Tree-Level Climate Sensitivity Reveals Size Effects and Impending Growth Decline in Silver Fir Affected by Dieback

Worldwide studies have related recent forest decline and mortality events to warmer temperatures and droughts, as well as pointing out a greater vulnerability to climate changes in larger trees. Previous research performed on silver fir (Abies alba Mill.) suggest an increasing decline and mortality, mainly related to rising water shortages. Here, we investigate these die-off events in two silver fir populations at the rear edge of the species in the western Pyrenees. We used dendrochronology to investigate tree age, size (diameter) and individual climate sensitivity (climate–growth relationships) as predisposing factors related to growth patterns and drought resilience indexes in canopy-level declining and non-declining trees. The regional climate was also investigated, including temperature trends, quantile regression in precipitation and frequency of extreme events in drought indexes (SPEI). The regional climate was characterized by an increase in mean temperatures and a higher frequency of extreme drought events in recent decades, without a decrease in total precipitation. Larger trees were more sensitive to temperature and prone to decline. Declining trees presented decreasing growth trends years ago, providing a robust predisposing trait. Both populations were not different in mean growth, despite the contrasting local climate and management legacies, although we identified a higher resilience to drought in the eastmost stand. A significant regression was found between growth trends and climate sensitivity, supporting that declining trees are more sensitive to warmer temperatures and drought. Hence, the results support a contrasting climate sensitivity related to tree size (but not to tree age), suggesting impending decline and mortality in large trees with higher temperature sensitivity (negative temperature–growth correlations). Nonetheless, contributing factors, such as the legacy of previous logging, should also be accounted for.