Integrating annual radial growth analyses and carbon isotope discrimination to forecast early warning of beech forest dieback across the Italian Peninsula

Tree mortality and forest dieback episodes are increasing due to drought and heat stress. However, a comprehensive understanding of the mechanisms enabling trees to cope with droughts remains lacking. Here, we employed a multi-proxy method utilizing tree-ring width, basal area increment (BAI) trends, and delta13C-derived intrinsic water-use-efficiency (iWUE) to unravel beech resilience against drought stress. We selected four sites spanning the latitudinal gradient and beech distribution in northern (Trentino-TRE), central (Lazio-LAZ), southern (Campania-CAM) and southernmost Italy (Calabria-CAL) with different climate conditions and soil water availability. First-order autocorrelation (AR1) analysis was performed to detect early warning signals for potential tree dieback risks during extreme drought events. Results revealed a negative correlation between vapour pressure deficit (VPD) and BAI, especially at southern latitudes. GAMM analysis showed a negative trend in BAI across most sites, stronger at the TRE site following the 2003 drought event. During this event, delta13C and iWUE increased with rising VPD, indicative of conservative water-use (lower stomatal conductance) and contributing to the decline in BAI. Conversely, CAM exhibited a steady increase in BAI and iWUE, likely influenced by rising atmospheric CO2 and water availability. LAZ site exhibited a decrease in delta13C, attributed to greater soil water holding capacity, enabling it to sustain higher transpiration rates. Conversely, southern sites presented higher iWUE, likely as high VPD initially reduces stomatal conductance but not the net assimilation rate, resulting in increased iWUE. Nevertheless, almost all sites exhibited a co-occurrence of increase in AR1 (except for CAM) and standard deviation, suggesting a reduction of resilience to future extreme events. Overall, multi-proxy, retrospective quantifications of BAI, iWUE and resilience provide a robust and complementary tool for differentiating water-use strategies and predicting tree growth decline and dieback, as well as identifying those that have the potential to survive in warmer and drier future conditions.