Historical and ecological controls on phylogenetic diversity in Californian plant communities

Background/Question/Methods

We addressed the classic question of whether community diversity is determined from the “bottom up” by the breadth and partitioning of niche space, or from the “top down” by historical and evolutionary forces. Specifically, we contrasted local and regional explanations for the diversity of Californian plant communities using phylogenetic and functional analyses. Our “communities” were sets of four field plots that sampled alpha (within-plot) and beta (among-plot) sources of variation in diversity. We sampled 93 such communities, nested within 78 larger regions for which regional species pools could be independently estimated, spanning the California Floristic Province. We measured phylogenetic and functional diversity within plots and between plots on neighboring soils (non-serpentine and serpentine) and slopes (north and south). We also measured the phylogenetic diversity of regional species pools and analyzed them in terms of biogeographic groups. 

Results/Conclusions

We found no evidence linking the phylogenetic diversity of communities to within-plot functional diversity or among-plot beta diversity. Instead, we found that the phylogenetic diversity of communities depends on that of regional species pools. In turn, phylogenetically diverse pools were those with high proportions of species of northern biogeographic affinity, which have relatively mesic distributions and traits. This supports what we call the “climatic refuge hypothesis”, rather than the “biogeographic crossroads hypothesis”.