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TRANSVERSE DRY BELT
The Transverse Dry Belt is an approximate climatically defined region mostly south of the Guianas, between central and far eastern Amazonia (Nimer 1977; Pires and Prance 1977). This general region extends from north-west of Boa Vista on the Upper Branco River in Brazil's Roraima Territory south-eastward beyond the Lower Xingú River in Pará State, crossing southern Guyana and south-western Surinam (Map 40). Although with an annual precipitation as high as 1750-2100 mm, the region has a distinct Köppen Aw climate or in the Thornthwaite system primarily B1rA´a´climate edged by the greater potential evapotranspiration of B2rA´a´ (SUDAM 1984), which is characterized by a seasonal (winter) drought (extending to as much as August-December) during which the rate of respiration exceeds photosynthesis. Drought coupled with deep sandy soils results in various types of savanna vegetation that are widely distributed in the region (Hoogmoed 1979), and characterize the Jari-Trombetas vegetation subprovince or phytogeographic region (Ducke and Black 1953, 1954; Rizzini 1963; Prance 1977, 1985; Whitmore and Prance 1987; IBGE 1990).
Except for flooded strips alongside rivers that were formed from Quaternary sediments, most of the region is Tertiary terrain. A remnant strip of Palaeozoic sand and siltstone occurs from the Jari River to the Rio Negro (Map 40), northward and parallel to the Amazon River over crystalline rocks of the ancient Uatum group (DNPM 1981).
The topography of the Transverse Dry Belt is mainly lowlands, which can be classified into three basic landforms: wide alluvial terrain, low Tertiary plains and tabletop hills. The highest areas (400-600 m) correspond to the Maracanaquará Plateau (west of the middle Jari River), where the more ancient sediments occur.
The major vegetation type in the mosaic of the Transverse Dry Belt is semi-open forest, although dense mesophytic forests are found in some areas of higher elevation and as well on some ravine sides and valley bottoms. The Transverse Dry Belt additionally includes many open dry forests, large savannas and a number of small savanna enclaves. The most frequent vegetation types (Prance 1977, 1985; IBGE and IBDF 1988) are given below.
1. Lowland terra-firme (i.e. locally upland) forests
The locally upland forests are called "terra-firme" forests to distinguish them from the forests in flood areas. Also referred to as lowland tropical moist forests (when compared with premontane and montane forests), they are the most common forest type in Amazonia. However their physiognomy in the Transverse Dry Belt differs from other regions by being mostly semi-deciduous. Such forests are sometimes becoming recognized as transition forests (Whitmore and Prance 1987; Prance 1989).
The amount of basal area rather than deciduousness better differentiates these types of terra-firme forest (J.M. Pires 1994, pers. comm.). Considering trees with dbh 10 cm or more, their basal area per ha in dense forest is c. 40 m² to over 25 m², in semi-open forest 30-20 m² and in open forest less than 20 m². Amazonian "caatinga" is a special type of open forest occurring mainly on white sands in the Upper Rio Negro region (CPD Site SA6), with trees of small basal area.
The dense forest includes Theobroma subincanum and several species of Manilkara and Protium. In the predominant semi-open forests, the following species are characteristic: Poupartia amazonica (Anacardiaceae); Pourouma minor (Cecropiaceae); Goupia glabra (Celastraceae); Bertholletia excelsa, Gustavia augusta (Lecythidaceae); Cedrela odorata (Meliaceae); Astrocaryum mumbaca, Bactris sphaerocarpa, Geonoma baculifera, Maximiliana maripa, Oenocarpus distichus, Orbignya (Attalea) spectabilis, Socratea exorrhiza (Palmae); and Cedrelinga cateniformis, Dinizia excelsa, Sclerolobium goeldianum (Leguminosae). The open forest includes Couma guianensis and Hancornia speciosa.
2. Inundation forests
As in the rest of Amazonia, the Transverse Dry Belt has its share of inundation forests. These can be separated into two main types based on the influence they receive from nearby water bodies: "várzea" forests occur with nutrient-rich white-water rivers and "igapó" forests with nutrient-poor black-water and nutrient-intermediate clear-water rivers. Each type can be subdivided into swamp forests that are permanent or seasonal.
Several notable species in swamp forests are Mauritia flexuosa and Virola surinamensis; in várzea forest also occur Euterpe oleracea and Hevea spp., whereas igapó forest also has Leopoldinia spp.
The lacustrine-riverine history of the Amazon Basin suggests that igapó forests are much older than várzea forests, with the igapó forests existing since the Late Cretaceous, whilst the várzea forests developed only after the Andean orogeny caused the Atlantic drainage of the Amazonas graben to form the Amazon River (Kubitski 1989; Clapperton 1993).
3. Savannas or "campos" sensu stricto
The savannas found in the Transverse Dry Belt region have an array of physiognomies (Ducke and Black 1953, 1954), but in essence were formed by similar processes involving imperfect soil drainage, impermeable subsoil layers and occurrence of sandy, gravelly or bleached soils on a deep water table.
The palaeohistory of Amazonia and the present distribution of ancient geological features of both Amazonia and Guayana suggest that the sands found in the present-day savannas ultimately originated from the Roraima sandstone, which underwent various erosion episodes by volcanism and weathering processes, with the resulting sediments being carried southward by extant tributaries of the Amazon River.
Typical savannas include Byrsonima crassifolia and various genera of Melastomataceae and Cyperaceae. While some of the savannas have been studied botanically, many have never been sampled (Daly and Prance 1989). The savannas of Roraima were studied by Takeuchi (1960), who reported finding vestiges of the use of fire to stimulate the growth of grasses. The savannas of Ariramba, near the Parú de Oeste (or Cuminá) River north of Oriximiná, were studied by Le Cointe (1922), Ducke and Black (1953, 1954) and Egler (1960). In the Jari area, a curious swampy savanna over a rock substrate was investigated by Pires-O'Brien (1992), and its flora compared with those of other savannas of Amazonia and Guayana.
During dry glacial periods of the Quaternary, the Transverse Dry Belt's deciduous forests and savannas may have served as corridors for exchange between the Llanos of Colombia and Venezuela and the savannas of Brazil's Central Plateau (Clapperton 1993; Daly and Prance 1989; Hoogmoed 1979). In the Trombetas area, moist forest probably persisted as a refugium (Prance 1982; Whitmore and Prance 1987).
The flora of the Transverse Dry Belt is best known along Highway BR-174, along the margins of the Amazon River, and around areas of greater development such as Pôrto Trombetas and Monte Dourado. Lists of the prominent species have been compiled for each of the vegetation types.
In the forests of the Transverse Dry Belt hundreds of tree species mix in various ways. Least rich are the dry semi-open forests and savanna-forests, which have from 120-140 different tree species in 1 ha. Many open forests have a large proportion of palm species. Most species-rich are the dense lowland forests, which have up to 240 different trees in 1 ha. These forests are found on terraces and in some ravines, and are characterized by fewer palms and more shade-tolerant trees.
The richest flora is probably over the geological strip of Palaeozoic sediments (Map 40). Along this strip are two centres of plant endemism summarized by Whitmore and Prance (1987) - in the Manaus region (CPD Site SA5) and in the Rio Trombetas Basin. The Jari Ecological Station, on the western side of the middle Jari River, also has high species richness and some vicariant taxa (Pires 1991; Pires-O'Brien 1992). A field survey from Trombetas to Jari along the Palaeozoic strip could determine whether the Trombetas area is a restricted centre of diversity or if such higher diversity occurs along the strip.
The region of the Transverse Dry Belt is the natural habitat of many plants producing spices and drugs that were exploited by the Portuguese from the second half of the 17th century to the first half of the 18th century. Among the economic products sought during that period were "wild clove" (Dicypellium caryophyllatum), an Amazon cinnamon (Aniba canelilla), vanilla (Vanilla planifolia), cassia (Cassia spp.), cocoa (Theobroma cacao) and "salsaparilha" (Smilax sp.). The exploration phase for spices and drugs was followed by an exploration phase for rubber. Raw rubber (Hevea brasiliensis), taken to Europe in 1744 by the French traveller and explorer Charles-Marie de La Condamine, soon came under study by various chemists. The world demand started in 1839 after invention of the process of vulcanization (von Hagen 1945).
The list of economic plants from the Amazon has increased substantially with the progress in chemical studies of plants suspected to have active compounds. For example, plants that yield the poison "curare" used on arrows by the Amerindians resulted in the discovery of three major groups of alkaloids as well as some glycosides with various pharmacological applications. Another example is the Lauraceae "pau-rosa" (Aniba rosaeodora), which has been heavily over-exploited for its fragrant oil linalol. Advances in phytochemical research on Amazonian plants continue to reveal species that have chemical compounds of scientific and economic interest (Gottlieb and Mors 1978). Interest in the rosewood oil from pau-rosa led to an extensive study of the essential oils of a large number of Aniba species, which resulted in new insights in plant evolutionary biology (Gottlieb 1985).
Since 1959 the Superintendência para o Desenvolvimento da Amazônia (SUDAM) has experimented with growing some useful species of trees south-east of Santarém, at the Curuá-Una Experiment Station. Good results have been found with Vochysia maxima, Jacaranda copaia, Simaruba amara, Goupia glabra, Virola cuspidata, Anacardium giganteum, Bertholletia excelsa, Parkia multijuga, Didymopanax morototoni and Bagassa guianensis (Pedrosso et al. 1982).
The Instituto Agronômico do Norte (IAN) in Belém, which is now called the Centro de Pesquisa Agroflorestal da Amazônia Oriental (acronym CPATU - since previously called the Centro de Pesquisa Agropecuária do Trópico Úmido), and which is linked with the Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA), has a tradition of research in forest management at the Tapajós Forest Reserve south-west of Santarém. In 1975 permanent experimental plots were begun for commercial and diagnostic inventories on natural regeneration following exploitation. Among the most desirable trees are Hymenaea courbaril, Piptadenia suaveolens, Didymopanax morototoni, Jacaranda copaia and Bixa arborea (Carvalho et al. 1984).
Social and environmental values
This vast area is very poorly known ornithologically. It is difficult to judge the range of many of the bird species as they are often recorded from only a handful of widely spaced localities. The Upper Rio Branco Endemic Bird Area (EBA B56) is located in the northern part of this region. This EBA consists of the gallery forests of the Upper Branco River and associated rivers of northern Roraima in northernmost Brazil and extreme western Guyana. There are two restricted-range species confined to the EBA: Rio Branco antbird (Cercomacra carbonaria) and hoary-throated spinetail (Synallaxis kollari). Both are considered threatened due to their small ranges which render them vulnerable to habitat destruction. Farther south, the Central Amazonian Brazil EBA (B43) overlaps with this region.
The northern part of the Transverse Dry Belt region, mainly in areas recognized officially, is home for several indigenous groups - particularly Jacamim, Wai-Wai, Nhamundá-Mapuera, Tirió and Yanomami. The Yanomami live between the headwaters of the Orinoco River in Venezuela and some tributaries of the Negro and Branco, and are considered the last relatively isolated large indigenous group in the Amazon. Their total population is c. 17,000, of whom c. 10,000 live in Brazil (Albert 1992). Heavy migration of gold prospectors to areas close to indigenous territory is a threat to the survival of the Yanomami and other tribes, by bringing diseases to which the Amerindians are not resistant. In the region of the Trombetas River, there are some black communities formed over 100 years ago by runaway slaves known as "Quilombos".
In the Transverse Dry Belt the greater proportion of the Western population historically has been concentrated alongside the Amazon and other major rivers. In Amazonia as a whole the building of roads and development projects have broken this trend, and populations now concentrate near such areas. The Transverse Dry Belt region is cut by a section of the Manaus-Rio Branco road (BR-174). However, the Transverse Dry Belt still has fewer roads than many other regions in the Amazon, which may be a blessing since the most disturbed areas are alongside the major highways.
"Ribeirinhos" commonly dwell alongside the rivers, living predominantly from fishing and a diversified subsistence agriculture. There is a ribeirinho culture, formed by the influence of European ways on indigenous populations. "Caboclo" is another term used to characterize such local dwellers, although this word is also used for those inhabitants who represent mixtures involving Europeans and Amerindians. Turtles are important in the local diet, as the Trombetas region is a breeding centre for them. Small-scale agro-industry is also significant for ribeirinho or caboclo dwellers, producing sugarcane, "aguardente" (a type of rum), manioc flour, salted fish and extractive forest products.
Populations from areas under the influence of major roads and development projects have broken away from the traditional activities of subsistence agriculture and small-scale agro-industry into larger scale production of fewer goods. Paralleling the independent family-run farming properties, there has been an increase in large properties, saw mills, mines and commerce, resulting in an increased demand for labour and for land.
The general region's macroeconomy is based mainly on mining activities, pulp production and extraction of timber. There also are many non-timber plant products that are commercially exploited (IBGE 1990) (see Table 54). Some of these native products already are cultivated for a larger economic return such as "açaí", grown for production of heart-of-palm.
By leading to large-scale deforestation, the new social order is more damaging to the environment than the traditional practices. In the Transverse Dry Belt the trends are just beginning. More efforts must be put into promoting socio-economic activities that allow maintenance of natural vegetation cover, discouraging those which promote deforestation and poor development.
The region of the Jari River has c. 1100 km² of converted forests cultivated particularly for production of some pulpwood species. The land is owned by the Companhia Florestal Monte Dourado, which acquired it from D.K. Ludwig's Projeto Jari that began in 1968 (Rankin 1985). Although perennial-crop plantations have been considered as a most suitable form of development for Amazonia (Alvim 1981), ecological considerations such as the difficulties with monocultures and the need for applied research in semi-natural reserves must be included in sensible planning for enduring development (Hecht 1988; Daly and Prance 1989; cf. Pires 1991).
During the United Nations Conference on Environment and Development (UNCED) in June 1992 in Rio de Janeiro, the Brazilian authorities showed a disposition towards sustainable development of the Amazon region. A large project promoting economic-ecological zoning of the State of Pará is underway by the federal government (through SAE, the Secretariat of Strategic Studies) in conjunction with the State of Pará (through IDESP, the Institute for Economic and Social Development of Pará). The project aims to gain a better knowledge of Pará in order to plan its long-term, sustainable development.
A northern east-west highway known as Perimetral Norte (BR-210) was planned in 1973 by the Brazilian government, but has not been fully constructed due to a lack of financial resources. The relative absence of roads at present makes the Transverse Dry Belt less threatened by deforestation than most other areas of Amazonia. The major threat is mercury pollution due to its increased use by prospectors to amalgamate gold particles (Martinelli et al. 1988). The areas most threatened by the pollution are the Upper Branco and the Lower Xingú, which have been subjected to heavy migration by gold and diamond prospectors. In the Alto Rio Branco area the migration has been facilitated by numerous airfields constructed by Brazil's Calha Norte project, aimed at patrolling the frontier. Corporate mining also could be a threat to the natural vegetation.
The area of Pôrto Trombetas has a large deposit of bauxite that is being exploited by the company Mineração Rio do Norte (MRN), which deforests 100-120 ha yearly; MRN makes efforts to regenerate forest on the stripped sites. The ore is transported on a 100-km railroad built for that purpose. An increasing migrant population is concentrated in a band south of the Amazon River, which has the highest demography. It is in this area where most forest-burning episodes have been reported.
The Transverse Dry Belt general region has several conservation units (IBGE and IBDF 1988), including a Biological Reserve, two Ecological Stations and as well a National Forest (Table 55). There are other areas semi-protected, such as the Curuá-Una Experiment Station and Forest Reserve (712 km²), and at least four Amerindian reserves: Jacamim, Wai-Wai, Nhamundá-Mapuera and Tumucumaque.
A centre of endemism has been proposed in the Trombetas River area (see Whitmore and Prance 1987). The region has many other areas that have been given priority for additional conservation efforts due to particular features (Prance 1990), such as the Palaeozoic strip.
Map 40. Central-eastern Amazonia (CPD Site SA4), showing approximate region of transverse dry belt (after DNPM 1981 and SUDAM 1984)
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This Data Sheet was written by Dra. Maria
Joaquina Pires-O'Brien ("Camelot", Mill Road, Mutford, Beccles, Suffolk NR34 7UR
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