Do niche-structured plant communities exhibit phylogenetic conservatism? A test case in an endemic clade

The growing literature on the phylogenetic structure of plant communities places great emphasis on the role of phylogenetic niche conservatism (PNC) in community assembly. However, the patterns revealed by such analyses are difficult to interpret in the absence of independent data on niche structure. While there is increasing evidence that plant coexistence does depend upon niche differences, it is still not clear in most cases what the relevant niche axes are. We address this problem by testing for PNC within the African Restionaceae (restios), a clade endemic to the Western Cape where we have shown niche segregation along soil moisture gradients to be common. Significant niche segregation on soil moisture gradients occurred among restios in 7 of 10 communities sampled, but PNC was detectable in only one of these and then only by one of three methods used. Phylogenetic analysis of the evolution of hydrological niche traits for the species pool of 37 Restionaceae in the study showed tolerance of drought to be convergent rather than conserved. Synthesis. The demonstration that clear niche segregation may occur among related species without PNC being detectable supports the hypothesis that hydrological niche responses are evolutionarily labile. More generally, the results demonstrate that phylogenetic analysis can be a poor guide to the process of community assembly. We argue that it may in future be better to apply ecological data to the interpretation of phylogenies, rather than to follow the current preoccupation with the application of phylogenies to ecology.