Widespread geographic and phylogenetic diversity of drought tolerance in grasses

Background/Question/Methods

Drought frequently reduces plant productivity, can induce widespread plant mortality, and limits the geographic distribution of many plant species. As climates warm and precipitation patterns shift in the future, understanding how drought has influenced the evolution of plants and their distribution will become even more central to our ability to predict future ecosystem functioning and patterns of biodiversity. These questions are especially pressing for the world’s eleven thousand grass species, which dominate a large fraction of the world’s land, yet are poorly characterized with respect to drought. To better understand the evolution and ecology of drought tolerance in grasses, we assessed physiological drought tolerance and a number of other leaf functional traits for 426 species of grass. To understand the potential for functional redundancy within grasslands, we examine the global bioclimatic patterns of grass physiological drought tolerance and its phylogenetic distribution. We then examine the relationships between physiological drought tolerance and other leaf functional traits to test whether changes in the prevalence of drought would impact ecosystem function beyond extending functioning to drier soils.

Results/Conclusions

Among grass species, there is a great range of ability to function at low water potentials and this physiological drought tolerance is well-distributed both geographically and phylogenetically. As such, changes in the prevalence of drought will not necessitate large migrations in grass species among grasslands or shifts among phylogenetic clades to maintain ecosystem function. Yet, the functional traits associated with physiological drought tolerance suggests that shifts in the relative importance of drought could alter the functional characteristics of grasslands. Physiologically drought-tolerant species did not conform to this general stress-tolerant syndrome as they had high not low rates of leaf gas exchange. In all, understanding the history and future of grasslands will require better understanding of the role of local environmental heterogeneity in the maintenance of functional diversity and physiological drought intolerance, whose benefits and ecological associations are unclear.