Crown–branching trade-offs mediate growth and resilience to drought, frost, and masting under moderate thinning in European Beech (Fagus sylvatica L.)

Abstract Key message Thinning boosts radial growth in European beech ( Fagus sylvatica L.), but more is not always better: moderate thinning offers the best vitality balance by keeping resilience comparable to heavily thinned trees, while reducing the sensitivity to late frost and the interannual growth variability and avoiding excessive branching in the crown. Context Forest management practices affect growth dynamics and crown architecture, yet their effects under increasing stress remain insufficiently understood. Aims This study compared the effects of thinning on basal area increment (BAI) of European beech at breast height, with a focus on growth sensitivity to fructification, drought, and late frost at the tree level. Specifically, we assessed whether moderate or heavy thinning improve drought resilience and we explored the role of crown architecture in growth responses. Methods The study was conducted in European beech-dominated stands in Rhineland-Palatinate, Germany, across three age classes (50, 70, 90 years) and three thinning treatments (control, moderate, heavy). In total, 125 trees were sampled. BAI was derived from increment cores, and crown structure from terrestrial laser scans processed into quantitative structure models. Linear mixed-effects modelling was used to analyse tree-level BAI and crown traits. We also assessed drought carry over effects and analysed the resilience index in relation to thinning. Results BAI, its relative and absolute interannual variation as well as crown volume increased with thinning intensity. Excessive branching promoted growth in small crowned trees, but limited it in large ones. Drought lag effects persisted for up to two years, particularly in unthinned plots, yet BAI resilience to early summer drought was equally high under both thinning treatments. Conclusion Thinning intensity strongly influenced crown architecture and stem growth, with heavier thinning promoting larger crowns and greater tree-level productivity, but at the cost of higher growth variability and frost sensitivity. Compared to moderate thinning, heavy thinning offers no clear vitality and growth resilience related advantages. Importantly, modelling radial growth as a function of crown volume and branching offers a more precise, tree-centred alternative to traditional thinning classifications.