Could local climate, forest site and stand conditions mitigate the defoliation in European beech stands? – The case of southern Belgium

Although climate change has been a major factor in the declining health of European beech (Fagus sylvatica L.) stands since the end of the 20th century, it seems that local site and stand conditions may play a key role within the beech’s climax region by exacerbating or mitigating this phenomenon. In this study, we assessed crown defoliation in 1,515 dominant trees distributed across 101 mature beech stands, located along bioclimatic and ecological gradients in the Ardenne region (southern Belgium) during July and August 2022. A large dataset covering past and present climate, local forest site and stand conditions was analysed using a Random Forest model, combined with a variable selection (VSURF) and interpretated by the Shapley values method. The final model explained 45% of the observed variability in stand defoliation and identified five key drivers. Warmer spring conditions and moderate increases in temperature during the growing season were associated with reduced crown damages, reflecting climate conditions that remained suitable to beech. Local site conditions also modulated the health status of beech: stands located on well-drained plateaus and soils with a high available water capacity showed lower defoliation, while the presence of waterlogged or shallow soils increased damage level. The stand structure also played an important role, as higher canopy density limited crown damages, probably due to a buffering effect on the forest understorey’s microclimate. These results provided practical guidance for adaptive forest management aimed at increasing the resilience of beech forests to current climate change.