Simulation of drought-induced forest mortality with the LandscapeDNDC ecosystem model

Drought stress causes widespread forest mortality globally, for instance recently in temperate forests of Central Europe during and after the extremely dry summers in 2018, 2019 and 2022. Trees may die in consequence of hydraulic damage from xylem embolism, but also due to long-term effects caused by allocation into repair and adaptation that may deplete carbon (C) reserves, reduce competition strength and lower resistance to subsequent insect and pathogen infestations. We review implementations of drought-induced tree and forest mortality in ecosystem models and test different implementations in LandscapeDNDC, a terrestrial ecosystem model designed for simulations of the C and nitrogen cycles at site and regional scales. Based on tree hydraulic processes recently integrated into the model, we simulate tree mortality either a) when a threshold in xylem hydraulic conductivity loss is exceeded, or b) when tree water storage is depleted. In addition, we consider c) tree mortality as a result of depleted C reserves and low growth efficiency caused by drought legacy effects. Direct and legacy effects of drought stress on tree mortality rates are parameterized for common European temperate tree species (Fagus sylvatica, Picea abies, Pinus sylvestris, Quercus robur). We evaluate our simulations of drought-related tree mortality rates by comparing them to estimates from forest inventory and remote sensing approaches covering recent drought events. An improved modelling of direct and lagged drought-induced forest mortality is essential to understand the response of the vegetation C cycle to climate change and the options of forest management to increase the resistance of European temperate forests to drought.