Location and General Description
The Uatuma-Trombetas moist forests ecoregion comprise the vast region north of the Amazon River in the eastern portion of Brazil, east of the Rio Branco-Rio Negro Basin. The region extends east almost to the Atlantic Coast and north to the Serra do Acarai Santa de Tumucumaque Mountains dividing the Guianas and Suriname from Brazil. The region spans the geological and floristic transition from the ancient Guayana Shield to the much more recent sedimentary Amazon Basin. The ancient crystalline uplands of the Guayana Shield consist of a highly weathered and ancient rock basement consisting mostly of quartzite or sandstone rocks formed during different geological times beginning 1.8 billion years ago. The upland terraces and mountains of the Guayana Shield are remnants this ancient basement. The lowland plains in the Amazon River Basin in the southern portion of the region emerged only recently, during the Tertiary (2 million to 5 million years ago), from lacustrine and marine environments. Beside the Rio Negro, Rio Branco and the Amazon Rivers which delimit two sides of this ecoregion, the Uatuma-Trombetas moist forest region is dissected by a number of blackwater or clearwater rivers, the Trombetas and Jari Rivers being the largest. Other rivers include the Uatumã, Curuapanema, Paru, and Araguari.
This region is diverse in climate and topography hosting high plains, lowlands, and undulating hills, which is translated into high biological diversity. Much of this humid rain forest ecoregion has a high canopy (30 to 40 m, with emergents to 50 m), but the region is broken by low canopy (less than 20 m) summer-dry forest with mesophyllous, semidecidous and xeromorphic elements, and some open campos, or meadows ; . The soils are generally nutrient-poor kaolinic soils and sandy podzols on the slopes . There are some areas of eutrophic (very fertile clay loam) soils . The climate is hot and humid with mean monthly temperatures ranging from 26° to 27° C. Average annual rainfall ranges from 1,700 mm east of Óbidos where the seasonal forests occur, to 3,000 mm in patches near the Rio Negro-Rio Branco Basin and is seasonal.
The forests to the west of the Trombetas River are characterized by a dense vegetation with a high number of small-diameter (less than 200 mm) to medium-diameter (200 to 600 mm) stems and a canopy height varying from 20 to 30 m, with emergent trees to 40 m. The most common tree families found in this region are Sapotaceae, Lecythidaceae, Burseraceae, Fabaceae, Rubiaceae, Chrysobalanaceae, Lauraceae, Annonaceae, Moraceae, Mimosaceae, and Caesalpinaceae. In the area north of Manaus, some of the more prominent tree species are Protium hebetatum, Eschweilera coriacea, E. wachenheimii, Manilkara bidentata, Rinorea guianensis, Pouteria engleri, Swartzia reticulata, Duckeodendron cestroides, and Qualea labouriauara . East of the Trombetas River, as in the Jari River Basin in the center of the region, the forests are more homogeneous in structure, but similar in stature and composition. Even the characteristically giant (up to 1,400 mm in diameter) Brazil nut tree Bertholletia excelsa and legume Dinizia excelsa are abundant here, but are not as large as normal . Epiphytes are not a strong component of these forests. An important endemic timber tree restricted to the eastern portion of the ecoregion is the legume acapú, (Vouacapoua americana).
This ecoregion contains the forests north and east of Manaus, which have some of the highest levels of local diversity in the world. They harbor a high number of endemic species of plants, animals and insects . Manaus is thought to be a "biotic crossroads," possibly a region of re-convergence of organisms disrupted during glacial periods , resulting in extremely high local diversity. North of Manuas, the forests contain up to 235 species of trees in a single hectare with only a few species being very abundant.
Some plants characteristic of the dry hills north of Óbidos are the primitive cycad Zamia lecointei, legumes such as Cynometra longifolia, Tachigalia grandiflora, Swartzia duckei, Ormosia cuneata, and Peltogyne paradoxa, as well as other trees such as Cusparia trombetensis, Vochysia mapuerae, Bonnetia dinizii, Lacunaria sampaioi, Lophostoma dinizii, Ctenardisia speciosa, Mostuea brasiliensis, Macairea viscosa, Buchenavia corrugata, Ferdinandusa cordata, Pouteria speciosa, and Lepidocordia punctata.
The mammal richness in this region may be slightly less than in western Amazonia with 175 species reported for the ecoregion as a whole, more than 80 of which are bats. Among the primates present, the endangered and endemic black spider monkeys (Ateles paniscus) and bare-faced tamarins (Saguinus bicolor) are found here as well as the bearded sakis (Chiropotes satanas), howler monkeys (Alouatta seniculus), and red-handed tamarins (Saguinus midas), all of which are limited to eastern Amazonia. Four cats, including jaguars (Panthera onca) and pumas (Puma concolor), anteaters, opossums, and many rodents inhabit these forests as well. Among the 482 birds reported for the Uatuma-Trombetas moist forests ecoregion are species of tinamous (Crypturellus spp.), parrots (Amazona spp.), macaws (Ara spp.), cookoos (Coccyzus spp.), pootoos (Nyctibius spp.), and tanagers (Tachyphonus spp.).
Fully 42 species of frog are reported from around Manaus, with rare or infrequent breeders such as Ceratophrys cornuta and Hyla sp. (Microcephala-group). The number of snake species totals 62, including pit-vipers (Bothrops atrox), bushmasters (Lachaesis muta), boa constrictors (Boa constrictor), and 23 species of lizards with many gekkos and iguanas (Iguana iguana) .
Much of the interior of this region remains intact, but there is considerable deforestation along the major rivers and roads, particularly between Óbidos and Monte Alegre along the Amazon River, around Manaus, and north along the road to Nova Paraiso. Several protected areas and biological research stations have been established near Manaus, and the Rio Trombetas Biological Reserve covers 3,850 km2 of dense tropical forest, riverine forest and lake vegetation along with the Jari Ecological Station further east which covers 2,271 km2. The Balbina Dam in the southwest of this region, along the Rio Uatumã, has drowned thousands of acres of upland forest.
Types and Severity of Threats
Development along the roads and rivers poses a major threat to the native habitat and species. Mining operations destroy upland areas and pollute the rivers. Urban sprawl around Manaus and other cities continues to cut into the natural landscape. Large expanses of forest have been eliminated by cattle ranchers, failed industrial plantation projects (Jari), and large-scale forestry projects in the eastern portion of this region. Wildlife trade, hunting, and selective logging may threaten some species.
Justification of Ecoregion Delineation
This moist forest ecoregion encompasses much of the eastern Brazilian Amazon Basin, north of the Solimões (Amazon River) – just touching Guyana and Suriname at their southern borders. The Brazilian delineation’s for these forests are derived from the IBGE (1993) map by lumping the "lowland ombrophilous dense forests", "submontane ombrophilous dense forests", "ombrophilous forest – savanna transition", and all encompasses human modified habitats. The western delineation follows the Branco River northwards from the confluence with the Solimões, and also includes a small portion east of the Branco. Northern linework follows the divide between the Amazon Basin and Guyanan drainage to the north, thereby following the borders of Guyana and Suriname. The northeastern line extending from the Guyanan Savannas ecoregion eastward follows the recommendation of expert opinion (da Silva, pers. comm. and 1998). Portions in Guyana were referenced with Huber et al (1995) and in Suriname with OAS (1988).
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Prepared by: Robin Sears
Reviewed by: In process