Thursday, August 6, 2009

PS 64-57: Phylogenetic scale of soil resource partitioning in tropical trees

Claire A. Baldeck, University of Illinois and James W. Dalling, University of Illinois.


Resource-based habitat partitioning is a prominent hypothesis for the maintenance of high species diversity in tropical forests. Previous studies have shown that soil nutrients are important axes for niche partitioning among tropical tree species, with the majority of species distributed non-randomly with respect to soil nutrients. However, the phylogenetic level at which niche partitioning mechanisms operate is not well understood. This study assessed the relationship between phylogeny and tree species sorting along soil nutrient gradients in five long-term tropical forest dynamics plots from the Center for Tropical Forest Science (CTFS) network. It was predicted that if a species’ soil resource requirement is a phylogenetically conserved trait, then closely related species will have more similar soil nutrient niches. The niche overlap (a value that indicates the similarity of two species’ soil resource use) was calculated for all pairwise combinations of co-occurring tree species for several soil nutrient concentration gradients, including nitrogen, phosphorus, and calcium. These pairwise niche overlap values were compared with the phylogenetic distance between pair members to determine whether more closely related species have higher niche overlap. 


Three of the five sites studied show that more closely related species pairs have greater niche overlap values than expected based on randomization tests. The remaining two sites show no conclusive pattern. The results show that closely related species tend to be more ecologically similar with respect to their soil resource niches, demonstrating phylogenetic niche conservatism. The results of this study also have implications for the phylogenetic stucure of tropical forest communities. This provides evidence that the sorting of tropical tree species along soil nutrient gradients is not phylogenetically independent, indicating that resource partitioning of soil nutrients occurs at higher taxonomic levels. The phylogenetic conservation of soil resource niches demonstrated in this study may explain the observed phylogenetic clustering of tree species at large scales in some CTFS plots: phylogenetic clustering of species is expected at spatial scales similar to that at which soil nutrients vary if soil resource niches are conserved traits.