Photosynthetic Performance of Green Alder (Alnus alnobetula) in Experimental Drought

Over the past decades, green alder (Alnus alnobetula) has spread rapidly across the Alps, expanding from moist, north-facing slopes into areas with limited water availability. Despite its growing ecological relevance in alpine environments, the species' physiological limits under drought are largely unknown. To evaluate the species' drought tolerance, we conducted a greenhouse experiment in which juvenile plants were subjected to drought periods of different durations, while monitoring stomatal conductance (gs) and quantum yields of photosystem II. Once the soil water potential fell below the wilting point (Θ_WP), gs declined to less than 25% of the control level. After re-irrigation, gs remained reduced for several weeks, indicating a post-drought legacy effect. Under prolonged drought, the maximum quantum yield of photosystem II (Fv/Fm) declined to 20% of control values. The correlation between effective quantum yield and radiation proved a useful indicator for moderate drought stress. During recurrent drought, plants showed an acclimation to dry conditions, and both gs and Fv/Fm declined significantly (p < 0.01) at moderate drought, indicating a short-term, drought-induced adjustment in stomatal regulation. Although green alder showed signs of acclimation under repeated drought, recurrent severe drought led to high mortality in juvenile plants, indicating vulnerability at drought-prone sites.