Opposing trait drivers of phylogenetic community assembly across a subalpine elevational gradient

Background/Question/Methods

Patterns of species diversity have long been used to ask questions about ecological assembly of communities.  However, species diversity alone does not illuminate the ecological and evolutionary mechanisms generating that diversity.  Recently, phylogenetic and functional diversity have been added to traditional species or taxonomic diversity to infer particular environmental or biotic factors that shape communities. Along an elevational gradient in the subalpine meadows of the Rocky Mountains, angiosperm communities become increasingly phylogenetically overdispersed at higher elevations, despite the arguably harsher climate near treeline.  What ecological mechanisms drive this pattern?  To answer this question, we assembled a trait database of more than 20 physiological, phenological and floral traits, representing major axes of plant diversification, for the species along the elevational gradient.  We first checked to see which traits were phylogenetically conserved. We then compared patterns of taxonomic, phylogenetic and functional trait richness, turnover and dispersion across the elevational gradient.  Including traits in our analysis allows us to infer which ecological processes create the species and phylogenetic diversity we observe along the elevational gradient.

Results/Conclusions

We find that traits are variably conserved, but suites of traits (physiological, phenological, and floral) are highly conserved.  Richness or alpha diversity patterns converge across elevation for all three diversity types (taxonomic, phylogenetic, and functional).  Turnover or beta diversity patterns are also consistent for each diversity type.  However, phylogenetic and functional trait dispersion patterns differ across elevation.  We find that floral and phenological traits become overdispersed at higher elevations, mirroring the phylogenetic pattern, while foliar %C becomes clustered.  This indicates that plant communities face multiple selective pressures. In particular, at the highest elevations where the growing season is very short, plants must have leaves that grow quickly and are relatively inexpensive.  At the same time, during the abbreviated growing season, there may be pressure to diversify floral traits in order to attract pollinators. These results suggest not only that changes in climate affect plant community assembly at high elevation, but that climate change may affect plant-pollinator interactions, which in turn structure high elevation plant communities.