Response of Green Alder (Alnus alnobetula) to experimental drought: Impacts on stomatal conductance, photosystem II efficiency, and plant vitality

Over the past decades, green alder (Alnus alnobetula [Ehrh.] K. Koch) 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 duration, while monitoring stomatal conductance (gs) and quantum yields of photosystem II. Gs declined sharply once soil water potential fell below wilting point (ΘWP). The recovery was rapid after reirrigation, however, due to a post-drought legacy effect gs remained reduced for several weeks. The maximum quantum yield of photosystem II (Fv/Fm) only declined following prolonged drought stress. However, the correlation between effective quantum yield and radiation proved to be 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 at moderate drought, indicating a short-term, drought-induced adjustment in stomatal regulation. Even though the second prolonged drought caused substantial defoliation and was followed by high mortality the next spring, we conclude that combined short-term adjustments and long-term modifications could enable green alder to cope with drier conditions.