Does conspecific negative density dependence act as a major driver of species composition in temperate forests?
Conspecific negative density dependence (CNDD), which posits that the survival of tree seedlings is directly related to the local density of trees of the same species, is often cited as a significant factor in how the species composition of forests is established. This theory is primarily applied to tropical forests (typically in relation to the Janzen-Connell hypothesis), and thus how it may affect temperate forests is not yet as well understood. In order to investigate what role CNDD may play in temperate forests, we investigated tree seedlings in two mid-Atlantic deciduous forests. Specifically, we analyzed tree seedling health as a function of the abundance of adult trees of the same species.
CNDD was found to be variant throughout the areas studied, as well as by species. At the site level, one area studied indicated that trunk basal area of adult trees (used as an indicator of tree density) did have a borderline significant relationship with seedling damage, while the other site studied had no such relationship. Individual species also displayed considerable variation in their adherence to CNDD, with some exhibiting no relationship, while others seemed to be at least partially dependent. Given these results, it seems unlikely that CNDD is the primary driver behind temperate forest species composition. The high degree of variance between species and across the area studied suggests that CNDD is just one of many processes that impact species composition. Nonetheless, our results provide evidence that CNDD may be an important determinant of spatiotemporal dynamics for some species in some locations.