AMAZON LANDFORMS AND SOILS IN RELATION TO BIOLOGICAL DIVERSITY1

Wim Sombroek2

ABSTRACT — Thirteen main landform units are distinguished for the whole of the forested Amazon region, each with its specific soil pattern and vegetation structure. These landform-soil-vegetation units are delineated on a small-scale map and illustrated by a schematic cross-section. Floristic diversity of the gamma type is to be highest on the steepland-and-valley complexes of the Andean fringe, on the crystalline shield uplands, on the inselberg complexes, and on the eutric variant of the western sedimentary plains. Endemism is expected to be highest on the sandy plains, and parts of the table lands and inselberg complexes. Speciation, linked to the concept of forest refuge areas, is likely to be highest on the sandstone table lands, on the stretches of Amazon planalto, and in the areas of relict valleys, in view of the prolonged geomorphological stability of these units.

Key-words: Amazon, major landforms, tropical soils, floristic biodiversity. Paisagens Maiores da Amazônia e seus Solos em Relação com a Biodiversidade RESUMO —Fora m distinguidas treze unidades de paisagens maiores, em toda região Amazônica florestada, cada unidade com padrão de solos e estrutura de cobertura vegetal específica. Estas unidades de paisagem-solos-vegetação foram delineadas num mapa geral da região e ilustradas por um corte transversal esquemático. A diversidade florística, do tipo gamma, deve ser a mais alta no complexo paisagístico da Selva Alta, bordeando a Cordilheira Andina; nas terras firmes colinosas dos escudos cristalinos; no complexo de inselbergs, e no variante eutrico das terras firmes sedimentárias ocidentais. Estima-se que o endemismo seja mais alto nas planícies arenosas, e em partes dos complexos de chapadões e de inselbergs. Especiação, ligada com o conceito de refúgios florestais, é provavelmente mais alta nos chapadões de areniscos antigos, nas extensões do planalto Amazônico, e nas áreas de vales relictos, considerando a estabilidade geomorfológica prolongada destas unidades.

Palavras-chave: Amazônia, unidades de paisagem; solos tropicais; biodiversidade florística

INTRODUCTION flora and fauna with geographically-ori­ ented studies on the Amazon climates, Two international workshops to geomorphology/landforms, soils, hy­ define priority areas for conservation in drology/limnology, and human popula­ Amazonia, on the basis of biogeo- tion patterns. The reasons are threefold: graphic maps of the region, were held -The characteristics and history of in Manaus, Brazil, January 1990, and the physical and human resources may Macapa, Brazil, September 1999 re­ explain some of the observed spatial spectively. They recognised the need to differences in the various aspects of flo­ augment the aggregated information on ral and faunal species diversity;

1 Ά preliminar version of this text appeared in ISRIC Annual Report over 1990, Wageningen, the Netherlands. Updating and digitizing of the coloured map took place in 1998. The digital version can be obtained separately from the author in Holland or from GTZ/PPG7/IPAAM, Manaus.

2 Consultant GTZ/PPG7/IPAAM; Manaus, Am. Brazil; e-mail: [email protected] Permanent adress: ISRIC, P. O. Box 353, 6700 AJ Wageningen, Holland. Fax: + 31 317 471700; e-mail: [email protected] -The geography of these re­ mation, a schematic synthesis of the sources may help in establishing landforms is presented, with postula- boundaries of biogeographic units in tions on their origin and degree of sta­ areas where the number of observa­ bility during the Cenozoic era. It tions on flora and fauna is scarce or draws, in part, on the comprehensive skewed; study on the land systems of the cen­ -The ecological fragility of the tral lowlands of tropical South physical resources themselves may America by Cochrane et al. (1985; constitute a reason on its own to in­ with map at 1:5M). It should be men­ clude certain areas in the map on pri­ tioned that for the Brazilian part a ority areas for conservation. geomorphologic map at scale The present text deals with the 1:2.500.000 has been published re­ aspects of landforms and soils. It is cently by SAI/IBGE (1998), present­ accompanied by a coloured sketch ing a more detailed scheme (43 tipos map on landforms (Fig. 1), a sche­ de modelados fisiográficos) matic cross section of the region With each main landform a non­ showing the relationships between technical description of the predomi­ landforms, vegetation and soils (Fig. nant soils is given. Only the FAO/ 2a), as well as a drawing of the main Unesco terminology for soil classifi­ soil profiles involved (Fig. 2b). cation (FAO, 1974; 1988) is used. For Knowledge on landforms, soil the naming in other systems such as conditions and their biota of the Ama­ the US "Soil Taxonomy" and the Bra­ zon region has long remained rudi­ zilian scheme, see the review of mentary (Sombroek, 1966; Ab'Saber, Sombroek (1984). The latter text gives 1967) because of difficult access of also more details on the physico- this largest area of tropical forest in chemical characteristics and proper­ the world. In recent years, however, ties of the soils, with an assessment of this has changed considerably. This their agricultural potential and refer­ not only because of the construction ences to the pertinent soil literature. of a network of roads, but especially The landform and soil descrip­ through the availability of airborne ra­ tions are accompanied by an indica­ dar images as produced by govern­ tion of the structure of the present-day ment entities of Brazil, Venezuela, Co­ vegetative cover. For soil-vegetation lombia, and Peru. For instance, the relationships see also Brown (1987) results of multidisciplinary surveys on for the Brazilian part, and Huber the basis of radar photography are (1982) for the Venezuelan part of the contained in a series of volumes, with region. 1:1.000.000 scale maps, of Radambrasil (1972-1981) and for Co­ Floristic species occurrence in lombia on 1:500.000 scale maps by relation to soil and landform condi­ Proradam (1979). tions is discussed in the second part On the basis of such new infor­ of the paper.

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Χ d Ο Ε Ü Ο Κ (/)(/)0. ο < Figure 2b. Schematic profiles of the main soils of the cross section o(Λf figur2 e 2a 2. LANDFORMS, SOILS AND The inselberg complexes have a VEGETATIVE COVER catenary sequence in soil conditions. Bare rock outcrops and very shallow 2.1. Mountainous Lands soils (Lithosols) of the inselbergs 2.1.1. Flat to gently undulating themselves and other mountainous high-level (> 500 m alt.) table lands, parts alternate with deep, often sandy mainly formed by weathering-resistant soils (albic and ferralic Arenosols) on arkosic sandstones (Proterozoic the colluvia at the feet of the Roraima formation and others), are inselbergs, and yellowish to reddish concentrated in the water divide re­ loamy soils on the bands of well- gions of the main tributaries of the drained uplands and alluvia in-be­ Amazon. Examples of these SAND­ tween (xanthic Ferralsols and STONE TABLE LANDS (symbol Τ of Acrisols). The inselbergs have a the maplet) are the "tepuis" of Venezu­ shrubby vegetation, the footslopes a ela, the "chapadões" of Brazil and the mosaic of forest and savannah, while "mesas" of the Colombian part. It is the uplands and alluvia support high assumed that these table lands are forest. geomorphologically stable since Cre­ 2.1.3. Steeplands, interhill val­ taceous times, if not earlier, although leys and colluvial fans are found in the the process of upheaval may have Andean fringe, on a variety of geo­ lasted until the Pleistocene (Kubitzki, logic materials: the SELVA ALTA (S) 1989). areas of Colombia, Ecuador, Peru and The soils of these sandstone table Bolivia, with geomorphologic dissec­ lands are predominantly shallow, tion still active. sandy and podzolised (albic Arenosols; These steepland-and-valley com­ Podzols; Lithosols). They support at plexes have an extremely strong varia­ present a savanna or low-forest veg­ tion in soil conditions, as a conse­ etation ("campina rupestre"). The quence of the prevalent short-distance scarps have often stony soils, with variation in lithology, slope position more vegetative cover than the flat and meso-climatic conditions (the area tops. around Tarapoto in Peru has even a 2.1.2. Steeplands occur on subhumid climate, with a long dry sea­ weathering-resistant parts of the crys­ son). A schematic subdivision is as talline shields. These are the follows: Rock outcrops or shallow and INSELBERG COMPLEXES (I) of the stony soils (Lithosols; shallow phases frontier zone Guyana-Brazil-Venezu­ of Cambisols) are found on steep up­ ela-Colombia, the mountainous lands per slopes; they often carry low forest of the Brazilian shield, and associated only. Well-drained, deep, reddish, colluvia and alluvia. Reshaping of the loamy to clayey and in part stony soils land has likely taken place all through with variable ion exchange capacity, the Pleistocene. base saturation, and mineral reserve (Ferralsols, Acrisols, Luvisols, textural differentiation with relatively Nitisols) are found on the middle compact subsoils, a reserve of slopes, and they support high forest. weatherable minerals and varying Bands of Pleistocene terrace lands clay-mineral activity (ferric or orthic may have a strong internal differentia­ Acrisols); these soils often sustain an tion in texture and acidity ("duplex" or open-canopy forest with a dense under­ claypan soils: Planosols), on which a growth. Other soils are more homoge­ shrubby vegetation prevails. Low up­ neous in their vertical build-up, with land and bottomland parts can have little or no weatherable mineral re­ base-rich cracking clay soils (Vertisols) serve and an inactive clay mineralogy with a scrub vegetation, while floodplain (orthic and rhodic Ferralsols); these stretches have base-rich silty soils carry predominantly closed-canopy (Fluvisols) with high forest. high forest. Areas with a substratum of dior- 2.2. Uplands itic crystalline rocks, basalts, or ferro- 2.2.1. Rolling to hilly dissected magnesian rich Paleozoic-Mesozoic lands and rounded hills with convex sedimentary rocks have dusky red, slopes are found on the crystalline deep and clayey soils of good struc­ basement rocks where weathering re- ture, with a high percentage of active sistance is smaller than in the clay-sized iron oxides (Nitisols). These inselberg areas, as well as on adjacent so-called "Terra Roxa Estruturada" outcropping Paleozoic-Mesozoic sedi­ soils of the Brazilian soil classification mentary rocks. They occupy the main system occur along parts of the part of the Guyana and Brazilian Transamazon highway; in the middle shield areas and can be denominated Xingu area, and in Rondônia (unit Uce as CRYSTALLINE SHIELD UP­ of figure 1; Camargo, 1981). They sup­ LANDS (Uc). Dissection has presum­ port a luxuriant forest cover, but are ably been active as recent as in the also much sought after for permanent Late Pleistocene. In places the terrain settlement (cocoa growing!) because may be only gently undulating, consti­ of their favourable physical and tuting remnants of early planation sur­ chemical properties. faces ("Sul-americana" and "Velhas" levels of King, 1957). 2.2.2. Flat, intermediate-level plateaux (100-300 m alt.) occur in the The shield uplands have often a eastern part of the sedimentary basin, short-distance variation in soil condi­ with a cover of kaolinitic sedimentary tions. Areas with steep slopes or resis­ clay. This is the AMAZON tant rocks have rather shallow soils, PLANALTO (A) with the so-called with a substantial content of weatherable minerals (Cambisols). Belterra clay, probably of lacustrine or Most of the soils are deep to rather inland-sea origin. These plateau lands, deep, well-drained, reddish, loamy to often with steep scarps, prevail be­ clayey and acid. Part of them have a tween the lower reaches of the Tapajós and Xingu rivers (see maplet in and Lips, 1995), while for the Peru­ Klammer, 1984), as well as in the up­ vian part of the sedimentary basin the per reaches of the Gurupi catchment "restos pequenos de acumulación (Sombroek, 1966, p. 18-26 and appen­ planos" on resistant layers of Tertiary dices 1, 4 and 5). The plateau rem­ deposits, as mentioned by Khobsi et nants have not been dissected in the al. (1980), would be concerned. The Pleistocene epoch. soils of these western Amazon The Amazon planalto has a mo­ Belterra clay plateau-rests are less notonous cover of very deep, very heavy in texture (50% clay size par­ clayey, acid, yellowish soils of very ticles instead of the 80-95% of east­ low physico-chemical activity and no ern Amazonia) while their subsoil mineral reserve (xanthic to geric compactness may have induced the Ferralsols). Most of them are porous formation of claypan soils (dystric Pl- and friable to great depth and then anosol of unit Sp2 of Duivenvoorden support high, closed-canopy forest. &Lips, 1995). Central parts of planalto stretches, far 2.2.3. Gently undulating, rela­ away from the scarps, can have com­ tively high lying land with concave pact subsoils; in these cases the veg­ slopes can be found on either crystal­ etative cover is a liana-rich line basement or Cretaceous-Tertiary open-canopy forest, or even pure liana deposits. These can be denominated forest or "cipoal" (Sombroek, 1966, RELICT VALLEYS (V) and prevail p. 194-196). The latter vegetation type in the southern fringe of the region predominates on the left bank of the (headwaters of the Xingu and the lower Xingu and may be the enduring Tapajós rivers in northern Mato result of past Amer-indian occupation Grosso, 400-500 m. alt.). They can (Balée & Campbell, 1989). also be observed locally in the central Contrary to early suppositions basin, for instance in the upper (Sombroek, 1966), the Amazon reaches of the Curuá-una river south­ planalto stretches with Belterra clay east of Santarém (Klammer, 1984, p. cover turn out to be of very limited 64-65), the upper reaches of the occurrence in western Amazonia. Only Capim river south of Belém, and in isolated patches have been identified, some sections of the sedimentary area for instance along the road from Rio between Manaus and Itacoatiara. In Branco to Sena Madureira (Acre, Bra­ the latter cases it apparently often zil) and north-west of Boca do Acre in concerns an Early Pleistocene remod­ Amazonas state (map unit LVd4 of elling of Belterra clay by erosion and Radambrasil, Volume 12). In the Co­ short-distance re-deposition, without lombian part some Amazon planalto- any dissection in later parts of the like plateau rests were recently Pleistocene. mapped for the middle Caquetá area The areas of the relict valleys (map units Spl/Sp2 of Duivenvoorden with their gentle concave slopes have predominantly very deep, sandy loam Carneiro Filho, 1991), it is believed to clayey and sometimes concretion­ that this divergent soil development is ary, acid, yellowish to reddish soils of due to a brackish-water lagoonal depo- low to very low physico-chemical ac­ sitional character of the parent mate­ tivity (xanthic or orthic Ferralsols and rials (cf. Sombroek et al, 1970, for ferralic Arenosols). They support high similar features in the Lagoa Merim forest of large timber volume unless area of southern Brazil). Dry climatic climatic conditions are marginal, such conditions during parts of the Pleis­ as in the uppermost reaches of the tocene, as evidenced by the occurrence Xingu and Tapajós catchments. of fossil sand dunes (Carneiro Filho & The savannah areas around Boa Zinck, 1994), may have contributed Vista (Roraima State of Brazil) may further to strong textural differentia­ constitute a variant (Vr) of these relict tion of most soils. valleys. In the past the area was ap­ 2.2.4. An alternation of flat and parently in direct hydrographic con­ undulating land, the latter often with tact with the Atlantic Ocean through convex slopes, is found in the Pleis­ the present-day Essequibo river valley tocene glacio-eustatic terrace levels in of Guyana, which forms part of the the eastern part of the Amazon basin. Takutu basin, an intra-continental NE- These stepped depositional plains, or SW running rift valley structure dat­ EASTERN SEDIMENTARY UP­ ing from the Jurassic (McConnell et LANDS (Uf), occur on fluviatile sedi­ ah, 1969). During the Sicilian or ments derived from crystalline shield Milazzian glacio-eustatic high sea material that was pre-weathered dur­ level of the Early Pleistocene (Zeuner, ing the Cretaceous and Tertiary. Dis­ 1959) the Boa Vista area may have section has taken place throughout the been a small inland sea or bay of this Pleistocene due to pluvial-interpluvial rift valley, during which time the sedi­ climatic alternations and base-level ments of the Boa Vista Formation oscillations, as demonstrated by were deposited. Klammer (1984). The Boa Vista relict valleys have The eastern sedimentary uplands predominantly a savannah cover be­ have well-drained, very deep, acid, cause the soils have a strong textural yellowish soils with textures that vary differentiation (sandy topsoils over laterally from loamy sand to light clay compact, clayey subsoils) and/or oc­ with low percentages of silt, but that currence of massive ironstone layers are vertically homogeneous. The soils (Arenosols, Planosols, Plinthosols, have no weatherable mineral reserve ironstone Lithosols). Also because of and are physico-chemically very inac­ locally high Mg/Ca ratios in the tive, especially on the flat parts of the deeper subsoil and the presence of upper terraces (Sombroek, 1966, p. "whale-wallow" or "olho-de-água" 167-171); but they are porous and fri­ type of terrain depressions (cf. able throughout (xanthic or geric Ferralsols; some ferralic Arenosols). mottling (pseudo-plinthite, i.e. not They support a closed-canopy high hardening upon exposure). These fer­ forest of large to fair timber volume. ric or haplic Acrisols and Alisols In part, the soils contain ironstone con­ (FAO, 1988) have a forest cover of cretionary layers, but this has normally predominantly rather open canopy no detrimental effect on the forest structure. structure. Very sandy patches on the A recent multi-disciplinary sur­ higher uplands may have a white-sand vey of the Peruvian Amazon Region soil profile, but then without any wa­ near Iquitos has shown that the soils ter-stagnating subsoil layer. Such of the Tierra Firme area are domi- patches of albic Arenosols or "Giant nantly strongly leached and very acid, Podzols" also support closed-canopy hence poor in plant nutrients forest, be it of low timber volume; at (Ferralsols, Acrisols, ferralic disturbance by present or past occupa­ Cambisols, some Podzols and tion there is, however, no forest re- Arenosols). A minor part of the Tierra growth. Firme soils has a substantial reserve of 2.2.5. Undulating to rolling land, plant nutrients, based on the presence often with convex slopes occupies the of non-weathered Tertiary clay depos­ western part of the basin, on sediments its of the Pebas formation in the shal­ derived from the Andean Cordillera by low subsoil. This situation seems to fluviatile and sometimes volcano-eo- be restricted to the denudated land lian deposition. Also these WESTERN along incising rivers, valleys and de­ SEDIMENTARY UPLANDS (Ua) pressions (Kauffman etal, 1998). Flo­ have undergone dissection throughout ristic observations show correlations the Pleistocene. with these soil groups, especially with their texture and the content of ex­ The soil conditions contrast dis­ changeable bases (Ruokolainen & tinctly with those of the eastern sedi­ Tuomista, 1998). mentary uplands described above, be­ cause most sediments were less pre- Soil mapping in the Colombian weathered at the time of their deposi­ part (Duivenvoorden & Lips, 1995) tion. The soils are also predominantly has revealed that two mega-level soil deep, well drained, yellowish red and associations are concerned: (i) the acid, but they have a different clay higher-activity Ali-Acrisol association mineral assemblage that results in being physiographically linked to the higher ion-exchange capacities. The lower terraces and the Pebas clays of over-all textures vary, but the silt con­ brackish-water depositional nature, tent is relatively high, and there is and (ii) the low-activity Acri-Ferralsol some reserve of weatherable minerals. association, linked to the higher ter­ Many of these soils show a substan­ races of pre-weathered fluviatile sandy tial textural differentiation and the to clayey sediments derived from the deeper subsoils may show reddish Guyana shield and the Proterozoic arkosic sandstone formation. ies, causing a selective removal of the In the south-western part, viz. clayey components of Early Pleis­ Acre state of Brazil and the adjoining tocene or Tertiary-Cretaceous sedi­ part of Peru (tentative unit Uae on the ments that may have covered the ar­ map), soils with high base status oc­ eas originally. Sandy plains also occur cur, due to the presence of richer sedi­ in a band between the crystalline up­ mentary deposits and in places also lands and the coastal floodplains of volcanic ash admixtures (Mõller & the Guyanas, notably near the mouth Kitagawa, 1982). The soils are either of the Essequibo river. deep (ferric Lixisols) or shallow The sandy plains of the Rio Ne­ (eutric Cambisols). The forest struc­ gro and middle Rio Branco areas and ture is determined by the occurrence of the Guyanas have year-round im­ of bamboo (Guadua superba), either perfectly drained soils. They are char­ scattered or in dense stands (Cardoso acterized by a subsurface horizon that da Silva et al, 1990); the dense stands consists of light grey to white sand or may be the result of erstwhile Amer­ loamy sand of single-grain structure. indian burning practises. This is underlain by a subsoil, at Some of the soils of the north­ strongly varying depth, that is homo­ western sedimentary uplands, notably geneously or banded dark brown to in the Ecuadorian part (also unit Uae), black, with a texture that is only little show little or no textural differentia­ heavier than the overlying layer but tion. They have either a high base satu­ with a firm consistence or even ce­ ration (eutric Cambisols, eutric mentation, causing low water perme­ Nitisols) or are acid (ferralic ability and poor root penetrability. Cambisols); in both cases high and This hardpan, or "Ortstein", may be closed-canopy forest prevails. continuous or show a very irregular lateral pattern to the extent that in 2.3 Plains places it may be completely absent. The soil is very acid throughout and 2.3.1. Flat to gently undulating, has no reserve of weatherable miner­ relatively low-lying lands with a sandy als at all (gleyic Podzols, or albic surface over basement rocks are preva­ Arenosols if the white layer is thicker lent in the Rio Negro and middle Rio than 200 cm). The natural vegetation Branco areas. It is speculated that on such soils is either a savannah with these denudational SANDY PLAINS common bare sand patches (Pa) were formed in the Late Pleisto­ ("campina" or "bana") or a closed- cene, after capture of some of the canopy low forest often with a former tributaries of the Orinoco and scleromorphic physiognomy ("caatinga Essequibo rivers by the Amazon river amazônica"). Clearing of this vegetation system. This capture would have re­ would result in permanent exposure of sulted in a sudden huge increase in the white sandy soil. The sandy plains river discharge of the Amazon tributar­ of the middle Rio Branco have traces of modern soil science, become irrevers­ past eolian deformation, visible on ibly hardened to slag-like material Landsat TM images of the area. upon exposure to the open air for sev­ 2.3.2. Flat to gently undulating, eral seasons and are then known as relatively low-lying lands with fine petroplinthite, laterite concretions, or sandy to silty sediments are prevalent "ironstone". The base saturation in the in the lower Juruá-Purus-Madeira river soil is low throughout, and the areas south-west of Manaus. These are physico-chemical activity of the clay apparently plains with Late Pleis­ minerals is low to very low. These tocene unstable sediments, recently soils are comparable with the "ground­ denominated "Içá" formation by the water laterites" of early pedological Brazilian Geologic Service (CPRM, literature (Marbut, 1932), but nowa­ 1998). Their unstable character has days denominated Plinthosols when resulted in exceptionally wide bands the plinthite is shallow and strongly (often 50 km) of low terrace land and developed, or plinthic Acrisols when floodplains along the rivers crossing the plinthic material is deeper or the area. The surfaces of these weaker. Their vegetation consists of LOAMY PLAINS (Pp) still show forest of relatively low biomass (tim­ relict features of meandering rivers, as ber volume of 100-150 m3 per ha), demonstrated by Klammer (1984) and often with a predominance of palms Irion (1976 et seq.) The deposits may on the gently sloping parts, and of a have originated from the postulated Late shrubby or open grass savannah Pleistocene or Early Holocene ("campo") on the flat interfluves. disengorgement of an antecedent of Clearing of the latter vegetation would Lake Titicaca in the Altiplano area of result in irreversible hardening of Bolivia and Peru, which would have some of the plinthitic subsoil, which resulted in a huge temporary interior may be aggravated upon erosion of the lake (Campbell et al, 1985; Frailey et unstable topsoil. ai, 1988). Late Pleistocene plinthitic plains The loamy plains have season­ are also found in the Beni area of north­ ally imperfectly drained soils of low eastern Bolivia, in the lower Gurupi- structural stability. A silty-loam topsoil Maracassumé area of northern overlies a silty-clay subsoil that has a Maranhão, on the west bank of the compact consistence with prominent, lower Tocantins river and the erstwhile coarse and abundant red mottles in a continuation there-of on the south-east­ light grey matrix. The centres of these ern fringe of Marajó island (Camargo, mottles are usually hardened, and at 1981). Most of them are less silty than the transition zone between topsoil those of the Içá sediments. and subsoil a thin layer of discrete 2.4. Lowlands iron-manganese concretions may oc­ cur. The mottles, called "plinthite" in 2.4.1. Flat lands and inland wa- ters alongside the major rivers consti­ 2.4.2. In the western part of the tute Holocene floodplain generations. Amazon region extensive BOTTOM­ Details of these FLOODPLAIN LAND COMPLEXES (Fb) occur. The COMPLEXES (Fa) are given by largest area is the Pastaza-Maranon Sutmõller et al. (1966) and Klammer basin of Peru-Ecuador, which is in fact (1984, p. 68). a huge Late Pleistocene-Holocene ag­ The floodplains along the major grading alluvial fan of approximately rivers have a strong short-distance 60.000 km2 (Ràsãnen, 1991). The soils variation in soil conditions. A broad of these bottomlands are predomi­ grouping as "alluvial soils" can be nantly poorly drained and in part peaty made because the sedimentary strati­ (Histosols; humic and eutric Gleysols) fication still overrides any pedogenetic and the vegetation is forest with a high profile development. However, the frequency of palms. soils vary much in texture, internal 2.4.3. Along the Atlantic coast drainage conditions, organic matter and on Marajó island in the mouth of content, acidity, and clay mineralogy the river system COASTAL LOW­ - depending on the local flooding con­ LANDS (Fc) occur. Their soils are, or ditions, the source of the sediments have been, subject to the influence of and the time elapsed since their depo­ brackish or salt water (Solonchak, sition or re-deposition (Fluvisols, thionic Fluvisols, Gleysols). The natu­ Gleysols, gleyic Acrisols, Vertisols, ral vegetation is mangrove forest or Histosols). grassland, the latter on the central and In general, the floodplains with northern parts of Marajó island. sediments from the Andean Cordillera ("várzeas" or "barriales" of "agua 3. SPECIES DIVERSITY, branca" rivers) are base-rich and have ENDEMISM AND REFUGIA a clay-mineralogy of high physico- 3.1. Definitions chemical activity. They support a luxu­ riant forest cover, often with many The foregoing descriptions and palm species, or rich natural grass­ tentative dating of major landforms- lands where the hydrological regime soiIs-vegetation structures, as spatial impedes tree growth. combinations of habitats, allow for a The rivers originating in the discussion of the geographical aspects crystalline shield areas or in the sedi­ of the Amazon biological diversity in mentary basin itself carry little or no general, and the species diversity in sediments and may contain high per­ particular, with some notes on species centages of humic acids ("várzeas" endemism and speciation. and "igapos" of "agua azul" or "agua BIOLOGICAL DIVERSITY, de­ preta" rivers). The resulting soils are fined as the variety and variability predominantly acid, with clay miner­ among living organisms and the eco­ als of lower activity, and the forest logical complexes in which they occur, cover is less luxuriant. has three levels: ecosystem diversity, species diversity and genetic diversity diversities; in other words, the rich­ (McNeeley et al, 1990). The ECO­ ness in species of a range of habitats SYSTEM DIVERSITY, reflected in the in the geographic unit, as the conse­ variety of habitats, biotic communities quence of the alpha diversity of the and ecological processes, has been im­ habitats together with the extent of the plicitly described in the preceding chap­ beta diversity between them); and EP- ter. The term SPECIES DIVERSITY SILON diversity (the total diversity of refers to the variety of living organisms, a group of areas, as applying to large while the GENETIC DIVERSITY re­ bio-geographic regions or "biomes"). fers to the sum total of the genetic in­ When taking the foregoing de­ formation contained in the genes of in­ scription of the major landform-soil dividual plants, animals, and micro­ units, i.e. landscapes or "land systems" organisms. within the Amazon region as frame­ Species diversity (or heteroge­ work, it should be evident that species neity) combines species richness with diversity in the following discussion is species evenness (Magurran, 1988). essentially of the gamma type. Species richness is the number of dif­ Related to beta diversity is the ferent species in an area expressed per concept of ENDEMISM: a species size unit of land (for instance per km2); occurs only in a certain area and no­ species evenness is the degree of equal where else, thus it has an ecological abundance of the different species niche specificity; it is native, restricted within this unit of land. Thus, the more or peculiar to a locality or region species and the more equally abundant (McNeely et al, 1990). they are, the greater the species diver­ Genetic diversity includes both sity. INTRA-SPECIFIC VARIATION (i.e. The measurement of species di­ a large variation in observable charac­ versity is habitat-related (habitat be­ ter among individuals belonging to the ing defined as the type of site where same species, or germ-plasm varia­ a plant or animal naturally and nor­ tion) and SPECIATION (i.e. the for­ mally lives and grows). Whittaker mation of a new species). According (1977) distinguishes ALPHA-diver- to Krebs (1984) such speciation can be sity (the within-homogeneous-habitat "allopatric" (having disjunct or mutu­ diversity); BETA-diversity (the be- ally exclusive areas of geographic dis­ tween-habitat diversity, or the degree tribution), "parapatric" (occupation of of difference between two contrasting a new habitat which is adjoining the habitats in their species composition original area of geographic distribu­ as measured along an ecological gra­ tion), or "sympatric" (having the same dient); GAMMA-diversity (the diver­ or overlapping areas of geographic sity of a large geographic unit such as distribution). Allopatric genetic diver­ an island or a major landscape, i.e. the sity in particular is supposed to be over-all diversity of a group of alpha stimulated by a long-time presence of a biome in the same geographic area, assemblage and trace elements occur­ with only minor changes in the envi­ rence. This would allow many flo­ ronment (stability of climatic and ristic species to find their required landform-soil conditions). It is there­ habitat for growth, reproduction and fore linked to the concept of forest seed dispersal. REFUGIA: small and disjunct areas From the descriptions of major where forest coverage would have landform-soil units in section 2, and been present throughout the Late Ter­ the schematic cross section of figure tiary and the Pleistocene (Whitmore & 2a it may be obvious that such condi­ Prance, 1987). tions can be found most pronouncedly The following short discussions in the steepland-and-valley complexes on species diversity, endemism and of the Selva Alta area (unit S of the speciation refer only to the plants, and maplet); on the crystalline shield up­ trees in particular, but it may be taken lands with their convex slopes (Uc); that faunal and micro-organism com­ on the inselberg complexes (I), and on munities are closely related. the eutric variant (Ue) of the western sedimentary plains. To a lesser degree 3.2. Species diversity it applies to the dystrophic eastern and Species diversity (of the gamma western sedimentary uplands (Uf and type as defined above) is bound to be Ua, respectively) because of the rela­ highest in areas with a short-distance tively monotonous nature of the par­ strong variation in habitat, in other ent materials. However, high words where landscapes or land sys­ biodiversity is to expected where tems are composed of land facets of these sedimentary uplands are crossed strongly different topography, soil, hy- by sand-filled former riverbeds, such drological and micro-climatic condi­ as in the Manicoré-Novo Aripuanã tions. These situations can be found in area east of the lower Madeira river. areas where geo-morphogenetic pro­ The same may hold for those parts of cesses were active in several episodes the loamy plains that have been re­ of the Pleistocene, as the result of pe­ modelled into low terraces, with intri­ riodic changes in pluviosity, linked cate patterns of black and white-wa­ with glaciations in the higher latitudes ter lakes as a result .An example is the and altitudes and concurrent lowering left side of the lower Purus river, in base level of the hydrographic which has an abundancy of primate catchments. Especially where such species (Roosmalen, pers.comm). A geomorphologic activity resulted in similar case is formed by the flood- exposure of fresh rocks of short-dis­ plain complexes (F) of the "agua tance contrasting lithology and miner­ branca" rivers originating in the alogy, the new pedogenesis resulted in Andean Cordillera: the fluvial dynam­ soils of different depth, drainage con­ ics during the Late Pleistocene and the dition, nutrient content, clay mineral Holocene has caused rapid temporal and spatial vegetation successions due and bottomlands; their influence on to changing sedimentological and hy- species endemism is discussed in de­ drological habitats, as demonstrated tail by Junk (1984) and others. by Saio & Ràsánen (1988). Amongst extreme soil conditions 3.3. Species endemism the white-sand soils of the sandy plains (Pa) and part of the table lands Endemism, or the restriction in (T) and inselberg complexes (I) stand occurrence of certain species to spe­ out. The specific floristic species com­ cific areas within the Amazon tropi­ position of the "campina" and cal forest biome, is likely to be linked "caatinga amazônica" vegetative cover to extreme climatic, hydrological or is rather well documented. It has a low soil conditions where such species degree of species diversity but a high have a relative advantage in the com­ number of endemic species (Ducke & petition with other species. Black, 1953; Murça Pires, 1978; Some tree species find their Anderson, 1981; Jordan, 1989). niche in conditions of year-long ex­ The floristic composition of the cessive rainfall such as the upper Co­ various shallow soils of the table lands lombian part of the Amazon region; and inselberg complexes is less well others apparently thrive in areas with known, with the exception of those of a strong dry season, such as large parts the Venezuelan "tepuis" (Huber, 1982 of the southern and southeastern et seq.). fringe. This applies for instance to the The author is not aware of any prized timber species Cedro {Cedrela endemism-oriented study of the spe­ odoratd) and Mahogany (Swietenia cies composition of the loamy plains macrophilla), and the nut-producing (Pp) with their imperfectly drained Castanha-do-Pará (Bertholletia Plinthosols or plinthic Acrisols. In the excelsa). Within this zoning a species parts with savanna vegetation, such as may still require a specific edaphic the "campos" of Humaitá-Lábrea, habitat, as shown by a special study sclerophyllous and fire-resistant tree on Mahogany occurrence carried out and shrub species are found that are by a FAO/SPVEA forest inventory also common in the "cerrado" areas of team in 1961 in the Lower Araguaia central Brazil (Ducke and Black, area. The Mahogany species was 1954); they would qualify as endemic found to be concentrated on well-de­ only within the context of the Amazon veloped hydromorphic soils with a forest region. high physico-chemical activity ("hy­ The forests in the areas with dromorphic grey podzolic soils", or Nitisols on the shield uplands (minor gleyic Luvisols; Sombroek, 1966, p. parts of the Uc unit, only partially de­ 203-208 and maplets). lineated on fig. 1 as Uce) reportedly Extreme hydrological conditions have an aberrant tree species compo­ are found in the various floodplains sition, viz. more light-timber species (Ducke & Black, 1954). The nutrient- 3.4. Speciation and refugia rich soils of the western sedimentary As discussed in 3.1, areas with uplands with a surmised or established large intra-specific variation and the influence of volcanic ash (sub-unit evolution of new species, as forms of Uae) may also have several endemic genetic diversity, are supposed to oc­ species. The rubber tree (Hevea cur where past climatic, geomorpho- brasiliensis) is especially frequent in logic and soil conditions were not the unit, unless the vegetative cover much different from those of today. It has become a bamboo forest. is rather commonly held that large Even in the areas with a seem­ parts of the present-day Amazon re­ ingly monotonous cover of well- gion were devoid of a forest coverage drained acid yellowish soils (xanthic during several parts of the Late Ter­ Ferralsols) of the eastern sedimentary tiary and the Pleistocene, due to the uplands (Uf) and the Amazon planalto occurrence of times of aridity, or (A) there are phyto-geographic differ­ interpluvials, which would have coin­ ences. This applies for instance cided with glacials in the higher lati­ (Sombroek, 1966, p. 198-203) to the tudes and altitudes. Only in isolated occurrence of the valuable tree species parts forests would have been able to Angelim-pedra {Hymenolobium survive as refuge areas or refugia, and excelsum). It is found predominantly in such areas genetic diversity of the on non-compacted soils of the Belterra allopatric type would be highest. clay, either in its original position on Maplets about those Amazon areas the Amazon planalto or in remodelled where humid tropical forest probably form in the associated relict valley persisted during the last glacial parts of lower level. Another sought- (Winconsin/Würm) are given by after timber species, the Pau-amarelo Brown & Ab'Saber (1979) and Brown {Euxilophora paraensis) is found only (1987). on soils with a substratum of plinthitic The validity of the location of or concretionary material of the these refugia is much debated these "Ipixuna" type, which immediately days, in part because of the geographi­ overlies bauxitic material as occur­ cally unbalanced amount of ground ring in the Paragominas area south of truth about the floristic and faunal spe­ Belém; in both cases a specific micro- cies (Whitmore & Prance, 1987; nutrient content of the substratum is Colinvaux, 1987). However, from a surmised to be the determining factor. geomorphologic point-of-view, sites of high genetic diversity are likely to be Detailed comparison of the data found on the sandstone table lands (T), on individual soil and forest inventory on the Amazon planalto stretches (A) samples of Radambrasil (1972-1981) and in the areas of relict valleys (V). would doubtless result in more of such These are all stable landscapes, were relationships. soils development continued uninter- ruptedly since the Early Pleistocene, pense for a seasonal shortage of rainfall. resulting in strongly weathered and of­ Detailed multidisciplinary studies ten very deep soils. The rather extreme are needed to establish geographic rela­ soil conditions of these geomorphic tionship between speciation, past cli­ units, especially the table lands and the matic conditions, stability of landforms Amazon planalto (sandiness, subsoil and uninterrupted soil development dur­ compactness, very low clay mineral ing successive geologic periods, before activity, strong acidity, lack of more definite conclusions can be drawn weatherable minerals) would be a det­ on the validity of the refugia concept rimental factor for the survival of a and their precise location. number of forest species in-loco, es­ pecially during times of droughty con­ ACKNOWLEDGMENTS ditions in the region as a whole. It is The author gratefully acknowl­ hereby postulated that, at the advance edges the comments and suggestions of such times, the forest species gradu­ of Prof. Salle Kroonenberg of ally moved to the edges and scarps of Wageningen University on geomor- the table lands and the Planalto phological aspects, and those of Dr. stretches. The occurrence of less ex­ Hanna Tuomisto of Turku University treme soil conditions, the presence of in Finland on species diversity aspects. seepage water, and the likelihood of more orographic rainfall at the wind­ Literature cited ward sides of these scarps would al­ Ab'Saber, A.N. 1967. Problemas low for survival of the species. It geomorfológicos da Amazonia Brasileira. would creep back to the flat tops at Atas do Simpósio sobre biota amazônica. the return of pluvial conditions in the Rio de Janeiro, Vol. I: 35-67. region-at-large. A case in point for Anderson, Α. 1981. White-sand vegetation of Brazilian Amazonia. Biotropica such a parapatric speciation may be 13(3):199-210. the relatively high and steep scarps Balée, W.; Campbell, D.G. 1989. Evidence for (100-200 m) of the planalto stretches the successional status of liana forest of the upper Gurupi catchment, on the (Xingu river basin, Amazonian Brazil). boundary of Pará and Maranhão states Biotropica (in press). See also: New York Botanical Garden's periodical "Garden" of Brazil, which is one of the sug­ 11(6) of 1987. gested refugia. Brown, K.; Ab'Saber, A. 1979. Ice-age forest refuges and evolution in the Neotropics. In the case of the relict valleys (V) Instituto de Geografia, Universidade de at the southern fringe of the phyto-geo- São Paulo. graphic Amazon region, the forest veg­ Brown, Κ. 1987. Soils and Vegetation. In: T.C. etation would have temporarily with­ Whitmore; G.T. Prance. Biogeography and drawn to narrow fringes of gallery for­ Quaternary History in Tropical America. 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Aceito para publicação em 23/02/2000