Relações entre a ocorrência de raízes acima do solo e fatores individuais e ambientais na Reserva Florestal Adolpho Ducke

Abstract

Determinants of the occurrence of buttress and stilt roots are still poorly understood. The mechanical stability required to reach large heights requires increasing diameter, and thus large construction costs that could be alleviated with reallocation of investments to support structures. Stressful environmental conditions such as exposure to wind, gravitational load and anchorage instability in shallow soils can place extra demands on the stability of trees. We here investigate how tree individual and environmental properties interact to determine the occurrence of support structures. Presence of buttress or stilt roots and tree diameter were recorded on 8.415 trees from 35 1-ha plots in central Amazon. On 67 trees of two target species distributed across topography, we also measured allometry and crown size. Proportion of support structures at the plot level and probability of occurrence at the individual level were modelled with multiple linear or logistic regressions, and boosted regression trees. The proportion of buttressed trees was higher in valleys and plateaus and stilt roots were more frequent in valleys, when more inclined. At the individual level, the probability of occurrence of any support structure increased with tree diameter and in valleys. Within species, diameter was the most important predictor of buttresses, but 30% of the species had varied and complex interactions with terrain slope and elevation. Occurrence of support structures was more likely on stout trees (lower H:D ratio), which had smaller crown areas. In summary, the most unstable environments, here represented by valleys with waterlogged soils, selected for a higher frequency of trees with support structures at the community level. However, coordinated allometric relationships among stem size and crown size also influence the need of support structures. Thus, support structures are not fixed species traits, their presence depending on individual plant´s allometric relationships and the instability conditions imposed by environment