Seedling Survival of Tropical Tree Species: Interactions of Dispersal Distance, Light‐Gaps, and Pathogens

Abstract

This study tested the hypotheses that seed dispersal enhances seedling survival by (1) escape of distance—dependent or density—dependent mortality or both, and (2) colonization of light—gaps. Spatial patterns of seedling survival for 1 yr in shade and in light—gaps and causes of seedling mortality for the first 2 mo were determined for one tree of each of nine species that use wind dispersal on Barro Colorado Island, Panama. Dispersal was advantageous to all nine trees, but for different reasons. The colonization hypothesis was supported by all nine trees. At some time interval during the 1st yr, the escape hypothesis was supported by eight of the nine trees; dispersal away from the parent to shaded conditions lowered the probability of seedling mortality. However, the trees differed greatly in (1) when the dispersal advantage became apparent, (2) whether the advantages persisted through time, (3) the distance at which survival was most enhanced, and (4) the strength of the advantage. By 1 yr only four trees showed a moderate to strong advantage of dispersal to shaded areas; two trees showed a very weak advantage, and three trees showed no advantage. Pathogens caused the largest proportion of deaths among shaded seedlings in six of the nine trees. For the two trees with the strongest support for the escape hypothesis, pathogens caused distance— and/or density—dependent mortality; those trees had high seedling densities near the parent. Four other trees had moderate to high mortality by pathogens, irrespective of dispersal distance; those trees had low seedling densities at all distances from the parent. For these six trees in which pathogens were responsible for the largest proportion of deaths in the shade, this mortality was significantly reduced in light—gaps, even though seedlings occurred in high densities. The study indicates that the location where seedlings survive is greatly influenced by pathogen activity. Only the four trees with moderate to strong support for the escape hypothesis had large numbers of seedlings survive to 1 yr in the shade. Seedling recruitment is expected to occur away from the parent tree, in shaded conditions for trees with distance— and/or density—dependent mortality, and in light—gaps for all trees studied here.