Ecological and evolutionary determinants of a key plant functional trait: wood density and its community-wide variation across latitude and elevation.

Wood density is an important trait in trees indicative of their life history and mechanical and physiological strategies. The following examines the evolutionary ecology of wood density using a large database for seed plants. In particular, we focused on the geographic and phylogenetic variation in wood density for both gymnosperms and angiosperms. A phylogenetic supertree was constructed for over 4600 taxa, allowing for comprehensive analyses of divergences across the seed plant phylogeny. Community-wide means and variances for wood densities were quantified for 171 standardized forest communities. Wood density was generally highly conserved across the entire seed plant phylogeny, yet large divergences were found within the rosid clade. Geographic and community variation in wood density, however, was significantly lower in temperate and high elevation communities, dominated by gymnosperms, than in tropical lowland communities, dominated by angiosperms, suggesting an increase in trait and, to some extent, clade filtering with latitude and elevation. Together, our results support the notion that both biotic and abiotic forces have been important in the evolution of wood density as well as in controlling the observed trait mean and variance across geographic gradients.