The ecological and economic consequences of shifting snowmelt dates and summer drought in the Alps

Alpine regions provide multifold ecosystem services that include an exceptional biodiversity, erosion control of soils, the regulation of the water balance, and energy production through hydropower. Moreover, their scenic views attract many tourists, adding to their economic value. In alpine regions, climate change advances particularly fast, with a 1.5-fold higher warming rate than the global average. In the temperate regions, a persistent snow cover is characteristic for the alpine zone above the upper natural treeline. This cover not only protects alpine plants from freezing air temperatures, it also determines the length of the growing season. Rising temperatures may therefore substantially affect these ecosystems. Additionally, the frequency and intensity of future summer droughts are expected to increase. Consequences could comprise reductions in the biomass production and a deceleration of the decomposition and mineralization, and thus, limited nutrient availability could arise.
The analysis of measured snow depth at 38 stations in the Swiss Alps (1055-2555 m asl) showed that between 1958 and 2019, the snowmelt date advanced by 2.8 days per decade. This advance was greatly driven by a strong shift during the late 1980s and early 1990s. Air temperature had a 1.5-fold higher impact on the snowmelt date than the height of the snowpack. At high elevations, where the snowpack is generally high, the effect of temperature was even more pronounced. Thus, the anticipated increase in winter precipitation will not counterbalance the rising air temperatures. As a consequence, a one-month earlier snowmelt at 2500 m asl by the end of the 21st century will prolong the alpine growing season by one third of its current length (high-emission scenario RCP8.5).
In a unique snow manipulation and summer drought experiment at 2500 m asl, a late successional alpine grassland was exposed to future climatic conditions. Shorter daylengths (photoperiod) at the start of the growing season may prohibit alpine plant species from profiting from earlier snowmelt. A time-to-event analysis disclosed two phenological types. The dominant sedge Carex curvula belongs to the group of species that tracks snowmelt dates directly. Flowering is induced a certain time after snowmelt or after the accumulation of a species-specific amount of thermal energy (degree hours). In the second group of species daylength (photoperiod) had a modulating effect on the amount of degree hours accumulated at the time of flowering. Hence, shorter photoperiods after early snowmelt delayed their flowering. The high importance of snowmelt dates for all species underlined the adaptiveness of alpine plant species to their microclimate and the large yearly fluctuations in snowmelt dates.
In contrast to the phenology, the biomass production clearly remained unaffected by a shift in snowmelt dates. But rain exclusion increased the below-ground allocation of biomass production. A drought during the first half of the growing season increased the below-ground production (+19%, in-growth cores) without affecting the above-ground production. A drought spanning almost the entire growing season (10.5 weeks) reduced the above-ground biomass production by 19%, with no effect on the below-ground biomass.
To address the important question of nutrient limitation under drought, litter bags (on-site litter, maize litter enriched in 15N and Rooibos tea bags) were buried. Most of the decomposition occurred during the winter months (37.1% mass loss for on-site litter), with no further mass loss over the subsequent growing season. By means of the labelled maize litter, it could be shown that the nitrogen uptake of Carex curvula was unhindered by summer drought, even in combination with early snowmelt.
Besides ecological consequences, climate change in the Alps will also induce economic consequences. An in-depth analysis of the ski resort Andermatt-Sedrun-Disentis revealed an overall high-resilience against climatic changes throughout the 21st century. The resort profits from multiple entry points and a recent expansion of their snowmaking facilities. Nevertheless, unabated greenhouse gas emissions (RCP8.5) would increase the water consumption for snowmaking by 23% by the mid-21st century, and by 79% by the end of the century. Even with this intensified snow production the resort will anticipate economic losses, particularly over the Christmas holidays.
Conclusively, the studied late successional alpine grassland is highly resilient against future climatic changes with earlier snowmelt and recurring summer droughts. This conclusion bases on observations of three years with snow manipulations, two of which were combined with summer droughts. A more direct impact of climate change in alpine regions is on the skiing industry. As most of the larger high-elevation ski resorts of the alpine arc, Andermatt-Sedrun-Disentis is well equipped to mitigate snow scarce winters by means of snowmaking. The intensified water consumption caused by ski resorts and other stakeholders calls for an overarching water management strategy that incorporates the future development of water usage.