One fundamental goal of ecology is to determine the ways in which organisms and their environments interact. Recent studies suggest that conspecific negative density dependence plays an important role in the stabilization of species populations and the maintenance of species diversity. Recent evidence also suggests that the strength of conspecific negative density dependence increases in warmer, wetter, and more productive environments. However, whether or not these patterns, which have been identified at landscape to regional scales, scale up to larger spatial scales remains unknown. We used a continental-scale dynamic dataset to test whether regional differences in climate across the continental United States influenced the strength of density dependence. We used data on forest trees collected through the US Forest Service Forest Inventory Analysis (FIA) program. Specifically, we measured density dependence in tree growth and survival as a function of increasing conspecific and heterospecific densities at neighborhood scales (7.3m radius).
Results/Conclusions
Conspecific negative density dependence in tree growth and survival differed among ecoregions and species. Regional differences in conspecific negative density dependence across the continental US were associated with differences in mean annual temperature and precipitation. Moreover, the effects of temperature and precipitation on conspecific negative density dependence depended on each other (i.e. an interaction). For example, conspecific negative density dependence in annual growth rates of trees was strongest in warm/wet regions, also strong in cold/dry regions, but weakest in warm/dry and cold/wet regions. These results suggest that differences in conspecific negative density dependence depend on climate at continental scales, providing a mechanism by which the demographic consequences of local species interactions can scale up to influence patterns of species diversity at much larger spatial extents. These results also have important implications for how community-level patterns like species diversity and relative abundance will respond to global climate change.