Forest tree neighborhoods are structured more by negative conspecific density dependence than by interactions among closely related species

Interactions among neighbors influence the structure of communities of sessile organisms. Closely related species tend to share habitat and resource requirements and to interact with the same mutualists and natural enemies so that the strength of interspecific interactions tends to decrease with evolutionary divergence time. Nevertheless, the degree to which such phylogenetically related ecological interactions structure plant communities remains unclear. Using data from five large mapped forest plots combined with a DNA barcode mega‐phylogeny, we employed an individual‐based approach to assess the collective effects of focal tree size on neighborhood phylogenetic relatedness. Abundance‐weighted average divergence time for all neighbors (ADT_all) and for heterospecific neighbors only (ADT_hetero) were calculated for each individual of canopy tree species. Within local neighborhoods, we found phylogenetic composition changed with focal tree size. Specifically, significant increases in ADT_all with focal tree size were evident at all sites. In contrast, there was no significant change in ADT_hetero with tree size in four of the five sites for both sapling‐sized and all neighbors, even at the smallest neighbourhood scale (0–5 m), suggesting a limited role for phylogeny‐dependent interactions. However, there were inverse relationships between focal tree size and the proportion of heterospecific neighbors belonging to closely related species at some sites, providing evidence for negative phylogenetic density dependence. Overall, our results indicate that negative interaction with conspecifics had a much greater impact on neighborhood assemblages than interactions among closely related species and could contribute to community structure and diversity maintenance in different forest communities.

Lei Chen, Liza S. Comita, S. Joseph Wright, Nathan G. Swenson, Jess K. Zimmerman, Xiagcheng Mi, Zhanqing Hao, Wanhui Ye, Stephen P. Hubbell, W. John Kress, Maria Uriarte, Jill Thompson, Christopher J. Nytch, Xugao Wang, Juyu Lian, & Keping Ma