Drought-Induced Dieback Emerges From Microsite-Driven Functional Divergence in European Beech

Why neighbouring trees exposed to the same climatic extremes follow contrasting decline trajectories remains a central question in understanding climate-driven forest dieback. Here, we reconstructed the processes underlying drought-induced decline in a rear-edge European beech (Fagus sylvatica) forest by integrating dendroecology, wood anatomy, stable isotopes, leaf physiology, fine-root traits and soil fungal community analyses. Tree vulnerability was primarily linked to long-term functional divergence driven by microsite heterogeneity rather than recent climatic stress. Vigorous trees occupied deeper soils and experienced higher local competition, exhibiting safer hydraulic architecture, greater stem storage capacity, deeper water uptake, lower long-term intrinsic water-use efficiency and stronger ectomycorrhizal mediation of resource acquisition. In contrast, trees on shallower soils and under low competition displayed more vulnerable hydraulic trajectories, reduced access to deep water sources and weaker drought buffering. These differences were detectable decades before growth trajectories diverged. Extreme droughts and compound drought-frost events exposed pre-existing vulnerabilities. Following a critical drought-frost event in 1995, declining trees failed to recover and entered a self-reinforcing cascade involving crown dieback, reduced carbon assimilation, constrained growth, diminished formation of conductive and storage tissues, and weaker carbon allocation to ectomycorrhizal fungi. The contrasting trajectories emerged from organ-specific adjustments, rather than from a uniform shift along a single whole-plant strategy. Our results show that forest decline often reflects the delayed expression of long-standing vulnerabilities shaped by fine-scale microsite heterogeneity.