Structural and compositional acclimation of forests to extended drought: results of the KROOF throughfall exclusion experiment in Norway spruce and European beech

Drought effects on tree growth and mortality are widely studied, but scant knowledge exists on its impact on stand density, size variation, or mixing proportions. Grasping drought's influence on structural and compositional diversity is crucial for stand dynamics, ecosystem services, and silvicultural adaptation. We relied on KROOF, a 5-year throughfall exclusion experiment in a mature Norway spruce (Picea abies) and European beech (Fagus sylvatica) stand, to analyze its impact on structural and compositional attributes, including Stand Density Index (SDI), Growth Dominance Coefficient (GDC), and species mixing proportion. Our study demonstrates that drought-induced growth reduction and tree loss decreased SDI by 27%, mixing proportion by 41% at Norway spruce's expense, and homogenized stand structure. Furthermore, we reveal that stand density, mixing proportion, and structural diversity were more affected in Norway spruce, stabilizing growth at the stand level. Extended drought significantly altered growth partitioning in favor of smaller trees, with a 70% reduction in growth-size relationship slope and a 157% decrease in GDC. Species-level analysis indicated a stronger partitioning shift towards smaller trees, particularly in Norway spruce. We discuss that longer drought periods may trigger acclimation at tree and stand levels, potentially underestimated when based solely on individual drought years. Sustained stress could induce acclimation across various levels, from the stand to the species cohort, tree, and organ. Maintaining structural and compositional diversity may mitigate future drought stress effects on growth, mortality, and stand structure, as exemplified by the extended experimental drought. We suggest silvicultural approaches better attuned to natural processes amid climate change.