The role of hydraulic failure and avoidance thereof in drought-induced mortality
Forest mortality appears to be accelerating non-linearly at regional to global scales, and predictions suggest this acceleration (increasing rate per unit time) will ramp up in upcoming decades. Our hydraulic framework proposed in 2008 suggests that two primary, interdependent mechanisms underly vegetation vulnerability to heat and drought: hydraulic failure and carbon starvation. Much research has occurred since 2008 that has shed light on these processes and their involvement with mortality. Much uncertainty exists, however, in the relative roles of these processes and other processes such as biotic agent amplification. Here I will review the state of the knowledge on these processes and their utility for modeling future forest mortality.
There is strong evidence for both hydraulic failure, carbon starvation, and their interaction from observational, experimental, and modeling based studies. The relative roles of these processes appears to vary though not necessarily in a (yet) predictable manner. Part of the unknown variation lies in semantics, some in methods (e.g. pot vs field studies), and part in true biological and climatic variation. Incorporation of these processes and their interdependencies into process models has yielded relatively accurate mortality simulation. Forecasts from these models consistently predict far more mortality of northern hemisphere conifers will occur in upcoming decades. Much more research is mandated to resolve the relative roles of these processes, to expand our knowledge into other more biomes, and to reduce the uncertainty in our predictions of future forest mortality.