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.