide recrea-tion areas for communities, and habitats for many animals and

8/211.(A.B. De-Campos), deadac@u.br (F.L. Tejerina-Garro),hotmail.com (G.T. Carneiro).

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plants. According to inventories in two watersheds in the State ofGoiás, a core state of Central Brazil, tropical wetlands mayencompass up to 47 percent of the total watershed areas (Fonsecaand De-Campos, 2011), thus they are of great importance interms of management initiatives. In the field, they are primarilyidentified by their vegetation type, hydrological regime, and soiltype. Hydrophytic Cerrado Biome vegetation in association withseasonal or permanent surface or subsurface waters and hydricsoils are the key landscape features which lead to the identificationof tropical wetlands in this part of the country.

Although vegetation, soil types, and hydrological regimes areused for the identification of tropical wetlands and underlie many

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wetland processes, it is known that wetland genesis and functionare also related to other abiotic and biotic factors which should betaken into account in order to understand wetlands, such asgeological and geomorphological settings and animal species(Mitsch and Gosselink, 2000). Geology and geomorphology play animportant role in wetland development. For instance, Tooth et al.(2004) and Grenfell et al. (2009), in local-scale studies, haveobserved that resistant outcrops of dolerite dykes in South Africafunction as a local base level and as a barrier to fluvial channeldevelopment. They therefore, control the nature of river activitiesand associated alluvial deposits both upstream and downstreamand impact wetland evolution. Tooth et al. (2004) have shown thatrivers form meanders associated with floodplain wetlands whendolerite barriers are present whereas less sinuous and deeplyincised channels have been formed and floodplain wetlands havebeen dessicated and abandoned in areas where dolerite dykes areeither absent or bleached. In another local-scale study, Stein et al.(2004) have shown that the presence of colluvialealluvial mate-rials and siltstone beds controls the subsurface water flow regimeand slope wetland formation and dynamics. Iriondo (2004) clas-sifies at large scale the large inland wetlands of South Americaaccording to the grain-size of sedimentary deposits, i.e. sandy ormuddy wetlands, which present different origins and functions.The large-scale study conducted by Cowan and Turner (1988)showed that the thickness and age of sediment layers as well asdrainage density control the evolution and loss of Louisiana’scoastal wetlands. The above-mentioned earlier studies demon-strate that the influence of geological and geomorphologicalsettings occurs in different spatial scales and is influential inwetland genesis and development.

Therefore, geological and geomorphological settings can influ-ence wetland origin and evolution. However, most research con-ducted in tropical wetlands or even wetlands present in otherclimate types does not give due emphasis to these abiotic settingsas factors which are influential in wetland development, asalready noted by Warner (2004) and Tooth et al. (2009). Thesignificance of geological and geomorphological settings has alsobeen underestimated by proposed classification schemes, such asthe Ramsar Classification System for Wetland Type and others(Brinson, 1993; Semeniuk and Semeniuk, 1995). The globally usedhydrogeomorphic classification system (Brinson, 1993), forinstance, only emphasizes the morphological aspect of thegeomorphological setting and occasionally mentions the geologicalsetting in the framework of the classification scheme. Many of thegeological and geomorphological variables that may influencewetland genesis, function, and spatial distribution, such as thechemicalemineralogical composition of geological substrates,structural discontinuities, erosion and sedimentation processes, areoften neglected or at least under-evaluated in most proposedwetland classification systems. Therefore, researchers should payattention to the influence of geological and geomorphologicalvariables on wetland development in order to consider thesevariables correctly in classification schemes.

This study set out to determine the influence of geologicalsubstrates and geomorphological units on the spatial distribution oftropical wetlands in Central Brazil. To conduct the study, tworepresentative watersheds of this region were investigated usingremote sensing, surveying data, and spatial and multivariate statis-tical analysis. Itwas also undertaken in an effort to understand betterhow wetlands function and to help to define classification schemes.

2. Study area

The study was conducted in the Caiapó and Piracanjubawatersheds located in the State of Goiás, Central Brazil, within the

Cerrado Biome (Fig. 1). The Central Brazil region is one of the mostimportant headwater areas in the country from which severaltributaries flow to the Amazon, Paraná, and São Francisco fluvialsystems. There are several small and large inland wetlands in thisregion. Some of them have international significance, e.g. BananalIsland located along the main channel of the Araguaia River,a tributary of the Amazon.

The Caiapó and Piracanjuba watersheds occupy an area of12,889 km2 and 4708 km2, respectively, and the dominant climatefor both watersheds is tropical wetedry with an annual precipita-tion of between 1500 and 1800 mm/yr and a rainy season fromOctober to March. The Piracanjuba and Caiapó watersheds connectto two large South American fluvial systems (Amazon and Paraná)through the Araguaia-Amazon and Paranaíba-Paraná river basins(Fig. 1). Planation surface landforms followed by escarpment andsteep slope landforms dominate the geomorphological settings inboth watersheds. Gentle slope landforms and aggradation areas arealso representative in the Caiapó watershed. In the aggradationzones, the occurrence of lakes associated with fluvial channels,such as oxbow lakes, is common. Another type of lake, small androunded, is found on planation surface landforms (Latrubesse andCarvalho, 2006). Two distinct geological settings representative ofCentral Brazil occur in the watersheds under review. The Caiapówatershed is dominated by sedimentary rocks of the Phanerozoicage belonging to the Paraná Basin, a large sedimentary basinextending from the center to the southern end of the country. Incontrast, metamorphic and igneous rocks of the Pre-Cambrian ageassociated with Central Brazil Proterozic mobile belts predominatein the Piracanjuba watershed. Pre-Cambrian NEeSW strike faultsand folds are found in the tectonically deformed areas dominatedbymetamorphic and igneous rocks. Joints and occasionally NWeSEstrike faults of Mesozoic and Cenozoic ages also occur in the areasdominated by sedimentary rocks (SGM, 2004).

A combination of dry and wetland areas dominates the land-scape in the Caiapó and Piracanjuba watersheds. Drylandsassociated with convex slopes, interfluves, dry-season deciduouslow-stature trees, and well drained soils (e.g. Oxisols) occurbetween river channels. In contrast to the drylands, wetlands occurin concave slopes, floodplains, and lowland areas and are associatedwith hydrophytic Cerrado Biome vegetation, springs, and poorlydrained soils (e.g. Hydric soils) or water bodies (e.g. tropical lakesand rivers). The landscape classification proposed by Ribeiro andWalter (1998) for the Cerrado Biome (Fig. 2) fits the type of land-scape found in both watersheds.

Cerrado vegetation dominated the two watershed areas prior todeforestation. Land cover change, with the conversion of nativeCerrado into cultivated pasture or tillage, has increased since the1970s. Land conversion has affected both dry and wetland areasand there is great demand for the revitalization of these areas.

3. Materials and methods

For both watersheds Landsat 5 TM images were used formapping the wetlands and Spring 5.0.6 software (Câmara et al.,1996) was used to treat the images (georeferencing, segmenta-tion, and classification). Images for the months of May and June2009 were selected as there was no excess of water in the drylandsto mask the wetlands. The identification of the wetland polygonswas visually based on the type of vegetation, as proposed by theRibeiro and Walter (1998) classification scheme (Fig. 2), and ongeomorphological features, such as lakes, fluvial channels, andbackswamp areas. Using a visual approach, seven types of wetlandswere identified in the watersheds under study (Fig. 3): campo limpoúmido (grassland-type savanna characterized by trees covering lessthan 10% of the land with a shallow water table), campo sujo úmido

Fig. 1. Location map of the study area.

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(shrub-type savanna characterized by a mixed flora consisting offorest and grassland vegetation types established in temporarily orpermanently waterlogged soils), mata galeria (riverine forestfringing both streammargins like eyelashes and which temporarilyfloods during the rainy season), vereda (valley-side marsh in whichthe water table reaches or almost reaches the surface during therainy season (Oliveira-Filho and Ratter, 2002). It includes palmgroves of Mauritia flexuosa), várzea (backswamp area periodicallyflooded during the rainy season), lake (tropical lakes found infloodplains, such as oxbow lakes, or in planation surface land-forms), and river (tropical rivers more than 30 m wide). There wasno selection of the size of the wetland polygon to be mapped so allwetland polygons were identified and classified. In the Caiapó andPiracanjuba watersheds a total number of 4546 and 745 wetlandpolygons were mapped, respectively.

Previous mapping by SGM (2004) and Latrubesse and Carvalho(2006) available at scale 1:250,000 provided the geological and

Fig. 2. Typical tropical wetlandsedrylands transect in Central Brazil wi

geomorphological spatial information, i.e. the description, classifi-cation, and spatial distribution of geological substrates andgeomorphological units. The mapped geological substrates weregrouped into five classes (sedimentary, low-grade metamorphic,high-grade metamorphic, igneous, and unconsolidated surfacematerial) while the mapped geomorphological units were groupedinto four classes (planation surface, escarpment and steep slope,gentle slope, and aggradation). The geological and geomorpholog-ical spatial information was also treated by the ArcGis 9.3 softwareto allow for statistical comparisons with the wetland spatialinformation. The geological, geomorphological, and wetland spatialinformationwas converted into numerical datasets and normalizedinto a percentage basis in order to calculate the proportionaldistribution of predictive and dependent variables.

Principal components analysis (PCA) by covariance (StatSoft Inc.,2008) was used to determine if the predictive variables (geologicaland geomorphological classes) accounted for a significant portion of

thin the Cerrado Biome (modified from Ribeiro and Walter, 1998).

Fig. 3. Pictures of the mapped wetlands as they appear in the satellite images. The black arrow and the red contour line indicate the mapped wetland polygon. The types ofwetlands are várzea (A), campo limpo úmido (B), vereda (C), mata galeria (D), campo sujo úmido (E), river (F), lake (G).

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the variability in the dependent variable (tropical wetland spatialdistribution). In the PCA the frequency of overlaps betweenpredictive and dependent variables was used, i.e. the number oftimes that a type of wetland coincided with a type of geological orgeomorphological class. Factors 1 and 2 were considered for thecovariance analysis. The highest scored values of factors 1 and 2were picked and the corresponding geological or geomorphologicalclass was selected. The selected class was visually analyzed againstthe type of wetland in the factors projection plane which was pre-sented in a graphic format. Proximity between the position of the

wetland type and the class in the graphswas chosen as a criterion fordetermining the influence of the geological and geomorphologicalvariables on the spatial distribution of the tropical wetlands.

4. Results

4.1. Geological and geomorphological settings

Five geological classes occur in the two watersheds understudy (Fig. 4). The Caiapó watershed area is dominated by the

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sedimentary class, representedmainly by sandstones and siltstones(Fig. 4; Table 1). Other secondary classes found in this watershedare metamorphic and igneous, represented chiefly by metavolcanicand intrusive acid rocks, such asmetabasalts and granodiorites. Thesedimentary rocks tend to present low dip strata (<15�) and areoften unfolded (SGM, 2004).

In contrast to the Caiapó watershed, low and high-grademetamorphic classes, represented mainly by schists, gneisses, andquartzites, predominate in the Piracanjuba watershed (Fig. 4 andTable 1). Granites occur secondarily and gneisses are tectonicallymore deformed than schists and quartzites. The schists andquartzites usually present a planar, unfolded, schistosity of low dip(<20�) that often coincides with the original sedimentary beddingplanes attitude. In both watersheds, the unconsolidated surfacematerial class is present and recognized in the field by a thick layer(up to 30 m thick) of, for instance, weathered bedrock, soil, laterite,or recent sedimentary deposits, which may overlay the hard rocksand predominate in some areas (SGM, 2004).

Four geomorphological classes occur in the two watersheds(Fig. 4 and Table 1). Planation surface is the by far the mostrepresentative class. Escarpment and steep slope class is alsorepresentative and covers about a fifth of the Caiapó and more thana third of the Piracanjubawatershed areas. Small areas of the gentleslope and aggradation geomorphological classes occur in theCaiapó watershed (Table 1).

4.2. Spatial distribution of tropical wetlands

There is a difference between the spatial distribution of thetropical wetlands in the two watersheds. The Caiapó watershedpresents a greater percentage area occupied by the tropicalwetlands under study than the Piracanjuba (Table 2). Thepercentage areas occupied by the campo limpo úmido, mata galeria,and vereda are always higher for the Caiapó whereas thepercentage area occupied by the campo sujo úmido in the Pira-canjuba watershed is about twice more than in the Caiapó. Thevárzea and lake occur only in the Caiapó watershed andthe percentage area occupied by the river is lowand about the samefor both watersheds.

4.3. Influence of geological and geomorphological settings on thespatial distribution of tropical wetlands

Geological and geomorphological classes influence the spatialdistribution of thewetlands surveyed for bothwatersheds, as foundfrom statistical analysis by PCA (Figs. 5e7; Tables 3e5). However,the effects of these abiotic variables are stronger on the moreterrestrial-type wetlands (campo sujo úmido, campo limpo úmido,and mata galeria) when compared with the more aquatic-typewetlands (vereda, várzea, river, and lake).

PCA of the geological classes alone showed that sedimentaryand high and low-grade metamorphic classes have a strong influ-ence on the spatial distribution of the more terrestrial-typewetlands for both watersheds (Fig. 5 and Table 3). In the Caiapówatershed, the sedimentary class controls the spatial distributionof campo limpo úmido, campo sujo úmido, andmata galeriawhereasthe low-grade metamorphic class has a strong influence on thespatial distribution of campo sujo úmido and mata galeria in thePiracanjuba watershed. Geological classes alone do not have a clearinfluence on the spatial distribution of the more aquatic-typewetlands.

From the PCA of the geomorphological classes alone (Fig. 6 andTable 4), planation surface and escarpment and steep slope are theclasses which influence the tropical wetland spatial distribution.The planation surface class has a strong influence on the spatial

distribution of the more terrestrial-type wetlands in the Caiapówatershed, i.e. mata galeria, campo sujo úmido, and campo limpoúmido, whereas the escarpment and steep slope class has an effecton the spatial distribution of the campo sujo úmido in the Pira-canjuba watershed. As was also seen for the geological classes, thegeomorphological classes have no clear effect on the spatialdistribution of the more aquatic-type wetlands, which tend to begrouped and isolated from the other types of wetlands (Fig. 6).

A similar trend found for the individual analysis of the influenceof either geological or geomorphological classes on wetland spatialdistribution was observed when all abiotic classes were consideredtogether (Fig. 7 and Table 5). Sedimentary, low-grademetamorphic,and planation surface classes showed the strongest effects on themore terrestrial-type wetland spatial distribution. In the Caiapówatershed, both sedimentary and planation surface classes have aneffect on the spatial distribution of the mata galeria, campo sujoúmido, and campo limpo úmido. The contributions of both classes,i.e. sedimentary and planation surface, to the observed covariancevalues are high. The sedimentary class contributes 56.2% of thecovariance values for the Y-axes (Factor 2) whereas the planationsurface class contributes 45.8% of the covariance values for the X-axes (Factor 1). In the Piracanjuba watershed, only the low-grademetamorphic class has an effect on the spatial distribution of thecampo sujo úmido and mata galeria. The low-grade metamorphicclass is responsible for 39.2% of the covariance values obtained forthe X-axes (Factor 1). Altogether, the abiotic variables considered inthis study, i.e. sedimentary, low-grade metamorphic, and planationsurface classes, are responsible for about 60% of the variabilityobserved in the spatial distribution of the more terrestrial-typewetlands. As seen in the ungrouped geological and geomorpho-logical classes analyses, when the abiotic variables are consideredtogether the geological and geomorphological settings have noclear effect on the spatial distribution of the more aquatic-typewetlands.

5. Discussion

At large scales, the spatial distribution of tropical wetlands inCentral Brazil is influenced by geological and geomorphologicalsettings. As shown in this study, the type of geological substrate andgeomorphological unit strongly affects the spatial distribution ofthe more terrestrial-type wetlands. Geological classes contributemore toward explaining the spatial distribution of the tropicalwetlands under study than the geomorphological classes (Fig. 7;Table 5). This is an interesting finding which deserves furtherinvestigation. One possible explanation is that with the large spatialscale approach used in this study, the geological substrates havea stronger influence on the spatial distribution of more terrestrial-type wetlands than the geomorphological units because geologicalsettings often extend over large distances and thus have a morehomogeneous spatial distribution than do landforms, whichmore frequently vary spatially. At local scale the influence of thegeomorphological units will probably be as significant as thegeological substrates.

Of all thewetlandsmapped, the campo sujo úmido is thewetlandtype most influenced by the abiotic variables analyzed here. Thegeological and geomorphological classes in both watersheds forthe ungrouped and grouped PCA affect the spatial distribution ofthe campo sujo úmido (Figs. 5e7; Tables 3e5). This indicatesa strong relationship between the genesis and evolution of this typeof wetland and the selected abiotic variables. It also suggests thata pre-existing environmental condition, which is controlled bygeological and geomorphological settings, is vital for the occur-rence of this type of wetland. Sedimentary and low-grade meta-morphic classes, represented mainly by sandstones, siltstones,

Fig. 4. Maps of the spatial distribution of the geological and geomorphological classes in the study area. Geological classes of the Caiapó (A) and Piracanjuba (B) watersheds;geomorphological classes of the Caiapó (C) and Piracanjuba (D) watersheds (modified from SGM, 2004; Latrubesse and Carvalho, 2006).

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schists, and quartzites, are the major geological classes whichinfluence the spatial distribution of the campo sujo úmido. The typesof rock found in these classes present a planar surface, schistosity orbedding plane which can control underground water seepage. This

happens because these geological structures can serve as ways forsubsurface water to migrate through the geological substrate. Thesmall slope breaks found in the landforms of the planation surfaceclass or the abrupt slope breaks found in landforms of the

Fig. 5. Principal Component Analysis of the influence of the geological classes (A) onthe spatial distribution of the tropical wetlands under review (B). The geologicalclasses are sedimentary (GS1), high-grade metamorphic (GS2), low-grade meta-morphic (GS3), igneous (GS4), unconsolidated surface material (GS5). Letters C and Pafter the wetland type refer to the Caiapó and Piracanjuba watersheds, respectively.

Table 1Percentage distribution of the areas occupied by the geological and geomorpho-logical classes in the Caiapó and Piracanjuba watersheds.

Classes Caiapó(%)

Piracanjuba(%)

Geological Sedimentary 60.6 e

Low-grade metamorphic 18.9 73.4High-grade metamorphic 4.6 13.3Igneous 10.2 7.5Unconsolidatedsurface material

5.3 5.5

Geomorphological Planation surface 63.8 63.9Escarpment and steep slope 22.6 35.7Gentle slope 12.2 <.1Aggradation 1.0 e

Water 0.4 0.3

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escarpment and steep slope class should create the environmentalcondition suitable for the discharge of subsurface water whichleads to seepage and the formation of the more terrestrial-typewetlands, in particular, those less associated with aquatic envi-ronments, such as the campo sujo úmido (Fig. 2). Stein et al. (2004)describe a similar discharge mechanism to explain the genesis ofslope wetlands. In their model, the infiltrated water migrates alongburied siltstone beds covered by colluvial deposits and dischargesat the slope break of the ground surface. This discharge mechanismresults in the formation of the slopewetlands at the footslope of thecolluvial deposits where the water seeps.

As demonstrated here, high-grade metamorphic or igneousclass which often contains no stratified and folded rocks does notinfluence the spatial distribution of the wetlands considered inthis study (Figs. 5e7; Tables 3e5). These findings suggest that theeffect of the geological substrate on tropical wetland spatialdistribution depends on the type of geological substrate.Unfolded, stratified rocks, or unfolded rocks with planar surfacefeatures, are particularly more influential in the development oftropical wetlands than non-stratified types of rocks, which areoften folded. Thus, it is likely that the presence of a geologicalsubstrate with stratified layers or planar surfaces and landformswith slope breaks which facilitate underground water seepage,are the pre-existing geological and geomorphological conditionsfor the occurrence of the campo sujo úmido and other moreterrestrial-types wetlands.

To a lesser extent than the campo sujo úmido, the geologicalsubstrates represented by low-grade metamorphic and sedimen-tary classes also influence the spatial distribution of the matagaleria (Fig. 5; Table 3). Typically the mata galeria is located alongstreams and forms riparian-type wetlands (Fig. 2). As observed inthe field by the authors of this paper, the mata galeria in thewetlands under study is usually connected to both undergroundwater discharge zones and surface water flow areas associated with

Table 2Percentage distribution of the areas occupied by wetlands and drylands in theCaiapó and Piracanjuba watersheds.

Caiapó(%)

Piracanjuba(%)

Wetlands Campo limpo úmido 5.0 1.3Campo sujo úmido 5.5 9.6Mata galeria 9.2 2.4Vereda 3.5 0.3Várzea 0.8 e

Lake <.1 e

River 0.2 0.2

Drylands 75.7 86.2

fluvial systems. A similar inference of the type of geologicalsubstrate found for the campo sujo úmido can be drawn for thespatial distribution of the mata galeria, i.e. rocks with stratifiedlayers or planar surfaces control the migration and discharge ofunderground water thus influencing the genesis and spatialdistribution of thewetlands. However, unlike the campo sujo úmido,themata galeria is also influenced by other factors related to a moreaquatic-type environment, such as seasonal flooding by streamchannels (Fig. 2). For this reason, the type of geological substrate

Table 3Variable contribution based on covariances for the geological classes. The geologicalclasses are sedimentary (GS1), low-grade metamorphic (GS2), high-grade meta-morphic (GS3), igneous (GS4), unconsolidated surface material (GS5).

Variable Factor 1 Factor 2

GS1 0.008740 3.344049GS2 1.907680 0.040769GS3 6.610200 0.225388GS4 1.340750 0.385603GS5 1.907100 0.566233

Fig. 7. Principal Component Analysis of the influence of both geological andgeomorphological classes (A) on the spatial distribution of the tropical wetlands underreview (B). The geological classes are sedimentary (GS1), high-grade metamorphic(GS2), low-grade metamorphic (GS3), igneous (GS4), unconsolidated surface material(GS5). The geomorphological classes are aggradation (GM1), escarpment and steepslope (GM2), gentle slope (GM3), planation surface (GM4). Letters C and P after thewetland type refer to the Caiapó and Piracanjuba watersheds, respectively.

Fig. 6. Principal Component Analysis of the influence of the geomorphological classes(A) on the spatial distribution of the tropical wetlands under review (B). Thegeomorphological classes are aggradation (GM1), escarpment and steep slope (GM2),gentle slope (GM3), planation surface (GM4). Letters C and P after the wetland typerefer to the Caiapó and Piracanjuba watersheds, respectively.

Table 5Variable contribution based on covariances for both geological and geomorpho-logical classes. The geological classes are sedimentary (GS1), high-grade meta-morphic (GS2), low-grade metamorphic (GS3), igneous (GS4), unconsolidatedsurface material (GS5). The geomorphological classes are aggradation (GM1),

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becomes less influential in this type of wetland than in the camposujo úmido. This may explain why the PCA showed a weakerinfluence of the geological classes on this type of wetland whencompared to the campo sujo úmido.

Surprisingly, the spatial distribution of the campo limpo úmido isinfluenced by both sedimentary and planation surface classes butonly in the Caiapó watershed. It was expected that this type ofwetland would behave in similar fashion to the campo sujo úmidobecause they often appear close to each other in the landscape(Fig. 2). However, that did not happen. It seems that the campo

Table 4Variable contribution based on covariances for the geomorphological classes. Thegeomorphological classes are aggradation (GM1), escarpment and steep slope(GM2), gentle slope (GM3), planation surface (GM4).

Variable Factor 1 Factor 2

GM1 0.299195 0.52890GM2 2.123279 1.02295GM3 0.414314 0.60569GM4 7.007977 0.25154

limpo úmido is influenced by the selected abiotic variables when thegeological and geomorphological settings are very favorable forthe development of the more terrestrial-type wetlands, i.e. whenthe rocks are well stratified and the landforms are planned with

escarpment and steep slope (GM2), gentle slope (GM3), planation surface (GM4).

Variable Factor 1 Factor 2

GS1 0.007835 0.561666GS2 0.033169 0.016492GS3 0.392030 0.219020GS4 0.019005 0.000002GS5 0.038869 0.000054GM1 0.000379 0.017333GM2 0.050039 0.009686GM3 0.000812 0.024141GM4 0.457862 0.151606

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small slope breaks. The reasons for the unexpected behavior of thistype of wetland are not clear and warrant further investigation.

One important contribution of this study is to show that atbroad spatial scale the variability in the spatial distribution of themore terrestrial-type tropical wetland can be explained bygeological and geomorphological variables. A similar influence ofgeological and geomorphological variables on wetland develop-ment in a large-scale type study has been reported by Cowan andTurner (1988) for coastal wetlands. They found that thicker andolder sediment layers as well as high channel density lead toincreased marsh loss rates thus controlling wetland evolution. Inagreement with their findings, this study found that older andthicker sedimentary rock layers of the PaleozoiceMesozoic ageshave a stronger influence on tropical wetland spatial distributionthan unconsolidated surface materials of the Cenozoic age (Fig. 5).This highlights the importance of the type and age of the geologicalsubstrate for tropical wetland development.

Therefore, in a large-scale type study, the geological substratestogether with regional geomorphological units control the spatialdistribution of the more terrestrial-type wetlands and these abioticvariables should be further considered when wetlands are beingclassified. Attempts to consider geological and geomorphologicalsettings in the framework of the proposed classification schemeshave been made by researchers (e.g. Semeniuk and Semeniuk,1995). However, a possible lack of understanding of the role ofboth geological and geomorphological variables on wetlanddevelopment has led most classifications to emphasize othervariables, for instance, hydrology and biota, instead of geologicaland geomorphological settings. As observed by other authors(Warner, 2004; Tooth et al., 2009), the influence of geological andgeomorphological variables on wetland origin, structure, andfunction needs to be further evaluated in order to stimulate interestin developing protocols for defining, delimiting, and classifyingwetlands.

An unexpected result from this study is that geologicalsubstrates and geomorphological units apparently do not influencethe spatial distribution of the more aquatic-type wetland, such asvereda, várzea, lake, and river. Other abiotic factors, such as struc-tural alignments, neotectonic movements, surface hydrology, aswell as biotic factors, which were not considered in this study, maydrive the development and spatial distribution of these types ofwetland. An example of the influence of other abiotic variables ontropical wetland morphology, genesis, and evolution in CentralBrazil is given by Latrubesse and Carvalho (2006). These authorsreport that the genesis of several rounded lakes located at thecrossing of structural NWeSE and NEeSW alignments is related tothe presence of these alignments and laterite crusts. These findingssupport the interpretation of this study, namely that the develop-ment and spatial distribution of the more aquatic-type wetlandsare controlled by abiotic and biotic variables which are differentfrom those evaluated in this paper.

6. Conclusions

This study demonstrated that at large scales, geologicalsubstrates and geomorphological units influence the spatial distri-bution of more terrestrial-type tropical wetlands. Unfoldedgeological substrates with stratified layers or planar geologicalsurfaces and geomorphological units dominated by planationsurfaces with small slope breaks which control groundwaterseepage, are the geological and geomorphological settings thatmostinfluence the spatial distribution of these types of wetland. Thefindings of this study highlight the importance of taking geologicaland geomorphological settings into consideration, if tropicalwetland spatial distribution, and, consequently, wetland genesis,evolution, and function are to be better understood.

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