OOS 36-7
A multiple species synthesis of tree mortality physiology – how prevalent are hydraulic failure and carbohydrate depletion?
Forests and other woody vegetation exert a strong influence on the earth’s carbon cycle, but these ecosystems are also vulnerable to die-off from climate change in many regions due to increased extreme events and drought in particular. As such, the feedback of a dynamic terrestrial carbon sink is a major uncertainty in projections of global climate change. For better assessment of woodland and forest loss and associated effects on the climate system, improved model inputs on how to predict tree mortality from drought are needed. Here we synthesize results for ~12 tree species from ~15 experiments and observational studies on the physiological process of tree mortality from drought. We examined the role of hydraulic failure (xylem cavitation by air embolism) and non-structural carbohydrate (NSC) depletion associated with carbon starvation in tree mortality caused by drought. We also examine the relative influence of these two physiological mortality processes as they relate to tree functional traits and drought resistance characteristics to provide a prediction framework for tree species where no data on drought mortality physiology is available.
Results/Conclusions
Hydraulic failure (defined as ~>75% loss of xylem hydraulic conductivity) was a nearly universal aspect of mortality physiology, while NSC depletion was not. However, a reduction of NSC in at least one tree tissue at death (relative to surviving control trees) was as common a response as unchanged or increased carbohydrates at mortality. Coniferous gymnosperm species were more likely to exhibit NSC reduction with mortality than angiosperm trees, but significant reductions were observed in some angiosperm tree tissues. The relative importance of NSC reduction was significantly correlated with wood density, especially for angiosperms. Trees with a higher wood density were less likely to experience NSC depletion, but instead died during drought only from hydraulic failure. We also explored the link between relative importance of NSC reduction during mortality and a species’ physiological ability to resist drought stress. This first global review of the tree mortality physiological mechanism can provide the information needed to parameterize and structure models of forest and woody vegetation response to climate change with an aim toward improved representation of terrestrial carbon budget dynamics.