Deconstructing the signal: Phylogenetic structure, elevation change, and the implications for species co-existence

Question: Is there evidence of temporal niche dynamics in the response of phylogenetic structure to environmental change? Data studied: We evaluated change in species composition of tree communities along a 600 m elevation gradient in the tropical deciduous forest of southwestern Mexico, censusing 43 transects of 100 individuals each, with transects conforming to strict criteria of similarity. Along this gradient, maximum annual rainfall increases threefold from low to high elevation, and temperature decreases by 3°C. Methods: (1) Within the regional-level phylogenetic tree, we located the source of the observed change in phylogenetic structure along the gradient. We did this by 'deconstructing' or splitting the full phylogenetic tree into two taxonomically distinct subtrees, and then conducting stepwise removal of taxonomic resolution and weighting (number of subtending taxa) for all three trees (full tree plus subtrees) starting at the tips. We then correlated all values of phylogenetic structure with elevation over the samples at each level of deconstruction. (2) We determined change in species composition with changing environmental conditions expected from an analytical model of temporal niche process, after which we correlated this change [mean and standard deviation of species-to-genus ratio (S/G)] with elevation and with the metric of changing phylogenetic structure. (3) We identified the genera most responsible for changing S/G and standard deviation of S/G. Conclusions: (1) Community phylogenetic structure responds significantly to the elevation gradient, with signal located at the genus level and below. The latter result suggests that species interactions are primarily responsible for the patterns observed. (2) The observed relationship between elevation and both S/G and standard deviation of S/G accounts for all or most of the variation in phylogenetic structure and is consistent with the action of temporal niche dynamics - a pattern well illustrated by four influential, multi-species genera that collectively contain more than 20% of the individuals included in the study.