Amazon Landforms and Soils in Relation to Biological Diversity1

Amazon Landforms and Soils in Relation to Biological Diversity1

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 — Foram 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. '•i ,1 : 1 Ί ι l't'l :·ϊ:ϊ%>.·κϊ-:·ϊχ^' ,·<1 1 li'!' -:;::ÍI::;:Í!Í:;;;M|:I:;:;;:,Í;I:I|Í;!]! 1111 ι ι ι ,|,|--(Vo----- s -------- 8 -------ii 11 C 1 £ I ;: 0 0 í-·'··· D 0 II II II -> I > Π S Ε Is 1Ε 1a Ê 1 2 < Mi1 ã .a s£ fc 5 η li O » ε σι O II PV; ,11.··; ;. ;: lil!.:::-:·: (•jl !> • o 2 -£.2 a b Β 8 II •Ι::;:-· ii ii [^,ΊΊ'ί'Ί'Ί'ίΊ'ϊ'Ί'^ί'ίΊ'ί^Ίãj!jjji!!{!i'i'!.il{'í|^ Ί > ι ι!ι 11 ι:'.ι 11' 1.1 Ms» Çf) fc õ o .y < Η 5 S υ 3 £O O |J Ο Ε Ι! jirío _J II II II II Õ «- o _J et ω < I υ (Λ ϋ) _ι 111 < 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.

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