Flowering phenology in alpine grassland strongly responds to shifts in snowmelt but weakly to summer drought

Abstract

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.

Growth of alpine grassland will start and stop earlier under climate warming

“…Nevertheless, it was speculated that an unusually short photoperiod may prevent growth in early spring 31 . But in contrast to flowering 6,7,32 , there is little evidence that vegetative growth of alpine plants is delayed by photoperiod in spring. We observed normal growth rates with a day-length of 14.5 h (1-1.5 months ahead of the natural season start) and previously even initiated typical spring growth using an 11.5 h-photoperiod for the same vegetation type (unpublished data).…”

“…By the end of the century, snowmelt is expected to occur up to one month earlier in the Swiss Alps 5 and autumn warming may further prolong the growing season length (GSL). Early release from snow cover commonly advances flowering phenology in many alpine species 6,7 , but less is known about how a longer growing season affects the temporal dynamics of growth and senescence 8,9 .…”

Land cover classification of treeline ecotones along a 1100 km latitudinal transect using spectral‐ and three‐dimensional information from UAV‐based aerial imagery

“…Thus, season seem to be of a greater importance at lower elevations and latitudes for distinguishing vegetation types. The length of the seasonal phenological cycle for plant communities at higher elevations is affected by snow cover duration and lasts normally between 2 and 3 months (Vorkauf et al., 2021). The shortness of the growing season might explain why season had no influence on the classification accuracy.…”

Why Is the Alpine Flora Comparatively Robust against Climatic Warming?

“…These species-specific phenology patterns may not be obvious in 'normal years' but become visible under extreme situations, with very early snowmelt or very late onset of winter. Photoperiod and, in part, an internal clock in winter-active species (e.g., [22]) ensure survival and fitness, but also constrain benefits of a warmer climate for which the genetic setting had not been selected for ( [23], see below).…”

“…In a recent alpine snow removal/snow addition experiment in combination with the natural year-to year variation of snowpack, Vorkauf et al [23] showed that alpine grassland plants in the Swiss central Alps that do not belong to either of the above habitat types largely fall into two categories, those that track the date of release from snow and respond to temperature sums, and those that follow temperature only once the photoperiod indicates a safe period. The earlier, that is, the further away from that ecotypic photoperiod window they become released from snow, the more strongly the photoperiod restricts the onset of flowering.…”