COS 37-9
Using functional traits to predict the vulnerability to drought of 40 species in diverse Mediterranean-type shrubland communities of South Africa
Attempts to understand the mechanisms underlying plant mortality during drought have led to the emergence of a hydraulic framework describing distinct hydraulic strategies. This framework distinguishes species that respond to dehydration by rapidly decreasing stomatal conductance (gs), thereby maintaining high water potential (isohydric), from those that maintain high gs, thereby maintaining carbon assimilation albeit at the cost of hydraulic dysfunction (anisohydric). However, portraying the hydraulic framework as a dichotomy between isohydry and anisohydry may obscure important continuous variation in hydraulic regulation, particularly in complex communities with high functional trait diversity. The purpose of this study was to investigate the range of hydraulic strategies present across a large sample of species from a diverse community within South Africa’s Cape Floristic Region. Using stomatal response curves from laboratory-based drydowns for 40 species together with vulnerability curves, we developed a novel framework that assessed co-ordination between measures of vulnerability (P50 and Pmin) and the water potential inducing stomatal closure (Pg12). We used the differences between Pg12 and P50 and Pg12 and Pmin as proxies for stomatal regulation of cavitation and carbon limitation, respectively, and the percent loss of conductivity (PLC) at Pmin as a proxy for hydraulic dysfunction.
Results/Conclusions
Our data showed that species varied in the degree to which they regulated cavitation through stomatal closure and experienced seasonal declines in water potential and that these differences resulted in distinct predictions of mechanisms of mortality. Restioids had combinations of vulnerability to embolism, Pg12 and Pmin that led to high hydraulic failure. Proteoids maintained relatively low hydraulic dysfunction and low carbon limitation by maintaining relatively high Pmin. Ericoid species varied in vulnerability and seasonal PLC, resulting in two distinct predictions of mortality within this functional type. Erica species had a combination of Pg12 and Pmin that led to both high hydraulic failure and carbon limitation, while species of Asteraceae displayed more negative P50 values and consequently were predicted to suffer carbon limitation only. We also showed that a continuum existed in the degree to which diverse species in our Mediterranean-type community regulated cavitation through stomatal closure. There was strong agreement between predictions from our framework and those from long-term field-based studies in the same communities. This, together with the relative ease with which trait data can be produced, suggests that our novel framework will be valuable for predicting the response of diverse communities to future climate-change type drought events.