Spatial patterns of sapling survival and mortality in the neighborhood of conspecific adult trees differ by species abundance in a Neotropical forest over 25 years
Previous research in the 50 ha plot on Barro Colorado Island (BCI), Panama has shown that seedlings of rare species tend to be more susceptible to negative density dependence than seedlings of common species, but whether this is also true of larger and older saplings of rare species has not been examined until the present study. In 55 species of different abundance, we tested whether saplings of rare species are more likely to die when nearer conspecific adults. To control for time since recruitment and for any potential census interval-specific effect (e.g. El Niño), we restricted the analysis to saplings that recruited in a given census year and then determined which of those saplings survived or died over the subsequent 5-yr census interval. We computed the cumulative distributions of living and dead saplings as a function of distance from nearest living conspecific adult at the end of each census interval. If saplings are more likely to die when they are nearer conspecific adults, the cumulative distribution of living saplings should be displaced farther from conspecific adults than the distribution for saplings that died during a given census interval.
The cumulative distribution function of saplings that died over the five 5-yr census intervals was not always or consistently closer to the nearest conspecific adult than the cumulative function for surviving saplings. Differences occurred among common and rare species, and between species that increased and decreased in total abundance. In common species, saplings that died over 5-yr census intervals were consistently farther from the nearest conspecific adult than those that survived. In species that increased in abundance over time, the pattern in common species was observed more often. In contrast, rare species and species that decreased in abundance over time more often showed the opposite pattern, in which the distribution of dead saplings was closer to the nearest conspecific adult. These differences suggest that survival in common species has a component of habitat preference that makes for better sapling survival near existing conspecific adults already in preferred habitats, and that this is less true of rare species. Alternatively, rare species may suffer greater negative density dependence that masks evidence for habitat preferences. Our results suggest that the effects of the nearest conspecific adult on sapling survival differ by species abundance and year.