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UPPER RIO NEGRO REGION
Geomorphologically, the Upper Rio Negro region correlates fairly well with the Rio Branco-Rio Negro Depression, which is the western portion of the Northern Amazonian Depression (Bezerra et al. 1990). To the region's south is the Western Amazonian Depression, beginning near where Colombia's Caquetá River joins the Japurá River of Brazil. Generally, this region is a broad plain between the somewhat higher western Colombian plateaux (and mesas) and the Guayana Highlands farther east in Venezuela and Brazil. The north-eastern limit in Venezuela is roughly from Yapacana National Park and the Orinoco River to its junction with the Casiquiare Canal. Excluding the highlands of the Pico da Neblina, which are in the Pantepui region (CPD Site SA2) (Huber 1987), altitudes vary generally from less than 100 m to 500 m above sea-level.
The regional geology is an extension of the Pantepui, consisting of a highly eroded early Precambrian (Archean) igneous basement of the Guayana complex, overlain by more recent Precambrian marine sandstones belonging to the Roraima Formation (Bezerra et al. 1990). The soils are very acidic, nutrient poor and periodically waterlogged quartzic podzols, or dystrophic red-yellow laterites (SNLCS 1981; Jordan 1987).
Rivers are important features of the region. The Guaviare and Inírida flow to the Orinoco, whereas the Guainía, Negro, Vaupés/Uaupés and Caquetá-Japurá flow to the Amazon. Three types of rivers occur (Sioli 1984): predominating are tea-coloured or black-water rivers, which drain white-sand vegetation - the most important is the Rio Negro (Goulding, Carvalho and Ferreira 1988); clear-water rivers, draining mainly quartzitic highlands; and white-water eutrophic rivers (e.g. the Guaviare), draining sediment-rich areas.
The climate is humid tropical (Köppen's Afi) or superhumid tropical (Thornthwaite's ArA´a´), with a mean annual rainfall of 2500-3500 mm, 180-240 days of rain per year and no month with less than 100 mm of rain. The temperature is isothermic, with yearly means of 24°-25°C, and the mean relative humidity is 80-90% throughout the year (SUDAM 1984).
The Upper Rio Negro region corresponds to the core area of tropical extremely moist forest as defined by UNESCO (1981), which extends approximately from 4°N to 2°50'S and 63° to 74°10'W.
Throughout the Amazon Basin, the vegetation is strongly correlated with the geomorphology, soils and climate (Salgado and Brazão 1990). Although the Upper Rio Negro is noted for its mosaic of unique Amazon caatinga forest and Amazon caatinga shrubland on sandy soils (Anderson 1981), most of this low-lying area is an ecological transition between extensive submontane rain forest (tropical moist forest) and black-water flood forest (Daly and Prance 1989). Amazon caatinga forest and shrubland are predominant on the hydromorphic podzols, submontane rain forest is most commonly found on the red-yellow laterites and black-water flood forest is associated with dystrophic alluvial gley soils.
1. Amazon caatinga forest ("campinarana") is a type of caatinga formation unique to Amazonia. Fairly frequent north of the Amazon River, these forests extend to north-eastern Peru and also have been found as far south as the Serra do Cachimbo (6°-8°S, 57°-58°W) (Lleras and Kirkbride 1978). Their core area, in the Upper Rio Negro region, is characterized by a tough-leaved, arborescent savanna vegetation adapted to very poor sandy soils that are periodically flooded (e.g. Klinge, Medina and Herrera 1977). Vegetation height correlates with the duration of annual flooding - shrubs and trees of c. 5 m characterize wetter sites, whereas trees up to 20 m tall occur on drier sites. The canopy is fairly open, with sufficient light reaching the ground so that a profusion of terrestrial "epiphytes" occurs. The topsoil is humic, with tufty patches formed by Cladonia lichens, Trichomanes filmy ferns, etc.
2. The Amazon caatinga forest phases into Amazon caatinga shrubland ("campina"), which is less high, more open and drier. The campinas are characterized by islands of vegetation less than 1 m² to several hundred m² surrounded by sandy open areas. The larger islands have one to several large trees. Species composition is similar throughout these shrublands.
3. Black-water flood forest ("igapó") borders all the watercourses and merges with the Amazon caatinga forest and shrubland. Based on data from around Manaus (CPD Site SA5), the black-water flood forests are very similar in species composition to these caatinga formations. The Amazon caatinga forests and shrublands may be relictual from black-water flood forests, having resulted from the change of river courses. Lleras and Kirkbride (1978) have found caatinga forest and shrubland atop bedrock as well, contributing to the soil development of the Serra do Cachimbo.
4. Evergreen submontane rain forest occurs dispersed through uplands ("terra firme"), to 1000 m above sea-level. The canopy rarely exceeds 30 m in height, with a few emergents such as Manilkara huberi, Caryocar villosum and Hymenaea parviflora. Other frequent species include Carapa guianensis, Sacoglottis guianensis, Pouteria surinamensis, Ocotea roraimae and several Vochysiaceae in Qualea, Vochysia and Erisma. Several genera of palms are associated with this type of forest - the Jessenia/Oenocarpus complex comprises some of the largest populations. Occurring also are Bertholletia excelsa and several species of Hevea.
Although a precise estimate on the size of the flora of the Upper Rio Negro region is presently impossible, 50-70% of the species in the Amazon Basin probably are represented. Therefore, using G.T. Prance's estimate (pers. comm.) of 30,000 vascular plant species in the Amazon Basin, the Upper Rio Negro and adjacent superhumid forests may have 15,000 to 21,000 species - which is probably up to ten times more than occur in the highly diverse area just north of Manaus. (The latter area coincides with the southernmost intrusion of the Guayana Shield and has the same basic geomorphology, geology and age as the Upper Rio Negro.)
Floristic collections are inadequate to compile a Flora for the Upper Rio Negro region; in comparison, the collecting intensity for the area around Manaus has been 10 to 20 times greater (Nelson et al. 1990). Nonetheless, in studying plant diversity throughout the Amazon Basin on the basis of existing collections, Lleras et al. (1992) recorded less than 5% difference between these two areas in the number of known species.
It is generally believed that most edaphic endemics in the region are associated with the white-sand vegetation. However, most research on this vegetation has shown a low species diversity when compared with the forests on richer soils (Pires 1957; Lleras and Kirkbride 1978). Moreover, many of the important taxa that originated in the region are components of the humid rain forests (Gentry 1982; Lleras et al. 1992). The importance of the Upper Rio Negro for plant diversity is not in the many endemics occurring on the white-sand vegetation, but that the region of which the Upper Rio Negro is the core, is the repository of the very old forest elements of the vegetation of the Guayana Shield. These high forests are much more diverse and with many more endemics than are in the white-sand vegetation, and they may be the centres of origin of a great part of the neotropical and palaeotropical lowland flora.
Lleras et al. (1992) proposed the Upper Rio Negro sensu lato (including most of the extremely moist forests of UNESCO 1981) as the centre of origin for many of its tropical families, including Caryocaraceae, Connaraceae, Sapotaceae, Meliaceae, Lecythidaceae, Dichapetalaceae and the tribe Henriquezieae of Rubiaceae. Gentry (1982) surmised that in Guayanan Pleistocene forest refugia the majority of families endemic to the neotropics probably originated, including as well the mainly neotropical Humiriaceae, Vochysiaceae and Bignoniaceae.
However, presumed refugia can be collection artefacts (Nelson et al. 1990). These extremely moist forests probably survived the climatic changes of the Pleistocene almost intact, which together with the great antiquity of the region as a whole, would argue further for excluding the concept of very recent Pleistocene forest refugia as the source of the biodiversity in the humid tropics. It is probable that the Upper Rio Negro and similar areas, and not such refugia, are the sites for the origin, evolution and long-term preservation of many neotropical taxa.
Henderson, Churchill and Luteyn (1991) probably are correct in proposing the Andes as the most diverse region of South America and probably of the world - the very many habitats and niches induced by the complex mosaic of soils, climate, geology, geomorphology and topography guarantee this. However, the Upper Rio Negro region, and indeed all of this extremely moist forest (UNESCO 1981) of which other important centres of diversity are covered in this volume, probably hold the highest floristic diversity on the planet for arborescent species.
This richness is due to three basic factors: (1) the region lies atop the continent's oldest geological formation, which is probably one of the world's oldest; (2) the region is more diverse in terms of biotic and abiotic factors than the rest of the Amazon Basin; and (3) it corresponds to an extensive transition zone where stress tends to cause diversification instead of intraspecific variation (Lleras, unpublished). Furthermore, Gentry (1986) has proposed a correlation between high rainfall and high diversity, and the Upper Rio Negro and its neighbouring regions are certainly the wettest portions of the Amazon; more significant than the high total rainfall is the lack of water stress throughout the year.
As one of the centres of origin of the neotropical lowland flora, the Upper Rio Negro region has many presently exploited as well as potentially useful species. A realistic assessment of economically important species is impossible due to the sparsity of data. However, important taxa found in the region include several species of Hevea (rubber), Caryocar, Jessenia/Oenocarpus, Bertholletia excelsa (Brazil nut) and Aniba rosaeodora (rosewood oil). Paullinia cupana var. cupana, a locally used wild form of "guaraná" with great potential for breeding purposes, is also found. Outside of the Upper Rio Negro core, the extremely moist forest is the centre of origin for Theobroma and Pourouma.
Social and environmental values
The Upper Rio Negro region straddles both the Orinoco and Negro river basins; 40% of the water in the Amazon River comes from the Rio Negro. Watershed conservation of this vital region should be a high priority.
The forests of the Upper Rio Negro regulate the hydrology of the Rio Negro Basin. Drastic changes in soil coverage in the Upper Rio Negro would affect much of the Amazon Basin. Variations in climate and rainfall in the Upper Rio Negro may cause flooding, drought or other climatic changes as far south as Manaus.
Many Amerindian communities in all three countries of this region have traditions and capabilities to manage the forest sustainably. These communities are presently the sole possessors of the knowledge and folklore that might permit humankind to exploit the floristic and faunistic resources of the region without destroying them (e.g. see Schultes and Raffauf 1990).
There have been few studies on the avifauna of the Upper Rio Negro and Orinoco white-sand forest (Endemic Bird Area B11), which supports 13 restricted-range species (two of them also occur in other areas). These birds are primarily confined to the humid tropical forest along the rivers, although some inhabit the adjacent Amazon caatinga forest and shrubland. Due to the relatively intact state of the vegetation in this region only the rare Orinoco softtail (Thripophaga cherriei) is considered threatened - but essentially because of a lack of knowledge rather than a perceived threat.
The unique mix of topography, waterways and vegetation make this region a valuable tourist asset, which could be exploited in conjunction with the Pantepui region.
The geology of the region is not well known. Iron ore and manganese are believed to be present, but it is not known if they are commercially exploitable (Bezerra et al. 1990). Gold is found in localized pockets along the borders of Brazil, Colombia and Venezuela.
The soils are poor, so extensive agriculture is not appropriate (cf. Saldarriaga 1994). A preliminary map of soil usage (SNLCS 1990) recommended that areas of podzols with Amazon caatinga forest and shrubland should be set aside for conservation, whereas the areas of lateritic soils with high forest should be used for extractive activities and light agriculture with perennials.
The greatest value of the Upper Rio Negro region may be its wealth of species, including its role as a repository for many useful species, along with the indigenous knowledge of them. With world recognition of the importance of maintaining biodiversity, the Upper Rio Negro may become a major economic asset for Brazil, Colombia and Venezuela.
Presently, the Upper Rio Negro is not threatened, because access from all three countries is difficult. A recent survey of deforestation in Amazonia based on trace-gas emissions (Skole et al. 1992) showed that most of the Upper Rio Negro area was untouched - with deforestation between 1978 and 1988 having occurred only very locally along the Colombia-Brazil border, where an extensive Amerindian reserve coincides with gold mining (0°-1°N, 69°-70°W).
Threats may come from an increase in illegal gold mining, the cutting down of forests to establish plantations of "coca" (Erythroxylum coca var. epadu), and selective logging. Furthermore, it is very difficult to convince governments or the general public that it is important to preserve areas that we know so little about.
The Upper Rio Negro region has one of the highest percentages of protected areas in the Amazon Basin north of the Equator (Map 42). These constitute the National Parks Serranía de la Neblina (13,600 km² in Venezuela and in adjacent Brazil Pico da Neblina (22,000 km²), as well as the Rio Negro Forest Reserve (37,900 km²) and two extensive Amerindian reserves. In Colombia two National Nature Reserves have been established, Puinawai (10,925 km²) and Nukak (8550 km²), and many Amerindian communities have strong participation in the administration of the region as a whole. In Venezuela there are two more National Parks, Yapacana (3200 km²) and Duida-Marahuaca (2100 km²).
Biologists meeting in Manaus in 1990 recommended a high priority for conservation of the large area from the border between Colombia and Venezuela (Rio Negro-Atabapo-Vichada, c. 4°N) extending south and joining with the existing conservation units in Brazil (CI 1991). At least one more conservation unit (Caparu) should be established south of the Equator, extending roughly from 0°20'N to 1°30'S and 69° to 71°30'W, following the Apaporis River to the Japurá River (CI 1991). Adjacent National Parks in Colombia and Brazil have been proposed for this area, which is fairly near Colombia's Cahuinarí Natural National Park (see CPD Site SA7).
The Upper Rio Negro region is part of a large area with high diversity, and meets with another - the Pantepui region (see CPD Site SA2); clear boundaries are not possible and many reserves coincide.
The most important priorities for conservation at present are extensive surveys and scientific collections of the whole area (Lleras et al. 1992). It is highly likely that less than 5% of the total flora is known, and it is extremely important to assess what is being conserved.
Map 42. Upper Rio Negro Region sensu Lato in Colombia, Venezuela and Brazil (CPD Site SA6), with existing and recommended conservation units
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CI (1991). Workshop 90. Biological priorities for conservation in Amazonia. Conservation International (CI), Washington, D.C. Map.
Daly, D.C. and Prance, G.T. (1989). Brazilian Amazon. In Campbell, D.G. and Hammond, H.D. (eds), Floristic inventory of tropical countries: the status of plant systematics, collections, and vegetation, plus recommendations for the future. New York Botanical Garden, Bronx. Pp. 401-426.
Gentry, A.H. (1982). Phytogeographic patterns as evidence for a Chocó refuge. In Prance, G.T. (ed.), Biological diversification in the tropics. Columbia University Press, New York. Pp. 112-136.
Gentry, A.H. (1986). An overview of neotropical phytogeographic patterns with an emphasis on Amazonia. In Dantas, M. (ed.), Proceedings 1st Symposium on the Humid Tropics, Vol. II, Flora and forest. EMBRAPA/CPATU (Empresa Brasileira de Pesquisa Agropecuária/Centro de Pesquisa Agropecuária do Trópico Umido), Documentos 36, Belém. Pp. 19-36.
Goulding, M., Carvalho, M.L. and Ferreira, E.G. (1988). Rio Negro: rich life in poor water. SPB Academic Publishing, The Hague, The Netherlands. 200 pp.
Henderson, A., Churchill, S.P. and Luteyn, J.L. (1991). Neotropical plant diversity. Nature 351(2 May): 21-22.
Huber, O. (1987). Consideraciones sobre el concepto de Pantepui. Pantepui 2: 2-10.
Jordan, C.F. (1987). Soils of the Amazon rainforest. In Whitmore, T.C. and Prance, G.T. (eds), Biogeography and Quaternary history in tropical Latin America. Oxford University Press, Oxford, U.K. Pp. 83-94.
Klinge, H., Medina, E. and Herrera, R. (1977). Studies on the ecology of Amazon Caatinga forest in southern Venezuela. I. General features. Acta Ci. Venez. 28: 270-276.
Lleras, E. and Kirkbride Jr., J.H. (1978). Alguns aspectos da vegetação da Serra do Cachimbo. Acta Amazonica 8: 51-65.
Lleras, E., Leite, A.M.C., Scariot, A.S. and de Sá Brandão, J.E. (1992). DefiniçÃo de áreas de alta diversidade vegetal e endemismos na Amazônia Brasileira. Final Report to UN Food and Agriculture Organization (FAO), Brasília. 65 pp.
Nelson, B.W., Ferreira, C.A.C., da Silva, M.F. and Kawasaki, M.L. (1990). Endemism centres, refugia and botanical collection density in Brazilian Amazonia. Nature 345(6277): 714-716.
Pires, J.M. (1957). Noções sobre ecologia e fitogeografia da Amazônia. Norte Agronômico 3(3): 37-54.
Saldarriaga, J.G. (1994). Recovery of the jungle on "Tierra Firme" in the upper rio Negro region of Amazonia in Colombia and Venezuela. Estudios en la Amazonia Colombiana, Vol. 5. Tropenbos-Colombia, Bogotá. 201 pp.
Salgado, L.M. and Brazão, J.E.M. (1990). Vegetação. In Projeto Zoneamento das Potencialidades dos Recursos Naturais da Amazônia Legal. IBGE/SUDAM, Rio de Janeiro. Pp. 189-211.
Schultes, R.E. and Raffauf, R.F. (1990). The healing forest: medicinal and toxic plants of the Northwest Amazonia. Dioscorides Press, Portland, Oregon, U.S.A. 484 pp.
Sioli, H. (1984). The Amazon and its main affluents: hydrology, morphology of the river courses, and river types. In Sioli, H. (ed.), The Amazon: limnology and landscape ecology of a mighty tropical river and its basin. Monogr. Biol. 56. Junk, Dordrecht, The Netherlands. Pp. 127-165.
Skole, D.L., Chomentowski, W.H., Nobre, A.D. and Tucker, C.J. (1992). A remote sensing and GIS methodology for estimating the trace gas emissions from tropical deforestation: a case study from Amazonia. Paper presented at the World Forest Watch Meeting, San José dos Campos, São Paulo, Brazil, 2730 May 1992. 19 pp. Unpublished.
SNLCS (1981). Mapa de solos do Brasil. EMBRAPA/SNLCS (Serviço Nacional de Levantamento e Conservaço de Solos). Rio de Janeiro.
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SUDAM (1984). Atlas climatológico da Amazônia Brasileira. SUDAM Publ. No. 39, Belém. 125 pp.
UNESCO (1981). Vegetation map of South America. Map and explanatory notes. UNESCO, Natural Resources Research. UNESCO Press, Paris. 189 pp. + map.
This Data Sheet was written by Dr Eduardo
Lleras [Centro Nacional de Pesquisas de Recursos Genéticos e Biotecnologia
(CENARGEN)/EMBRAPA, Caixa Postal 02-372, 70.849 Brasília, D.F., Brazil].
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