Alpine plants complete their seasonal phenological cycle during two to three snow-free months. Under climate change, snowmelt advances and the risk of summer droughts increases. Yet, photoperiodism may prevent alpine plants from benefiting from an earlier start of the growing season. To identify the drivers of flowering phenology in the seven main species of an alpine grassland, we experimentally shifted the snowmelt date through snow manipulations, and excluded precipitation during summer. With “time-to-event” models, we analysed the beginning of main flowering with respect to temperature sums, time after snowmelt, and calendar day (photoperiod). We identified two phenology types: four species tracking snowmelt dates directly or with a certain lag set by temperature sums, including the dominant sedge Carex curvula, Anthoxanthum alpinum Helictotrichon versicolor, and Trifolium alpinum, and three species tracking photoperiod: Geum montanum, Leontodon helveticus and Potentilla aurea. Photoperiodism did not act as daylength threshold but rather modulated the thermal sums at flowering. Hence, photoperiod delayed flowering after earlier snowmelt. The grass A. alpinum was the only one of seven species that clearly responded to drought by earlier and longer flowering. The remarkably high importance of snowmelt dates for both phenology types suggests an earlier onset of flowering in a warmer climate, particularly for non-photoperiod-sensitive species, with an increasing risk for freezing damages and potential disruptions of biotic interactions in the most frequent type of alpine grassland across the Alps. Consequentially, the distinct microclimate and species-specific responses to photoperiod challenge temperature-only based projections of climate warming effects on alpine plant species.

高山植物在两到三个无雪月份中完成其季节性物候周期。在气候变化下，融雪不断发展，夏季干旱的风险增加。然而，光周期作用可能阻止高山植物从生长季节的较早开始受益。为了确定高寒草原的七个主要物种中开花物候的驱动因素，我们通过雪的操作实验性地改变了融雪日期，并排除了夏季的降水。利用“事件发生时间”模型，我们针对温度总和，融雪后的时间和日历日（光照周期）分析了主要开花的开始。我们确定了两种物候类型：四种直接追踪融雪日期或由温度和温度设置一定滞后的物种，包括优势莎草草（Carex Curvula），黄花茅异燕麦云芝和车轴茅，和三种跟踪光照：今montanum，Leontodon瑞士和蕨麻罗非鱼。光周期不作为日长阈值，而是调节开花时的热量总和。因此，光周期在较早融雪后延迟了开花。高山草是七个明显对干旱产生早期和较长开花响应的物种中的唯一一种。对于两种物候类型而言，融雪日期的重要性非常高，这表明在较暖的气候下，尤其是对非光周期敏感物种而言，开花开始较早，在最常见的两种类型中，冻害和生物相互作用潜在破坏的风险增加。横跨阿尔卑斯山的高山草原。因此，对光周期的挑战仅基于温度，而对气候变化对高山植物物种的预测则表现出独特的微气候和物种特异性响应。