Tree size diversity can enhance the drought resilience of Abies alba Mill. in the European mountain forests

Abstract

The increasing frequency and severity of extreme events, such as drought, are expected to disturb forest ecosystems worldwide. Stand structure, including tree size diversity, may play a crucial role in how forests respond to these changes. This study examines the effect of tree size diversity on the drought resilience of silver fir (Abies alba Mill.) using data from 138 circular plots in even- and uneven-aged mountain stands across Germany, Italy, and Poland. Increment cores from nearly 600 trees were evaluated to calculate complementary resilience indices. Generalized linear mixed-effect models were fitted to assess how tree size diversity affects individual tree growth response to drought stress under varying environmental conditions, mediated by admixture of broadleaf species, stand density, and individual tree size. We found that tree size diversity improves the growth response of silver fir to drought stress, expressed by higher resistance and stress-driven deviation (SDD) indices, in more water-limited sites. However, this benefit diminishes or becomes slightly negative with increasing climate humidity. Similarly, smaller trees demonstrated higher resistance and SDD, although these effects also weakened with more favorable water conditions. The admixture of broadleaf species and stand density did not mediate the impact of tree size diversity on growth resilience. Our results are in line with the stress-gradient hypothesis. Ecophysiological adaptations to prevailing conditions can lead to different competition regimes (symmetric and asymmetric), causing variations in the impact of stand structure on drought resilience. Since tree size diversity is crucial in water-limited environments, it can be considered a strategic forest management tool for adapting silver fir-dominated forests to anticipated global changes.