|The Fragile Tropics of Latin America: Sustainable Management of Changing Environments (UNU, 1995)|
|Part 1 : The ecological outlook|
|Rich and poor ecosystems of Amazonia: an approach to management|
One of the general characteristics of the upland forests found in Amazonian
terra firme is their biotic diversity and environmental patchiness. Figure 3.4
illustrates just one type of patchiness present, in this case vegetational
patchiness, in an eutrophic area of the lower Tapajos valley. One way in which
human populations cope with extreme diversity is by simplifying the initial
heterogeneity present, giving preference to resources to which they give
particularly high value. Indeed, human beings do not simply adapt to the
environment but also modify it so that its limitations are replaced with
for future use. As we will see in this section of the paper, prehistoric populations of terra firme brought about major transformations in the vegetation formations in the more productive areas of terra firme. These anthropogenic vegetations serve as indicators of areas of high potential for human occupation and agriculture. Indeed, there is much we can learn from indigenous peoples of Amazonia about how to manage the ecosystems.
The more favourable conditions of these eutrophic areas are reflected in the lower proportion of total biomass in the root component, with only about 20 per cent of the total, in contrast to 34-87 per cent of the total in the blackwater basins (see table 3.1, p. 51). Thus, the above-ground biomass is nearly twice that of the blackwater regions.
These forests' natural diversity has been simplified by management to the point where some of them have overwhelming dominance of some species, particularly of some valuable palms. In areas that have not been notably modified by human management, one finds 80 to 100 tree species per hectare with 10 cm dbh (Black et al., 1950; Prance et al., 1976; Takeuchi, 1961; Uhl and Murphy, 1981). In a study by Uhl and Murphy, 67 per cent of the species were represented by fewer than four individuals. This diversity is reduced in areas that have been modified by prehistoric populations.
Soils in terra firme include some of the poorest and some of the richest soils in the world. The poorest we examined in the earlier section of this paper. The richest soils of terra firme are the alfisols and the mollisols. The alfisols are soils of medium to high fertility, resulting from basaltic intrusions, and are high in cation exchange capacity and of near-neutral pH. They occur in a highly dispersed pattern throughout the basin but are often associated with vine forests and with anthropogenic black soils. Areas like the Guaporé valley in Rondônia, the lower Tocantins and Xingú, and the north of Mato Grosso have sizeable areas of these good soils (Moran, 1990, 1993). Figure 3.5 illustrates visually the fertility of these soils in comparison to "average" soils of terra firme (the proverbial oxisols or latosols) and in contrast to the spodosols typical of the black-water basins with which we are contrasting the eutrophic areas. The soil samples for the eutrophic and average areas were taken by the author along the Trans-Amazon highway near Altamira, and were taken at a depth of 0-20 cm (i.e. fertility cores). The oligotrophic soil samples were taken by Rafael Herrera (1979).
At least 11.8 per cent of the terra firme forests are now thought to be anthropogenic in nature, resulting from prolonged management by prehistoric populations (Balée, 1989). The population did not eliminate diversity entirely in this process, as this would have been foolhardy and ineffective, but promoted, instead, islands of concentrated resources within a sea of diversity. They did this by creating favourable conditions for the dominance of species that they valued highly and with characteristics that facilitated their competitiveness over time in an environment where succession is remarkably vigorous. Among the vegetations that are likely to be anthropogenic in contemporary Amazonia one may cite palm forests, bamboo forests, Brazil nut forests, and vine forests.
Palms are excellent indicator species of archaeological sites. Pupunha (Bactris gassipaes), inajá (Maximiliana maripa), and buritÍ (Mauritia flexuosa) have been used as indicative of prolonged human occupation sites (Balick, 1984; Boer, 1965; Heinen and Ruddle, 1974; Pesce, 1985). BuritTucumã is found most often in areas of flooded forest and várzea, in contrast to the other two. Tucumã(Astrocarium vulgare), caiaui (Elaeis oleifera), and babaçú (Orbignya phalerata) are important, favoured species in the terra firme forests.
Boer (1965: 132) views Astrocarium vulgare, or tucumã, as a species that "never is found in virgin" areas, but only in areas that have been extensively disturbed by human managers. The Urubú Ka'apor of the state of Maranhão value it as a tree that attracts tapir and cutias, making hunting easier and more productive. It is also valued for its fibres, which are used to make hammocks, child-carrying slings, and other articles of daily use (Balée, 1989).
Andrade (1983: 23) found an association between the presence of Elaeis oleifera and anthropogenic black soils in areas of the Madeira river and south of the city of Manaus. Its uses are similar to those of the African palm and it seems to have come originally from Central America.
Perhaps the best-known palm of the palm forests is Orbignya phalerata or babaçú. It has been estimated that forests dominated by babaçú cover 196,370 Km² in the Brazilian Amazon (May et al., 1985: 115). This degree of dominance is rare for any palm, and seems to be a result of its manner of germination (Anderson and Anderson, 1985). The babaçú palm lives about 180 years and its presence is indicative of human disturbance. Balée (1984: 94-5) found forest islands of babaçú of up to three hectares in areas occupied today by the Guajá, Tembé, and Urubú Ka'apor. Among the Suruí of Rondônia, areas near groves of babaçú are preferred as residential sites. babaçú forests themselves do not occur on areas of high fertility, but they seem to occur near patches of high fertility.
Bamboo forests (Guadua glomerata) are important to indigenous populations because of their value in making flutes for rituals and arrows for hunting and fishing. Bamboo forests have been estimated to cover about 85,000 Km² in the Brazilian Amazon (Braga, 1979: 55). Balée (1989) has noted that the Guajá occupy today areas of bamboo forest created earlier by the activities of the Guajajara in the area of the Pindaré river.
Forests dominated by the presence of the Brazil nut tree (Bertholletia excelsa) occupy approximately 8,000 Km² in the lower Tocantins (Kitamura and Muller, 1984: 8) and have also been observed in Amapá, in the basin of the Jari river, and in Rondônia, although their areal extent has not been mapped. Some observers have noted that these forests seemed to be associated with archaeological sites and with areas of anthropogenic black soils (Araújo-Costa, 1983; Simoes and Araújo-Costa, 1987; Simões et al., 1973). The Kayapó at Gorotire plant Brazil nut trees because of their importance as food (Anderson and Posey, 1985; Posey, 1985). Besides its local food value, this free is also an important economic resource for export (Laraia and da Mata, 1968). Brazil nut trees live even longer than babaçú, and are important indicator species of anthropogenic forests.
Perhaps most extensive of all, after the babaçú palm forests, are the vine or liana forests (matas de cipó), estimated to occupy 100,000 Km² in the Brazilian Amazon (fires, 1973: 152). They occur in particular concentration in the lower Tocantins, Xingú, and Tapajos river basins (see the area of vine forests in figure 3.4). Vine forests have a lower basal area than presumably "virgin" forests of terra firme (18-24 vis-à-vis 40 m² per hectare, respectively), an unusually high density of woody vines, and a tendency towards dominance by a few species (fires and Prance, 1985: 120-2).
In these vine forests one finds a concentration of resources that is suggestive of complex, long-term management: food items, fruits that attract game, construction materials, medicinal plants, insect repellents, and firewood. The number of useful species is larger than in other types of upland forest. Observers have long noted the preference of farmers for vine forests (Moran, 1977, 1981; Sombroek, 1966: 195). Although no simple correlation between these forests and a particular soil order has been made, they seem to be more frequently associated with anthropogenic black soils (Heindsdijk, 1957; Smith, 1980) and with alfisols (Falesi, 1972; Moran, 1981). Balée (1989) found anthropogenic black soils in vine forests of Maranhão, of 39 cm depth, and with 31.8 meq/100 g of phosphorus - levels much higher than in adjacent soils. He also noted the higher pH (5.8) and the high concentration of calcium.
In contrast to the black-water regions of Amazonia, in these anthropogenic forests one finds an unusually high frequency of corn cultivation, in some cases leading to the near-total loss of bitter manioc cultivars and to the abandonment of manioc flour production. By contrast, in the black-water regions, one sees a predominance of extremely bitter varieties of manioc and a near-total absence of corn cultivation. One should note that in the white-water basins of the western Amazon, one also finds a low incidence of bitter manioc varieties and more corn cultivation. Corn is much more demanding of nutrients than can be sustained in many parts of Amazonia, particularly in the black-water regions.