Ecologia e extrativismo da castanheira (bertholletia excelsa, lecythidaceae) em duas regiões da amazônia brasileira

Abstract

The Brazil nut tree, Bertholletia excelsa (Lecythidaceae), is an emergent tree species that occurs in stands called castanhais in non-flooded, upland forests of the Amazon. The castanhais are seasonally visited by castanheiros (nut harvesters) during the period of fruit fall. The Brazil nut represents an essential income source for local communities living near the castanhais . This study aimed to examine the population ecology and the demographic structure of castanhais located in two regions of the Brazilian Amazon, with different historical occupation and contemporany exploitation regimes: the region of the Trombetas River (Lower Amazon, Pará State), and the Lake Capanã Grande (Madeira River, Middle Amazon, Amazonas State). Demographic and regeneration data were obtained from 35 stands, 25 in the Trombetas region and 10 in Capanã Grande, by means of long transects (50 x 1,000 m) placed approximately along the walking trails of harvesters. Three regeneration categories (seedlings, saplings, and juveniles) were correlated with possible explanatory variables related to ecological, demographic, and extractive factors. Additionally, we carried out a field trial planting Brazil nut saplings in three habitats (open field, early secondary vegetation, and forest understorey) in order to monitor the saplings performance in different light conditions. The contrasting population structure found between Trombetas and Capanã Grande populations suggest that ancient and contemporany anthropogenic activities have played important roles in shaping the demographic and regeneration patterns of B. excelsa in these regions. In Trombetas, a region in which historical data suggest the occurrence of a rapid human depopulation from the XVI century until the recent past, the B. excelsa stands are dominated by medium- and large-sized trees, with little presence of juveniles or seedlings. In contrast, in the Madeira River, the stands exhibit a younger and denser structure, with higher recruitment, especially those near human settlements. The experimental plantations showed a highly significant difference between treatments, with higher rates among saplings planted in the open fields than in the semi-shaded secondary growth or in the understorey. The data suggest that the older structure in Trombetas resulted from the limited chances of regeneration in the dense, stable, and mature forest of this region. On the other hand, regeneration of Brazil nut trees, a long-lived pioneer tree, seems to be favored by agricultural and extractive activities such as those occurring in the Capanã Grande region during the last decades. There was no evidence, for either study site, of decreasing regeneration in more intensely visited and harvested areas. The data suggest that the long term maintenance of the castanhal regeneration and productivity depends on some level of human-mediated forest disturbances. In the areas with very mature forests and ancient populations of B. excelsa, such as in the Trombetas region, the adoption of enrichment measures to compensate the low regeneration rate is recommended. In these areas, Brazil nut saplings should be planted in strategic sites such as large gaps, along of the forest edges, and in cleared areas nearby the local communities. However, the success of these initiatives, depends on the awareness and participation of the local communities in all of their phases, such as the planning, germplasm selection, nursery, sapling production, planting and monitoring of juvenile survivorship and development.