Decline of European beech (Fagus sylvatica L.): impact of the local environment and climate change on beech forests in southern Belgium

Fagus sylvatica L. is one of the most widespread tree species in temperate forests of Western Europe which plays an important role in both ecological and economic terms. European beech stands were especially adapted to the past climate but have suffered a decline in last recent years. Climate change and the increased frequency of extreme events (droughts, heatwaves but also extreme rainfall) seem to be impacting their vitality even if the exact description of this phenomenon is still unclear.

In this context, we have monitored around 97 plots in southern Belgium in which we have evaluated the state of health of ten to fifteen dominant or co-dominant beech trees during the growing season (2022). To describe the state of health, we used the French method DEPERIS and a simplified version of the European ICP-Forests protocol. The aim of this study is to understand the decline phenomenon in its globality and how local and regional factors can influence it. For each plot, topographic, pedological, past and present climate, local tree environment and sylvicultural data were collected (raster layers or in the field) and considered explanatory variables for beech decline. Stand decline, described as the mean defoliation of trees on the plot, and individual tree defoliation are both considered in the analyses. A Random Forest model combined to an analysis of Shapley values allow to explain more than 40% of the variability in beech decline. The results show that (i) climatic factors, such as the rise of mean temperatures compared with the long-term climate of the past and the change in rainfall distribution, and (ii) abiotic factors, such as available water capacity and the trees’ direct local environment, are the most relevant explanatory factors. This research confirms the complexity and interactions between climate change and abiotic conditions in the decline of European beech.