Independent evolution of suites of leaf and root functional traits in grassland plants

A suite of correlated leaf traits known as the leaf economics spectrum explains most of the worldwide variation in leaf morphology and physiology, and fine root morphology and physiology follow a similar set of resource economic tradeoffs. While data on root functional traits are relatively scarce, some studies have found that corresponding leaf and root traits (tissue density, nitrogen concentration, lifespan) are correlated, and have assumed that these correlations are the result of selection for a coordinated above- and belowground resource uptake strategy. Traits vary both among and within communities of co-occurring species, but very little is known about how the among and within community components of leaf and root trait variation have arisen, how they covary along environmental gradients, or about the adaptive significance of trait correlations among and within communities. In this study, I used phylogenetic comparative methods to test for correlated evolution of the among and within community components of leaf and root trait variation of plants in grassland habitats in Alberta, Canada. Corresponding leaf and root traits related to tissue density, tissue thinness and plant size evolved in a correlated fashion when considering trait variation among co-occuring species within communities. However, suites of leaf and root traits evolved independently among species in different communities along a climate gradient. Within-community trait differentiation evolved earlier than did habitat associations or among-community trait variation. These results suggest that differential above- and belowground effects of selection may have imposed contrasting evolutionary pressures on leaf and root functional traits in different habitats. I discuss the importance of these findings for our understanding of the adaptive significance of leaf and root trait correlations, and the origins and importance of trait variation for species coexistence in different habitats.