Can we expect more from Alnus glutinosa (L.) Gaertn. in a changing climate?

Black alder (Alnus glutinosa [L.] Gaertn.) is recognized as a key tree species in moist habitats, riparian ecosystems, and wetland restoration projects. The future success of A. glutinosa establishment is believed to be at risk due to the increasing frequency of high-temperature and drought events associated with climate change. However, some studies have indicated that alder may tolerate greater drought stress than previously assumed. In this study, a series of greenhouse experiments was conducted to assess the sensitivity of black alder to drought during seed germination and seedling establishment in its first year. The results showed that alder seeds can withstand unfavourable germination conditions (e.g., radiation, precipitation, heat) for periods exceeding 16 weeks and still germinate successfully thereafter. Consequently, the expected increase in spring and summer droughts due to climate change will not affect the success of alder germination in years with extended periods of precipitation. Additionally, germination success appears unaffected by the absence of cool spring temperatures. Warmer spring temperatures are necessary for germination and the presence of open mineral soil. Covering dry soil with litter and moss proved problematic, and as drought conditions worsen, there are no safe alternatives to exposed mineral soil. Shading was found to significantly enhance germination success. This allows alder to germinate in a changing climate in shaded locations that remain moist for longer. During germination on open soil, alder seeds did not exhibit specific requirements for soil moisture compared to other deciduous pioneer tree species. Furthermore, drought conditions did not significantly affect growth parameters, including shoot and root growth, root collar diameter, and biomass; however, there was a tendency toward reduced shoot length and belowground biomass under drought stress. Importantly, alder seedlings cannot survive dry periods exceeding 3-6 weeks without soil moisture during spring, summer, or autumn, consistent with other pioneer species such as Betula pendula, Salix caprea, and Populus tremula. These findings support the hypothesis that alder trees are more drought-tolerant than previously thought.