Coupled decline in growth and fine-branching signals the onset of drought-induced European beech mortality.

Severe and recurrent droughts have caused widespread canopy decline and mortality in European forests. Drought-induced mortality occurs when structural damage and physiological dysfunction impair a tree's ability to recover, yet it remains unclear when trees transition from reversible stress to irreversible vitality loss at the whole-tree level. We combined detailed information on fine-branching architecture derived from terrestrial laser scanning with radial growth data from 174 trees to assess how structural and growth decline relate to drought-induced mortality in European beech (Fagus sylvatica). We found that European beech trees cross a critical tipping point under drought stress when pronounced declines in fine-branching and radial growth occur together. Mortality probability increased sharply once relative twig length (= fine-branching) dropped below c. 325 m m-3 and relative growth fell below c. 7%, equaling a ≥ 75% mortality probability within 3 yr, whereas neither indicator alone reliably captured this transition. Our results demonstrate that drought-induced vitality loss in an angiosperm follows distinct trajectories but converges on a critical zone of coupled crown and growth decline that marks an 'area of no return.' Linking crown deterioration with growth performance provides a mechanistic early-warning signal of irreversible vitality loss under intensifying climate extremes.