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Recruitment Near Conspecific Adults and the Maintenance of Tree and Shrub Diversity in a Neotropical Forest

According to the Janzen-Connell hypothesis for the maintenance of species diversity, recruitment is inhibited in the immediate vicinity of adults by herbivores and pathogens. This reduces the per capita ability of abundant species to reproduce, relative to less common species, and gives rare or competitively inferior species a greater chance to persist. We tested this hypothesis in a 50-ha mapped plot of tropical moist forest on Barro Colorado Island, Panama, by investigating the spatial patterns of sapling recruitment in 80 species of trees and shrubs. Two censuses of adults and saplings were carried out, in 1982 and in 1985. Recruits were defined as saplings of 1-8 cm dbh (diameter breast height) appearing in the 1985 census that were not present in 1982. The distance from each recruit to its nearest conspecific adult neighbor was measured. At various distances from adults, the number of conspecific recruits and the number of recruits of all species were tallied. The ratio of recruits of species i to all recruits was taken as an estimate of the probability that species i would occupy that site as an adult. A few species showed a significant reduction in recruitment probability close to adults, but more species showed a significant increase, and many other species showed no significant spatial pattern. Among canopy trees, about a third of the species showed some sign of local reduction in recruitment, but the distance over which the effect extended was usually less than 5 m; however, the most abundant canopy tree, Trichilia tuberculata, showed a sharp reduction in recruitment probability up to 10 m from adults. In treelets and shrubs, most species showed strong peaks in recruitment probability close to adults. Thus, most recruitment patterns did not fit the prediction of Janzen and Connell; however, two to three of the most common species may have reached densities at which a depression in local recruitment is regulating abundance.