Edge structure determines the magnitude of changes in microclimate and vegetation structure in tropical forest fragments

Edge structure is one of the principal determinants of the extent and magni tude of edge effects in forest fragments. In central Amazonia, natural succ ession at forest edges typically produces a dense wall of vegetation domina ted by Cecropia spp. that buffers the forest interior. Fire encroachment in to forest edges, however, eliminates the soil seed bank, enhances plant mor tality, and promotes succession to an open, Vismia-dominated edge that does not buffer the forest interior. Contrasting open, fire-encroached forest e dges and closed, non-fire-encroached edges were examined in central Amazoni a to assess the effects of edge structure on microclimate and vegetation st ructure in tropical forest fragments. Edge penetration distances for most m icroclimate and vegetation structure variables were as much as two to five times greater at open edges than at closed edges. The magnitude of these di fferences suggests that edge structure is one of the main determinants of m icroclimate and vegetation structure within tropical forest fragments. Edge effects also varied systematically with fragment area. For a given edg e type, 100-ha fragments had consistently lower canopy height, higher folia ge density, higher temperature, a higher rate of evaporative drying, lower leaf litter moisture content, and lower litter depth than continuous forest , at all distances from the forest edge. These differences, however, were r elatively minor compared to the striking differences in edge penetration be tween open and closed forest edges. For organisms in small fragments, the d ifference between open and closed edges may be the difference between total edge encroachment on one hand and an effective nature reserve on the other , relatively independent of absolute fragment area.