Physiological Responses of Green Alder (Alnus alnobetula) to Drought

Due to land-use change and the abandonment of mountain pastures, green alder (Alnus alnobetula (Ehrh.) K. Koch; former Alnus viridis (Chaix) DC.) has been reported to invade abandoned grassland in the Alps on a wide scale. Once restricted to north-facing slopes, with high water availability, it is now expanding into sites with impaired water availability. To identify possible restrains for a further expansion of the species, we evaluated drought tolerance using a greenhouse experiment where saplings were exposed to drought periods of different lengths, monitoring transpiration (E) and maximum (Fv/Fm) and effective (ϕ) quantum yield of photosystem II. E declined markedly once volumetric soil water content (SWC) dropped below 10%. After reirrigation E recovered quickly, but remained reduced for several weeks, indicating a post-drought legacy effect. Fv/Fm was rather insensitive to drought showing no significant changes until SWC fell below 5%. However, in correlation with photosynthetically active radiation (PAR), ϕ proved to be a useful indicator to detect moderate drought stress. When the plants were exposed to a second drought period, E reacted more sensitive to reduced soil water availability and was significantly reduced at a moderate SWC of 28%. Fv/Fm also showed an early decline at SWC of 15%, both indicating a short-term adjustment in stomatal regulation induced by the first drought. Plants lost 70% of their leaves after 12 days of SWC < 15% in the first drought. However, about 6 days after re-irrigation they started to grow new leaves. After the second prolonged drought the saplings had lost most leaves and less than a quarter survived the following winter. Green alder has shown the capacity to adapt to moderate drought, indicating a potential to persist on drier sites. This research was funded in whole by the Austrian Science Fund (FWF) (Grant-DOI: 10.55776/P34706).