Simulating forest drought legacy-effects from tree hydraulic damage: An integrated modelling approach

The inclusion of tree hydraulic processes into ecosystem models provides opportunities to better capture instantaneous tree drought responses as well as drought legacy effects. Here we are presenting a simple tree hydrologic approach implemented into a process-oriented ecosystem model that simulates instantaneous tree water potential dynamics based on soil water availability and transpiration demand. Reductions in tree water potential are then calculated into a loss of hydraulic functioning leading to sap wood and leaf area losses. This affects within-tree allocation as tissue becomes damaged, and finally may result in tree death if either hydraulic function is impaired beyond repair or tissues for resource acquisition cannot be sufficiently recovered anymore. This approach further provides potential explanations for various medium- and long-term legacy effects of drought, as well as mortality rates in dependence on environmental conditions.

Here we describe the model and evaluate the approach at a number of different sites over several decades, illustrating the species-specific sensitivity to drought stress and the dependency on precipitation pattern, potential soil water storage, and specific tree physiological traits such as xylem vulnerability. The importance of considering stem water storage and depletion as well as the possibility to link this water pool to micro-dendrometer measurements for evaluation is emphasized. Also, we indicate a possible way to integrate the hydraulic failure hypothesis with the theory of carbon starvation and discuss which process may be dominating under specific environmental conditions.