Pará is the largest state in the Brazilian Amazon, covering 1.25 million km2 or an area nearly two times the size of Texas. Its geography is shaped by the Amazon River, which bisects the state from west to east, with major tributaries draining the slightly upraised Guianan and Brazilian Shields to the north and south respectively. Urban centers like Belém, Santarem, and Marabá were founded at river junctures to reap the profits from river commerce. More recent urban development has coincided with overland highway construction in the 1960s and 1970s with its associated influx of settlers and business interests. The southeastern city of Redenção, the supply base for this study, was founded in the early 1970s by ranchers, goldminers, and loggers specializing in mahogany extraction.
The study region straddles the southeastern limits of closed Amazon forest between 6.5°–8° S and 49.5°–52° W. Considering the general continuity of landform, climate, and forest within southeastern Pará, the study region encompasses an area extending east to the forest-cerrado edge approaching the Araguaia River, north to the base of the Carajás mountain massif, west to the Xingu River, and south to the furthest extent of mahogany’s distribution into Mato Grosso. This region once contained mahogany stands of exceptionally high density on its eastern edge.
The study region is underlain by the Brazilian Shield, a Precambriancrystalline basement that began drifting apart from the once-contiguous Guiana Shield to the north roughly 600 million years ago, opening the intercratonic depression which now comprises the alluvial Amazon Basin. Shield bedrock is composed of metamorphic and igneous materials—gneisses, schists, andesites, granites, basalts—which, in their more weather-resistant forms, crop out extensively as isolate or aggregate inselbergs (‘island mountains’) at intervals tens of kilometers wide. According to Klammer (1984), the most widespread relief type on the crystalline Shields is a “high-level erosion plain with inselbergs of all sizes and a basin and swell topography in which the basins are mostly leveled out by sediment from the ranges” (p. 55). This description aptly describes the study region, where scattered inselbergs rise 100–250 m above the essentially flat surrounding plain which is itself 200–300 m above sea level. Shield bedrock commonly protrudes on high ground where soils thin markedly, either as massive concretions or in extensive boulderfields.
The Brazilian Shield is tectonically stable. The last orogenic cycle to affect it occurred > 600 million years ago. Two major ranges of outcropping materials which flank the study region, the northern Carajás and southern Gradaús Ranges, are granite-greenstone belts of volcanic origin intruded by Archean granitoids dated at nearly three billion years. Denudation processes—weathering from rainfall at high year-round temperatures decomposing bedrock into soils, with loss of solid materials and nutrient cations to drainage—have shaped topographic relief since the last orogenesis.
This terrain’s crystalline nature translates into structural stability insofar as stream and river channels are effectively permanent within narrow limits on lateral migration. This contrasts with the more fluid nature of landscape dynamics in the western alluvial Amazon Basin. The Brazilian Shield’s great antiquity means that, since bedrock-derived soils are highly weathered, with primary materials receded far below the biotic (surface) zone except where bedrock stands exposed, nutrient inputs must be largely atmospheric through precipitation.
Larger aseasonal rivers typically flow through wide, slightly raised, flat valley plains. Within the eastern portion of the study region these flow west-to-east towards the Araguaia River, which traces Pará’s eastern border with the state of Tocantins; within the western portion of the study region, flow is north and west towards the Xingu River. There is no diurnal flooding comparable to varzea systems along the Amazon River. In general, streams averaging five or more meters wide maintain some level of year-round flow. Progressively smaller (lower-order) streams cease flowing earlier during the dry season until, reaching first-order streams at the tops of watersheds (called cabeceiras, the headwaters), these run dry days after the wet season’s last rains in May or early June. The pattern of subsurface drainage on the seasonal landscape can be observed from the air where corridors of evergreen vegetation penetrate cerrado grasslands during the dry season.
Soils of this region are richly diverse. Relatively recent sedimentary formations contact Shield bedrock as well as slowly weathering extrusions to create a mosaic of red-yellow latosols, eutrophic podzols, and yellow podzols (oxisols, ultisols) interspersed with patches of bedrock-derived nutrient-rich red latosols (alfisols) moving west and north towards the Xingu.
At local scales, soil color and texture varies predictably with topographic relief. Reddish-brown to red-yellow sandy clay soils tend to be found on slightly higher ground relative to pale brown, gray, or white sandy soils associated with seasonal streambeds and drainage systems. Drainage on low ground may be excessively rapid or impeded, depending on texture and depth to the water table. Lateritic and iron concretions are common in red sandy clay soils on slopes. Lateritic gravel mixes into soil horizons at unpredictable depths in soil profiles, forming the ‘stone lines’ that Ab'Saber (1982) attributed to soil exposure under reduced vegetative cover during arid periods coinciding with glacial eras.
Forests and cerrado (grasslands) interdigitate along an irregular northeast-to-southwest zone across the eastern end of the study region, in complex patterns shaped by soil water and nutrient status and dry season fires. This transition zone is extremely fluid, and may be easily observed—closed forest abutting open grassland—or obscured by intermediate zones of cerradão, a shrubby, closed-canopy, woody community whose composition, while showing some forest affinities, is dominated by fire-tolerant cerrado species. Forests extend east and south into cerrado as galleries along larger water courses, and as isolated closed canopy islands associated with drainage. Cerrado and cerradão formations likewise occur as isolated patches within closed forest, gradually diminishing in size and frequency moving north and west towards the Xingu River. Mahogany does not occur in cerradão or cerrado formations, nor to any great extent in gallery forests flanking seasonal streams that drain grassland landscapes.
Forests here are evergreen with a deciduous component, with low (12–24 m height), highly irregular canopies punctuated by emergent trees like mahogany rising to 35 m or more. These forests are essentially two-storied, with frequent patches of nearly impenetrable vine forest. Moving west towards São Felix on the Xingu River, taller forest overstories are punctuated by emergent trees exceeding 50 m tall. In between, high plateaus harbor extensive cerrado formations, as can be seen on the eastern portion of the Kayapó Indigenous Area. Palms are important components of all forest stories, with babaçu (Attalea speciosa) and inajá (Attalea maripa) occurring at high densities across extensive areas.
Occupation and transformation of the south Paraense landscape by ranchers, gold miners, colonists, loggers, and agribusiness began in the late 1960s concurrent with construction of the Belém-Brasília highway (BR-010) east of the Araguaia River. Within the study region, little forest remains 25–50 km either side of any significant road. Undisturbed forest is rare east of the Kayapó Indigenous Area. The east-west corridor along state highway PA-279 from Xinguara to São Felix has been essentially cleared of forest. A new landscape is being created by these economic and social forces, one in which forest cover at scales larger than a few thousand hectares is rare. The scale and intensity of land clearing declines with distance from roads and market centers.
Ab'Saber AN (1982) The palaeoclimate and palaeoecology of Brazilian Amazonia. In: Prance GT (Ed.), Biological Diversification in the Tropics, pp. 41-59. Columbia University Press, New York, NY, USA.
Bigarella JJ & Ferreira AMM (1985) Amazonian geology and the Pleistocene and the Cenozoic environments and paleoclimates. In: Prance GT & Lovejoy TE (Eds.), Key Environments: Amazonia, pp. 49-71. Pergamon, Oxford, UK.
Clapperton C (1993) Quaternary Geology and Geomorphology of South America. Elsevier Science Publishers BV, Amsterdam, The Netherlands.
Cole MM (1960) Cerrado, caatinga and pantanal: distribution and origin of savanna vegetation of Brazil. Geography Journal 126: 168-179.
Coutinho LM (1982) Ecological effects of fire in Brazilian cerrado. In: Huntley BJ & Walker BH (Eds.), Ecology of Tropical Savannas, pp. 273-291. Springer-Verlag, Berlin, West Germany.
Kalliola R, Salo J, Puhakka M, Rajasilta M, Häme T, Neller RJ, Räsänen ME & Danjoy Arias WA (1992) Upper Amazon channel migration. Naturwissenschaften 79: 75-79.
Klammer G (1984) The relief of the extra-Andean Amazon Basin. In: Sioli H (Ed.), The Amazon: Limnology and Landscape Ecology of a Mighty Tropical River, pp. 47-83. Dr. WJ Junk Publishers, Boston, MA, USA.
Klink CA, Moreira AG & Solbrig OT (1993) Ecological impact of agricultural development in the Brazilian cerrados. In: Young MD & Solbrig OT (Eds.), The World's Savannas: Economic Driving Forces, Ecological Constraints, and Policy Options for Sustainable Land Use, pp. 259-282. Man and the Biosphere Series Volume 12: UNESCO Parthenon Publishing, New York, NY, USA.
Macambira MJB & Lancelot JR (1996) Time constraints for the formation of the archean Rio Maria crust, southeastern Amazonian Craton, Brazil. International Geology Review 38: 1134-1142.
Putzer H (1984) The geological evolution of the Amazon Basin and its mineral resources. In: Sioli H (Ed.), The Amazon: Limnology and Landscape Ecology of a Mighty Tropical River, pp. 15-46. Dr. WJ Junk Publishers, Boston, MA, USA..
Ratter JA (1992) Transitions between cerrado and forest vegetation in Brazil. In: Furley PA, Proctor J & Ratter JA (Eds.), Nature and Dynamics of Forest-Savanna Boundaries, pp. 417-429. Chapman & Hill, London, UK.
Salo J, Kalliola R, Häkkinen I, Mäkinen Y, Niemelä P, Puhakka M & Coley PD (1986) River dynamics and the diversity of Amazon lowland forest. Nature 322: 254-258.
Schmink M & Wood C (1992) Contested Frontiers in Amazonia. Columbia University Press, New York, NY, USA.
Sombroek WG & Sampaio JB (1962) Reconnaissance soil survey of the Araguaia mahogany area. FAO, Commisão de Solos, Belém, Pará, Brasil, 61 pp.