Predicting plant vulnerability to drought in biodiverse regions using functional traits.

Attempts to understand mechanisms underlying plant mortality during drought have led to the emergence of a hydraulic framework describing distinct hydraulic strategies among coexisting species. This framework distinguishes species that rapidly decrease stomatal conductance (gs), thereby maintaining high water potential (Px; isohydric), from those species that maintain relatively high gs at low Px, thereby maintaining carbon assimilation, albeit at the cost of loss of hydraulic conductivity (anisohydric). This framework is yet to be tested in biodiverse communities, potentially due to a lack of standardized reference values upon which hydraulic strategies can be defined. We developed a system of quantifying hydraulic strategy using indices from vulnerability curves and stomatal dehydration response curves and tested it in a speciose community from South Africa's Cape Floristic Region. Degree of stomatal regulation over cavitation was defined as the margin between Px at stomatal closure (Pg12) and Px at 50% loss of conductivity. To assess relationships between hydraulic strategy and mortality mechanisms, we developed proxies for carbon limitation and hydraulic failure using time since Pg12 and loss of conductivity at minimum seasonal Px, respectively. Our approach captured continuous variation along an isohydry/anisohydry axis and showed that this variation was linearly related to xylem safety margin. Degree of isohydry/anisohydry was associated with contrasting predictions for mortality during drought. Merging stomatal regulation strategies that represent an index of water use behavior with xylem vulnerability facilitates a more comprehensive framework with which to characterize plant response to drought, thus opening up an avenue for predicting the response of diverse communities to future droughts.