Drought induced tree mortality - new concepts to understand mortality mechanisms

Drought‐induced tree mortality is likely to increase in future as climate models forecast increased frequency of drought events together with higher air temperatures. It is thus important to understand if particular trees in given forest stands are predisposed to mortality and which factors cause in the end tree death. There are two main physiological mechanisms that are supposed to be the drivers for tree death: (1) hydraulic failure, mainly caused by embolisms in the hydraulic system when the plant and atmospheric water demand cannot be met anymore because the soil is too dry (2) Carbon starvation, which is caused when trees close their stomata in the leaves to avoid water loss. This, however, also restrict CO2 influx - as a consequence carbon assimilation gets limited. Both factors interact and can occur together. Additionally, biotic factors such as pests and diseases contribute to death in the weakened trees.
We have now proposed a conceptual model (Gessler et al. New Phytologist 2018) where we combine long-term tree ring (i.e., growth) information, isotope signals in the tree rings as well as information on the hydraulic system of dying trees to reconstruct the causes of mortality (i.e., hydraulic failure vs. carbon starvation).
With this "backcasting" approach it is possible to identify trait combinations that allow predicting vulnerability or resistance of trees to future drought conditions.