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Abstract:
Widespread tree mortality associated with drought has been observed on all forested continents and global change is expected
to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere–atmosphere interactions of
carbon, water and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires
improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of droughtinduced
mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies
have investigated these mechanisms, but data have not been consistently analysed across species and biomes using a standardized
physiological framework. Here, we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at
drought-induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed
theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at
mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were
more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing
hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates
that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function.
