2 Characterization of the oligotrophic environment

Unlike other areas of terra firme, the characteristic vegetation of this habitat is camping or cautinga amazônica rather than tropical moist or rain forest. Caatinga is a dwarfed scrub-forest vegetation of about 6 to 20 metres above ground level growing on hydromorphic quartzy sands (Klinge, 1978; Takeuchi, 1961). It is found chiefly in the Guianas and portions of the Rio Negro basin and its effluents (chiefly the Vaupés and the Içana). This type of vegetation develops in areas with a humid tropical climate where there is no dry season and which is dominated by podzolic soils or spodosols. The vegetation varies along a gradient from high caatinga to low caatinga or bana, which is the poorest of all. With increased oligotrophy, the herbaceous cover increases in dominance and the proportion of roots in total biomass increases. Figure 3.3 illustrates the interrelation between hydrology, soils, and vegetation in an area of the upper Rio Negro.

Leaves of caatinga vegetation are hard and leathery (sclerophyllous). Leaf area index is smaller in this type of vegetation (5.2) as compared with the mean of 8.0 in tropical forests, probably to reduce the effects of the drought stress period. Wood volume is less and the canopy is smaller than in areas less limited (Jordan, 1982: 395). Vines are uncommon, in contrast to epiphytes. In oligotrophic areas, there is a tendency for species dominance, in contrast to the pattern, more common in Amazonia, where dominance is rare in native forests. In a region near San Carlos de RÍo Negro in Venezuela, Micrandra spruceana and Eperua leucantha constituted 50.3 per cent of the biomass (Klinge, 1978: 260).

Caatingas are not restricted to the Amazon. In Sarawak this vegetation is known as kerangas, growing on poor soils where rice cultivation is not possible (Jacobs, 1988: 188). In other areas of Asia they are known as padangs. Areas with xeromorphic vegetation in the humid tropics have also been noted in Borneo, Sumatra, and Malacca. Richards (1952) compared the padangs of Malaysia to wallaba vegetation in the Guianas. In all these cases, these vegetations are associated with extremely leached white sandy soils. When cleared of their native vegetation, the areas take an uncommonly long time to return to their original state (perhaps as much as one hundred years), confirming the poverty of the environment (Jacobs, 1988: 189; Uhl, 1983; Uhl et al., 1982).

Low bane reaches a height of 3-7 metres, with dwarfed trees and bushes occurring mixed with grassy vegetation. In high caatinga the height may reach 20 metres, approximating the structure of upland tropical forest. High bane can be seen as a transitional zone between low bane and caatinga amazônica (Klinge, n.d.: 20). Biomass in caatinga is twice that of bane (i.e. 28 kg/m² vis-à-vis 10-17 kg/m²).

Table 3.1 Ecological characteristics of types of forest

Source: Modified from Klinge, 1982 and Uhl and Murphy, 1981: table 3.

Other indices of the differences between bana/caatinga and upland tropical forests elsewhere in Amazonia clearly indicate the substantial differences between them (see table 3.1). Of particular note is the substantial increase in species diversity when moving from bane/ caatinga to upland forests in the Rio Negro. The species diversity is comparable in upland forests of the Rio Negro to tropical moist and rain forests elsewhere in Amazonia. However, it is important to observe that the upland forests in the Rio Negro have a shorter canopy, a considerably smaller basal area, lower above-ground biomass, and a higher proportion of total biomass in the root layer than tropical moist and rain forests elsewhere in Amazonia. Thus, not all terra firme tropical forests are the same. The Rio Negro terra firme forests, because they are at the extreme end of the gradient of poverty, are more geared to nutrient conservation and recycling than they are to producing net yield available to herbivores or humans.

Also implicated in the presence of this type of vegetation are the hydrologic patterns. Medina et al. (1978) demonstrated that even with 3,600 mm of annual rainfall and rains of more than 200 mm each month, seasonal drought is common in these areas. By the time rainfall declines to the range of 200 to 300 mm in a month there is a high probability of wilting due to the high evapotranspiration (5.411.5 mm/day), high albedo, and the excellent drainage of the sandy soils. The water table is near the surface, due to an impermeable B horizon, characteristic of podzolic soils (Herrera, 1979). With every rain the water can flood an area immediately, and draining will occur only slowly over the next few days.

The soils are sandy and composed of near-pure quartz. They have a high erosion potential. These soils are known variously as spodosols, podzolic soils, and hydromorphic podzols. They have a superficial layer with a great deal of undecomposed organic matter, pH below 4.0, and a diagnostic B horizon (i.e. a spodic horizon) of greyish colour of impermeable clays.

Further evidence for the oligotrophy of this environment is evident in the heavy investment in root biomass. Above-ground biomass is lower in this ecosystem but total biomass is comparable to other parts of terra firme. Roots constitute between 34 and 87 per cent of total biomass, as compared with 20 per cent in most areas of forested terra firme. Eighty-six per cent of the roots are found in the A horizon of the soils, and 70 per cent are considered extremely fine (Klinge and Herrera, 1978). In an experiment, 99.9 per cent of calcium and potassium was absorbed by the fine roots (Stark and Jordan, 1978). This is due to the mutualistic relation between mycorrhizae and the vegetation. In other areas of Amazonia the nutrient capture mechanisms are more porous and more nutrients escape from the immediate area of leaf-fall near a tree. The micro-organisms in the root layer incorporate the available nitrogen, the nutrient which seems most limiting in this ecosystem. Denitrifying bacteria are almost non-existent in this system, as compared to other areas of Amazonia. The poorer the ecosystem, the greater the development of the fine root layer, and the greater the presence of toxic substances which inhibit predation of leaves. Herbivore populations are very low in these areas, owing to the lack of palatable biomass. Research in Africa and in Venezuela confirms the presence of bacteriostatic and fungistatic substances like alkaloids and polyphenols in oligotrophic areas (McKey et al., 1978). These chemical defences are of considerable importance for research in both medicine and agriculture.