Impact of forest fragmentation on seedling abundance in a tropical rain forest

To evaluate the impact of fragmentation on forest regeneration, I measured the abundance of shade-tolerant, mature-phase tree seedlings (individuals 5-100 cm tall) in unfragmented and fragmented vegetation in three sites near Manaus, Brazil. The habitats studied were (1) continuous forest (control, n = 5); (2) 100-ha fragments (n = 2); (3) 10-ha fragments (n = 4); and (4) 1-ha fragments (n = 5). For 10- and 100-ha fragments, seedling density was measured in the center, the edge, and the corner of the fragments, and at 20-m intervals up to 100 m away from the fragment's edge. The density of seedlings declined significantly from continuous forest to forest fragments. Corners of 100-ha fragments had lower densities of seedlings than plots in centers and edges. In both 100- and 10-ha fragments, edge seedling density increased toward forest interior, but the increment was significant for only one site. Edge effects were more important than area effects per se in affecting seedling abundance. Overall, the centers of larger fragments (100 ha) did not have significantly higher densities of tree seedlings than smaller ones (10 and 1 ha). I suggest that a decrease in seed rain produced by increased tree mortality, reduced seed output and dispersal, high seed predation, and lower seedling establishment might explain the lower seedling numbers observed in forest fragments and fragment edges. These results suggest that forest fragmentation at Manaus may effect the regenerative potential of the forest.