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

Over the past decades, green alder (Alnus alnobetula (Ehrh.) K. Koch) has spread rapidly across the Alps. Once restricted to moist north-facing slopes, it is now spreading into areas with limited water availability. Despite its growing ecological relevance in alpine environments, its physiological limits under drought conditions are largely unknown. To assess the species’ drought tolerance, we conducted a greenhouse experiment in which saplings were subjected to drought periods of varying duration, while monitoring transpiration rates (E) and the maximum (Fv/Fm) and effective (Y(II)) quantum yields of photosystem II. During drought, transpiration rates (E) declined markedly once volumetric soil water content (SWC) dropped below 10%. Recovery occurred quickly, with the rate depending on the length of the drought period. A. post-drought legacy effect caused E to remain reduced for several weeks after reirrigation. Fv/Fm proved to be rather insensitive to drought showing no significant changes until SWC fell below 5%. However, the correlation between Y(II) and photosynthetically active radiation (PAR) proved to be a useful indicator to detect moderate drought stress. When recovered samples were exposed to a second drought period, an adaptation in stomatal control was detected. E significantly decreased already at moderate drought (SWC 27%) and approached zero at SWC of 13%, indicating a short-term adaptation in stomatal control induced by the preceding drought. Also Fv/Fm decreased already at SWC of 13%, which also indicates an adaptive response after the initial drought. Although the second drought led to substantial leaf loss and resulted in mortality rates exceeding 85% by the following spring, we conclude that long-term adaptation could enable green alder to persist under drier conditions and establish on sites with higher soil moisture variability.过去数十年间，绿桤木（Alnus alnobetula (Ehrh.) K. Koch）在阿尔卑斯山脉快速扩散。此前该物种仅局限于湿润北坡，如今已蔓延至水资源有限的区域。尽管其在高山环境中的生态重要性日益凸显，但目前对其在干旱条件下的生理极限仍知之甚少。为评估该物种的耐旱性，我们开展了一项温室试验，对幼树施加不同时长的干旱处理，同时监测蒸腾速率（E）以及光系统II的最大量子产额（Fv/Fm）与有效量子产额（Y(II)）。干旱处理期间，当体积土壤含水量（SWC）降至10%以下时，蒸腾速率（E）显著下降。复水后植株恢复迅速，恢复速率取决于干旱时长。干旱遗留效应会使得复水数周内蒸腾速率（E）仍维持在较低水平。光系统II的最大量子产额（Fv/Fm）对干旱相对不敏感，在体积土壤含水量（SWC）降至5%之前均无显著变化。然而，有效量子产额（Y(II)）与光合有效辐射（PAR）的相关性可作为检测中度干旱胁迫的有效指标。当复水后的植株再次经受干旱处理时，我们检测到其气孔调控产生了适应性变化。此次试验中，蒸腾速率（E）在中度干旱（体积土壤含水量SWC为27%）时即出现显著下降，当SWC降至13%时趋近于零，这表明前期干旱诱导了气孔调控的短期适应性。光系统II的最大量子产额（Fv/Fm）在SWC为13%时即出现下降，这同样表明初始干旱后植株产生了适应性响应。尽管第二次干旱导致了大量叶片脱落，并在次年春季死亡率超过85%，但我们认为，长期适应性可使绿桤木在更干旱的条件下存活，并在土壤水分变异性更高的生境中定植。