Do differences in xylem cavitation resistance and leaf turgor loss point explain differences in drought tolerance among tropical rainforest species?

Background/Question/Methods

Considerable uncertainty surrounds the fate of Amazon rainforests due to shifting precipitation patterns across the basin resulting from global climate change and regional land transformation.  Two long-term large-scale ecosystem drought experiments in the eastern Brazilian Amazon observed ~30% reductions in aboveground biomass in the experimental plots after three years with ~50% reductions in precipitation.  In this study, we analyzed the differing degrees of drought resilience exhibited among the species in the experimental plots.  Stem xylem vulnerabilities to cavitation and leaf turgor loss points were measured in upper canopy branches and leaves of four genera found at both experimental sites.  Leaf turgor loss point was used as a proxy for determining the sensitivity of stomatal conductance to a reduction in leaf water potential.  Each genus was placed a priori into one of four functional categories: drought tolerant versus intolerant based on the drought experiment results and early versus late successional based on wood density.  We tested the hypothesis that these two important traits for regulating the plant hydraulic system would be significantly different between each of the plant functional groups.

Results/Conclusions

Both xylem cavitation and stomatal closure occurred at a significantly higher water potential in the drought intolerant functional group compared to the tolerant functional group.  However, there were no significant differences in xylem vulnerability and stomatal sensitivity between the early- and late-successional functional groups.  These results suggest that these two traits are important for defining trade-offs associated with drought tolerance, but appear to be orthogonal to traits associated with successional status.  This more detailed understanding of how stomatal conductance and hydraulic failure varies between drought tolerant and intolerant tropical tree species will facilitate a more mechanistic representation of plant hydrology in terrestrial biosphere models, which in turn will improve our ability to accurately predict drought responses of tropical ecosystems as precipitation patterns shift in the future.