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 drought-induced 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.
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This research was supported by the US Department of Energy, Office of Science, Biological and Environmental Research and Office of Science, Next Generation Ecosystem Experiment-Tropics, the Los Alamos National Laboratory LDRD Program, the Pacific Northwest National Laboratory LDRD Program, The EU Euforinno project, the National Science Foundation LTER Program and EF-1340624, EF-1550756 and EAR-1331408, ARC DECRA DE120100518, ARC LP0989881, ARC DP110105102, the Philecology Foundation of Fort Worth, Texas, the Center for Environmental Biology at UC Irvine through a gift from D. Bren and additional funding sources listed in the Supplementary Acknowledgements. We thank A. Boutz, S. Bucci, R. Fisher, A. Meador-Sanchez, R. Meinzer and D. White for discussions on study design, analysis and interpretation of results, and T. Ocheltree for helpful comments on the manuscript. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the US government.
The authors declare no competing financial interests.
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Adams, H.D., Zeppel, M.J.B., Anderegg, W.R.L. et al. A multi-species synthesis of physiological mechanisms in drought-induced tree mortality. Nat Ecol Evol 1, 1285–1291 (2017). https://doi.org/10.1038/s41559-017-0248-x
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