Increasing forest mortality and its drivers: Simulating central European forests under climate change

Abstract: Increasing tree growth and mortality rates in Europe are still poorly understood and have been attributed to a variety of drivers. This study aimed to relate increasing forest mortality rates in six central European countries to climate drivers (CO2 concentration, temperature and precipitation) from 1985-2015, using a process-based vegetation model. For this, the direct (e.g. mortality due to water scarcity) and indirect (e.g. enhanced competition leading to increased mortality) effects of changes in climate drivers on mortality were assessed. Using the LPJ-GUESS dynamic vegetation model (DVM), a dataset showing increased canopy mortality rates was aimed to be reproduced. Factorial simulations excluding changes in individual drivers were run to identify causes of simulated trends. The lack of relationship between simulated and observed canopy mortality rates were suggested to be linked to simplifications of the model. Also, a link to land use changes and increased harvest intensity, which cannot be captured by a model simulating natural vegetation is suggested. In most countries examined, an increased tree mortality could partially be attributed to increased competition, caused by faster tree growth and crowding. An exception was Switzerland, where a negative trend in canopy mortality was associated with its vegetation being simulated at a higher altitude and treeline advance was suggested to be responsible for decreasing mortality rates and increasing NPP. The main driver of increased competition mortality was identified to be the increase in CO2 concentration. Interestingly, changes in seasonal precipitation patterns caused an increase in water availability and a small increase in competition. Increasing nitrogen deposition partially increased competition trends, but it remains uncertain at which scale. Temperature (mainly increasing stress mortality) was the largest driver of mortality followed by CO2 concentration (mainly increasing competition mortality due to higher productivity induced by ‘CO2 fertilisation’), however, this assessment includes a large uncertainty. Further studies distinguishing between vitality mortality caused by direct and indirect resource stress (through neighbourhood crowding) are recommended to decrease that uncertainty. This study contributes to the understanding of the current changes in the global carbon cycle and sink, can help to adapt forest management practices to those changes and improves the understanding of the LPJ-GUESS model.