Location and General Description
The Xingu-Tocantins-Araguaia moist forest ecoregion lies in eastern Amazonian Brazil south of the Amazon River. It spans the interfluve between the Xingu and Lower Tocantins Rivers. It encompasses the middle and lower Araguaia River Basin, which branches to the southwest from the Tocantins. The Xingu and Tocantins Rivers are major tributaries to the mighty Amazon. The region extends southward into the uplands of the Brazilian Shield. The southern reaches are fragmented by savanna woodlands creeping in from the cerrado vegetation zone on the undulating topography of the Central Brazilian Plateau, an upland area formed one billion years ago of hard crystalline rock. The northern half of the region lies on the lowland Amazon Basin. The environment is heterogeneous with an undulating terrain and numerous smaller rivers that dissect the interfluve. These rivers include the Pacajá and Anapu which drain directly into the Amazon River, the Bacajá and Fresco that feed the Xingu, and the Parauapebas and Catete that feed the Tocantins River. These are all stable blackwater rivers. They carry few or no suspended sediments, and their banks sometimes flood, creating white-sand igapó forest. The soils are eutrophic, or nutrient-rich, in many areas, but other areas are characterized by oligotrophic soils having low nutrient availability. Elevation ranges from sea level on the Amazon River to 400 m in the uplands to the south in the Serra dos Carajás. Annual rainfall in middle Xingu is 1,500 to 2,000 mm; the region is drier than the central and western parts of the Amazon.
The topographic variability begets biological heterogeneity, and the eutrophic soils sustain a rich and diverse biota with a high rate of endemism. Some of the biota is restricted to certain mineral formations that underlie the region. The forests are generally evergreen tropical rain forest on terra firme (non-flooded land). They grade from lowland Amazon forest (very tall to 40 m in height) in the north and south through submontane dense forest to submontane open forest in the south. Two interesting elements appear in these forests: liana forests and babaçu palm (Attalea speciosa)/Brazil nut (Bertholletia excelsa) forests.
The liana forests occupy hundreds of thousands of square kilometers on the high relief in the south and southeast portion of the region. These are typically open forests that occur on richer soils (Prance and Brown 1987) with a lower (less than 25 m) and more open canopy as compared to the typical humid terra firme forest. The difference from the typical open forest is that many huge woody vines occur throughout the forest strata. Their presence gives the forest a structural complexity unequaled elsewhere. The most important family containing lianas is Bignoniaceae, followed by Leguminosae, Hippocrateaceae, Menispermaceae, Sapindaceae, and Malpighiaceae (Gentry 1991). Nevertheless, these forests host large trees such as Apuleia molaris, Bagassa guianensis, Caryocar villosum, Hymenaea parvifolia, Tetragastris altissima, Astronium graveolens, A. le-cointei, Apuleia leiocarpa var. molaris, Sapium marmieri, Acacia polyphylla, and Elizabetha sp. Numerous endemic species occur, including Cenostigma tocantinum, Ziziphus itacaiunensis, and Bauhinia bombaciflora. Characteristic trees of the middle Tocantins upland are Cenostigma tocantinum, Bombax tocantimumi, B. macrocalyx, Matisia bicolor, Strychnos melinoniana, and S. solimoesana. Some Swietenia macrophylla occurs in the moist areas. Epiphytes and orchids are lacking in these forests as compared to the western Amazon.
The second type of special forest comprises stands of Brazil nut (Bertholletia excelsa) and babaçu palm (Attalea speciosa) which occur together or alternately dominating different locales in the eastern portion of this region (Pires 1984). The babaçu stands are thought to be anthropogenic in origin (Anderson, 1991).
For Amazonia, this region is not exceptionally rich in mammals, hosting 153 species. Eight primates are found here including the bearded saki (Chiropotes albinasus), red-handed tamarin (Saguinus midas), and tiny titi monkey (Callicebus moloch), which is found in few other places. The gray-necked night monkey (Aotus infulatus) and spider monkey (Ateles marginatus) are endemic to this region and the interfluve to the west. Other large mammals include white-lipped peccaries (Tayassus pecari), collared peccaries (T. tajacu), pumas (Puma concolor), panthers (Panthera onca), tapir (Tapirus terrestris), brocket deer (Mazama spp.), opossums, such as the woolly opossum (Caluromys philander), and Brazilian lesser long-nosed armadillos (Dasypus septemcinctus).
Scientists have logged 90 species of bats and 21 rodents. River wildlife includes spectacled caimans (Caiman crocodilus), black caimans (Melanosuchus niger), yellow-spotted sideneck turtles (Podocnemis unifilis), American manatees (Trichechus inunguis), and river dolphins (Ina geoffroyensis, Sotalia fluviatilis).
At least 527 species of birds have been identified here. Many egrets, herons, hawks, and falcons grace the waterways and meadows. Also gracing the skies are Brigida's woodcreepers (Hylexetastes brigidai), hyacinth macaws (Anodorhynchus hyacinthinus), scarlet macaws (Ara macao), many parrots (Amazona spp., Pionus spp.) and parakeets (Paratinga spp., Pyrrhura spp., Brotogeris spp.).
This is one of the most deforested and degraded regions in Amazonia, second only to the Tocantins-Araguaia-Maranhão moist forest region to the east. Extensive deforestation occurs on the frontier with the drier, more populated zone to the south. Urban development radiates from the Amazonian cities of São Felix do Xingu on the Xingu; Porto de Moz, Oeiras do Pará, and Gurupá on the Amazon; and Marabá on the Tocantins. The Transamazon Highway traverses the middle of the region east to west and another highway flanks the Tocantins River. Most of the forest along these roads has been felled, burned, and replaced by cattle ranches, municipal infrastructure, or agricultural fields. Most of the valuable timber has been removed from the remaining forest. Many insects, birds, mammals, plants, and fish are locally extinct. The protected area system is weak here with only the Tapirapé Biological Reserve that covers 1,030 km2.
Types and Severity of Threats
Extensive deforestation and intense land degradation follow the roads though this region. Commercial logging operations also provide inroads to the interior where cattle ranches and agricultural projects are then established. Anthropogenic fire is a major threat to the environment with regard to both habitat loss and degradation of water and air quality. Large-scale mining operations of the rich stores of mineral deposits near the city of Marabá and elsewhere have disrupted the natural vegetation. To run the smelters alone in Marabá requires the wood from 2,000 km2 of forest each year.
Justification of Ecoregion Delineation
This interfluvial ecoregion is bound by the Amazon Delta to the north, the Xingu River to the west, and the Tocantins River to the east. These large river systems act as formidable barriers to the dispersal of many species and thus have isolated it for surrounding forests. Because of this, many endemic species occur here. The southern boundary is porous, and these moist forests make a gradual transition savanna-forest/ seasonal dry forest mosaic. Linework for this ecoregion follows the IBGE (1993) classifications of "lowland ombrophilous dense forests", "submontane ombrophilous dense forest", "submontane ombrophilous open forest", and all human modifications which fell within this broader classification. Further justifications are offered by de Silva (1998).
Anderson, A.B., P.H. May, and M.J. Balick. 1991. The subsidy from nature: Palm forests, peasantry, and development on an Amazon frontier. New York: Columbia University Press.
Ducke, A., and G.A. Black. 1953. Phytogeographical notes on the Brazilian Amazon. Anais da Academia Brasileira de Ciências 25: 1-46.
Fundação Instituto Brasilero de Geografia Estatástica-IBGE. 1993. Mapa de vegetação do Brasil. Map 1:5,000,000. Rio de Janeiro, Brazil.
Gentry, A.H. 1991. The distribution and evolution of climbing plants. Pages 3-49 in J. Putz and H. Mooney, editors, The biology of vines. Cambridge: Cambridge University Press.
Pires, J.M. 1984. The Amazonian forest. Pages 581-602 in H. Sioli, editor, The Amazon - Limnology and landscape ecology of a mighty tropical river and its basin. Junk, Dordrecht.
Prance, G.T., and K.S. Brown Jr. 1987. The principle vegetation types of the Brazilian Amazon. Pages 30-31 in T.C. Whitmore and G.T. Prance, editors, Biogeography and Quaternary History in Tropical America. Clarendon Press, Oxford.
Silva, J.M. C. 1998. Um método para o estabelecimento de áreas prioritárias para a conservação na Amazônia Legal. Report prepared for WWF-Brazil. 17 pp.
Prepared by: Robin Sears
Reviewed by: In process