RAIN FOREST FRAGMENTATION AND THE DYNAMICS OF AMAZONIAN TREE COMMUNITIES

Abstract

Few studies have assessed effects of habitat fragmentation on tropical forest dynamics. We describe results from an 18-yr experimental study of the effects of rain forest fragmentation on tree-community dynamics in central Amazonia. Tree communities were assessed in 39 permanent, 1-ha plots in forest fragments of 1, 10, or 100 ha in area, and in 27 plots in nearby continuous forest. Repeated censuses of >56000 marked trees (≥10 cm diameter at breast height) were used to generate annualized estimates of tree mortality, damage, and turnover in fragmented and continuous forest. On average, forest fragments exhibited markedly elevated dynamics, apparently as a result of increased windthrow and microclimatic changes near forest edges. Mean mortality, damage, and turnover rates were much higher within 60 m of edges (4.01, 4.10, and 3.16%, respectively) and moderately higher within 60–100 m of edges (2.40, 1.96, and 2.05%) than in forest interiors (1.27, 1.48, and 1.15%). Less-pronounced changes in mortality and turnover rates were apparently detectable up to ∼300 m from forest edges. Edge aspect had no significant effect on forest dynamics. Tree mortality and damage rates did not vary significantly with fragment age, suggesting that increased dynamics are not merely transitory effects that occur immediately after fragmentation, while turnover rates increased with age in most (8/9) fragments. These findings reveal that fragmentation causes important changes in the dynamics of Amazonian forests, especially within ∼100 m of habitat edges. A mathematical “core-area model” incorporating these data predicted that edge effects will increase rapidly in importance once fragments fall below ∼100–400 ha in area, depending on fragment shape. Accelerated dynamics in fragments will alter forest structure, floristic composition, biomass, and microclimate and are likely to exacerbate effects of fragmentation on disturbance-sensitive species.