The cumulative effect of phylogenetic relatedness on the assembly of forest tree neighborhoods
Closely related plant species tend to share habitat and resource requirements and to interact with the same mutualists and pests so that the strength of interspecific interactions tends to decrease with evolutionary divergence time. However, the degree to which such phylogenetically conserved ecological interactions structure plant communities remains unclear.Using data from five large mapped forest plots combined with DNA barcode phylogenies, we employed an individual-based approach to explore the cumulative consequences of evolutionary relatedness on forest community structure.Specifically, for each individual of each focal species, we calculated abundance-weighted average divergence time for all neighbors (ADT) and for heterospecificneighbors only (ADT'). We then tested whether these metrics of neighborhood relatedness changed with increasing focal tree size.
We observed greater ADT in the neighborhoods of large trees relative to those of small trees (i.e., a decrease in phylogenetic relatedness), but it was only significant over short distances (<10m). In contrast, we did not find a universal relationship between tree size and ADT' (i.e. when only heterospecific neighbors were included), suggesting that decreases in neighbourhood relatedness with tree size are due predominantly to conspecific negative density dependence and not to negative interspecific interactions. Nevertheless, when we divided neighbors into different phylogenetic categories, we found larger trees not only had significantly fewer conspecific neighbors, but also had significantly fewer closely related heterospecific neighbors and significantly more distantly related heterospecific neighbors. These relationships were consistent across five contrasting sites from Asia and the Americas, including temperate, subtropical, and tropical forests. By contrasting neighborhood composition over tree ontogeny, our findings suggest that phylogenetic-dependent negative interactions do structure tree communities at local spatial scales, but the effect of conspecific neighbors largely drives observed patterns.