Assessing drought-driven mortality trees with physiological process-based models

- A process-based model was used to simulate drought-induced mortality.
- Spatial variability in climate and soil conditions, and in functional traits were taken into account.
- Six different proxies related to carbon starvation and hydraulic failure were tested.
- Tree mortality was not caused by xylem embolism, but rather by depletion in carbon reserves.

(1) This study describes how physiological process-based models can be used to assess the mortality risk of forest trees under global change. (2) Using the CASTANEA model, we simulated the development over time of tree functioning with different ontogenetic and phenotypic characteristics (age, diameter, Leaf Area Index, leaf traits) and growing in different site conditions (elevation, soil water content). Based on this set of simulations, we determined the carbon and hydraulic physiological thresholds associated with tree mortality that best reproduce the observed mortality rate. (3) We tested this methodology on a long-lasting and patchy drought-induced mortality event of silver fir (Abies alba Mill.) in South-Eastern France. (4) We found that tree mortality was not caused by a massive summer xylem embolism, but rather by depletion in carbon reserves probably associated with bark beetle attacks. Simulation outputs also revealed that trees with high diameter and Leaf Area Index and growing on shallower soils were more prone to die. (5) This study highlighted that physiological process-based models can be of high interest to determine the factors predisposing and inducing tree death.