Seed Dispersal and Potential Forest Succession in Abandoned Agriculture in Tropical Africa

Natural forest succession on human-disturbed lands is often slow because the resources necessary for succession are depleted. In such landscapes, forest succession may be dependent on arrival of seeds from off-site, many of which are dispersed by fruiteating animals. We studied bat and bird seed dispersal in a deforested agricultural area adjacent to Kibale National Park, Uganda. Seed rain was monitored for 6 mo in short (<0.5 m) and tall (2-5 m) grassland, and below short (1-3.5 m), medium (3.5-10.0 m), and tall (-10.0 m) isolated trees within grassland. Seed rain (numbers of seeds and seed species) under trees of all heights was greater than in short or tall grasslands. Bats dispersed seeds mostly below tall trees, while birds dispersed seeds mostly below both tall and midsized trees. More seeds and seed species were dispersed under tall trees than below short trees. Within 150 m of the forest there was no relationship between the seed rain under trees and the distance to the forest edge. Nearly half of the > 11 200 seeds collected were from hemiepiphytic Ficus that are unable to grow from the ground. One-third of collected seeds were from shrubs, and most of the remainder were from trees. Nearly all tree and shrub seeds collected (>99%) were species typically found in disturbed grassland, not in forest. These results suggest that in this African region, forest succession may proceed very slowly on degraded lands.