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

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