Demographic and Structural Variability Modulate Growth Dynamics in European Beech Primary Forests

Intensifying droughts and heatwaves are increasing hydraulic constraints on European beech (Fagus sylvatica L.) and driving recent tree vitality losses at its warmer and drier range margins. Network-wide growth models predict widespread beech growth declines under future warming, yet these models rarely account for intraspecific demographic variability, potentially underestimating the adaptive capacity of natural beech forests. We leveraged a network of 530 plots and ~11,000 trees from primary beech forests covering a broad environmental gradient in central and southeastern Europe to assess how demographic variability in growth dynamics modulates forest productivity. We explored demographic differences in temporal and spatial variation in radial growth patterns and growth sensitivity to climate warming. Previous summer maximum temperatures and climatic water balance were the dominant climatic constraints of radial growth in xeric regions, whereas growth in mesic regions was more closely associated with current spring and summer conditions. Large trees exhibited stronger sensitivity to prior-summer heat stress, while small trees had stronger associations with spring and summer moisture availability. Severe summer droughts caused pronounced multi-year growth legacies across the demographic strata only in xeric regions. Long-term growth trend analysis revealed substantial regional heterogeneity, with growth declines in mid-sized and large trees contrasted by gains in small and mid-sized young (mesic regions) and old trees (xeric regions). Recent growth declines were linked to rising summer temperatures and declining water balance across the forest strata. Overall, productivity losses outweighed gains across the study network under climate warming. Stand structural complexity and demographic variability in growth dynamics were key drivers of ecosystem productivity, but their positive effects diminished under severe climatic stress. Our findings highlight the critical role of demographic and structural heterogeneity in shaping beech growth dynamics under climate warming, and caution against inferring future ecosystem productivity trajectories without explicitly accounting for demographic variability.