Tree-ring stable isotopes and growth trajectories reveal early warning signals of drought-induced Scots pine mortality.

Summer droughts have affected tree growth in central Europe since at least the 1940s, yet the physiological mechanisms behind why some trees die whilst others survive remain poorly understood. Here, we present absolutely dated and annually resolved tree-ring width, carbon (δ13C) and oxygen (δ18O) stable isotope chronologies from 18 Scots pines (Pinus sylvestris L.) growing near the species' climatic and edaphic limit in one of Germany's driest regions (Rhine Hesse, near Mainz). Spanning the period 1930-2019, sampled trees were assigned to three post-2018 vitality classes based on crown transparency: vigorous, intermediate and poor vigour (including dead individuals). We assessed long-term growth and isotopic trajectories in relation to climate variables, with a focus on pan-European summer drought extremes in 1947, 1976, 2003 and 2018. We found that trees with consistently higher growth rates and elevated long-term δ13C values were more susceptible to dieback, whereas surviving trees maintained lower δ13C values. This pattern suggests differences in long-term water-use strategies and/or drought exposure, whilst also partly reflecting canopy position and light-driven assimilation. In contrast, δ18O values sharply increased during drought events, especially in poor-vigour trees, indicating greater reliance on shallow, evaporatively enriched water sources and heightened hydraulic strain under extreme drought. These isotopic trajectories differentiated vitality classes well before visible canopy decline. Our findings indicate that the 2018 drought was not the sole trigger, but rather a tipping point in a long-term dieback process driven by repeated droughts and heatwaves. Tree-ring stable isotopes, particularly when interpreted alongside growth trajectories, provide valuable early warning signals of physiological stress and drought vulnerability. Since trees with conservative growth strategies were more resilient, long-term physiological stability, rather than maximum productivity, may enhance forest resilience under an increasingly warm, dry and variable future climate.