Tree community diversity changes by two orders of magnitude from the boreal zone to the tropics. The degree to which intra- and inter-specific interactions contribute to the structure and dynamics of these communities and how the strength of these interactions may change with latitude are of fundamental interest. Previous work has indicated that conspecific individuals in the tropics experience highly dissimilar heterospecific neighborhoods. This dissimilarity may prevent strong selection for negative interspecific interactions and could lead to neutral dynamics. Conversely, it may be expected that in species poor assemblages heterospecific neighborhoods will be highly repeatable and will, perhaps, lead to deterministic dynamics. Here, using a network of large forest dynamics plots arrayed across latitude ranging in species richness from less than 20 to greater than 1000, we quantify the repeatability of tree neighborhoods between conspecific individuals across latitude. Specifically, we ask, are the heterospecific neighborhoods of individuals within a species decreasingly similar as the community richness increases and whether individuals are more or less likely to encounter a conspecific individual in their neighborhood than expected from a random point process model.
Results/Conclusions
The results show that as we move towards highly diverse tropical forests from temperate forests, heterospecific neighborhoods become highly dissimilar between conspecific individuals. In other words, in highly diverse forests, two individuals of the same species share very few heterospecific neighbors in common. Thus, the heterospecific selective environment is increasingly inconsistent as species richness increases. Additionally, we show that conspecific individuals are non-randomly clustered leading to a higher than expected encounter rate between conspecifics across latitude. In sum, tree species across latitude can rely upon interactions with other conspecific individuals, but in tropical forests there is little repeatability of heterospecific neighborhoods and selective environments. These results are of relevance to recent research pointing towards the importance of conspecific negative density dependence across latitude and a decrease in the relative strength of heterospecific negative density dependence towards the tropics. Furthermore, while inconsistent heterospecific neighborhoods in tropical forests have been used to argue for diffuse competition and neutral dynamics, these prior analyses have missed the fact that there is one thing an individual tree can almost always relay upon - interacting with a conspecific with whom strong negative interactions may promote deterministic community dynamics.
