Consistent growth responses of silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.) to drought in mixed and monospecific forests: Insights from Central European forests

Ongoing climate change, characterized by an increased frequency of extreme weather events, including severe droughts, negatively impacts forest ecosystems. However, tree responses differ across tree species and stand composition (mixtures vs. pure stands), which may serve as an important management strategy for forest adaptation to climate change. To investigate the effect of stand composition on drought resilience, we conducted research on forest ecosystems in the Polish Outer Carpathians at the eastern edge of the distribution range of two important European tree species, silver fir (Abies alba) and European beech (Fagus sylvatica). The aim of our study was to identify climatic factors influencing the growth of these species and to examine their drought sensitivity and productivity. For this purpose, we sampled 100 fir and 100 beech trees from both mixed and monospecific stands. We applied a dendroclimatological approach to determine temporal stability of climate-growth relationships. Lloret's resilience indices were used to quantify the response of tree growth to episodic drought stress, while to derive differences in productivity, we applied the generalized additive mixed models (GAMMs). Our results showed species specific climate sensitivity which remain consistent regardless of mixture only for fir. Fir responded positively to higher temperatures, especially during spring, but required more precipitation, particularly in summer. Beech exhibited lower sensitivity to climate in mixed than in monospecific stands and exhibited higher resistance than fir. Importantly, species responses to droughts were driven by the duration of specific drought event and the timing in which it occurred. Furthermore, our results revealed that mixed stands were more resistant to extreme drought events. Fir showed higher resistance to extreme drought events in mixed stands than in pure stands. Overall, silver fir exhibited higher productivity than European beech. Especially in younger cohorts, it was higher for pure stands, but fir from mixed stands exhibited increasing productivity with age and delayed culmination. Our findings indicate that both species can form resilient stands in the studied region, making them viable for forest management in the face of climate change. Mixtures of fir and beech are particularly valid option to promote forest sustainability and productivity in mountain regions. Managed mixed stands of Abies alba and Fagus sylvatica demonstrated advantages in productivity and stability under extreme drought conditions. However, varying intensities of management interventions and structural differences between stands and among studies complicate direct comparisons, highlighting the need for more standardized approaches to analyzing species responses and management strategies in the future.