Erika Hiltbrunner

Aspect and steepness with ridges and depressions of different spatial extent, hence, the topography of high-elevation terrain affects the incidence angle of sun rays, the influence of wind, the redistribution of snow and the sorting action of gravity for water, nutrients, diaspores and sediments. These factors jointly shape life in the treeless alpine world, beyond the influences related to the strong vertical profile of the air temperature with a mean lapse rate of 0.55 K 100 m ⁻¹ . On top of diverse soil types, the resulting thermal habitat conditions within a given elevation belt substantially exceed the current (+2.9 K) and future (+4 K) climatic warming in the European Alps. To account for the full spectrum of life conditions, the spatial scale of analysis is essential. We show that pronounced topography effects on microclimate on nunatak-type cliffs above the trimline of glacial ice shields could most likely not fully counteract the then lower and hostile temperatures. Life conditions across snowbeds exemplify the effect of season length on alpine plant’s phenology and growth over short distances, with halved aboveground biomass production for a shortening of the season by two to three weeks. Furthermore, we highlight potential pitfalls in microclimate assessments of topography effects in open terrain, but also explain how to improve such measurements. Throughout this review, we emphasize the interactive effect of plant canopy structure (height, density) with topography, mutually creating life conditions not predictable from standard meteorological sources, but responsible for the over-proportional plant species richness of the alpine flora globally.

Earlier snowmelt and more frequent summer drought due to climate warming are considered particularly influential for extratropical alpine plants, which are adapted to a short growing season and high water availability. Here, we explored the combined effects of the two drivers with a field experiment in late‐successional alpine grassland in the Swiss Alps (2500 m a.s.l.) over 6–7 years. We advanced and delayed snowmelt by removing and adding snow to experimental plots prior to natural snowmelt for 7 years and combined this treatment with 5 and 10 weeks of summer drought for 6 years. We measured plant biomass formation, community composition and ecosystem respiration, and monitored soil moisture as well as soil temperature. Natural snowmelt dates varied by 42 days across years. Snow manipulations advanced and delayed snowmelt by 4.6 and 8.0 days on average but did not affect annual growth (peak biomass) above‐ nor below‐ground. Interactions between snowmelt and drought were nonsignificant, implying that drought effects were independent of snowmelt. Drought reduced total annual above‐ground biomass in the 10‐week treatment by 16 ± 7% across years, while the 5‐week treatment lowered biomass in the last year only. This decline in biomass was accountable to high drought sensitivity of biomass production in a few forb and graminoid species. In contrast, drought did not affect the biomass production of the dominant sedge Carex curvula, whose proportion of total plant cover increased from 36% in controls to 48% in 10‐week drought. Below‐ground biomass slightly increased under drought (5‐week treatment only), resulting in a higher root mass fraction (both treatments). Despite continued root formation, drought reduced ecosystem respiration by 13%–23% per season, assessed nine times during three growing seasons. Since more than 85% of ecosystem respiration stemmed from below‐ground activities and roots continued growing under drought, we assume that soil microorganisms were heavily constrained by the drought treatments. Synthesis. We conclude that snowmelt timing is unrelated to productivity, while recurrent drought will shift biomass allocation from shoots to roots in this typical alpine grassland, with potential implications for grazers but also for nutrient and carbon cycling. Species‐specific drought‐sensitivity will considerably alter community composition under more frequent drought.

What is addressed asgrowing seasonin terrestrial ecosystems is one of the main determinants of annual plant biomass production globally. However, there is no well‐defined concept behind. Here, we show different facets of what might be termed growing season, each with a distinct meaning: (1) the time period during which a plant or a part of it actually grows and produces new tissue, irrespective of net carbon gain (growing season sensu stricto). (2) The period defined by developmental, that is, phenological markers (phenological season). (3) The period during which vegetation as a whole achieves its annual net primary production (NPP) or a net ecosystem production (NEP), expressed as net carbon gain (productive season) and (4) the period during which plants could potentially grow based on meteorological criteria (meteorological season). We hypothesize that the duration of such a ‘window of opportunity’ is a strong predictor for NPP at a global scale, especially for forests. These different definitions have implications for the understanding and modelling of plant growth and biomass production. The common view that variation in phenology is a proxy for variation in productivity is misleading, often resulting in unfounded statements on potential consequences of climatic warming such as carbon sequestration.

Rising air temperatures threaten the snow reliability of ski resorts. Most resorts rely on technical snowmaking to compensate lacking natural snow. But increased water consumption for snowmaking may cause conflicts with other sectors’ water uses such as hydropower production or the hotel industry. We assessed the future snow reliability (likelihood of a continuous 100-day skiing season and of operable Christmas holidays) of the Swiss resort Andermatt-Sedrun-Disentis throughout the twenty-first century, where 65% of the area is currently equipped for snowmaking. Our projections are based on the most recent climate change scenarios for Switzerland (CH2018) and the model SkiSim 2.0 including a snowmaking module. Unabated greenhouse gas emissions (scenario RCP8.5) will cause a lack of natural snow at areas below 1800–2000 m asl by the mid-twenty-first century. Initially, this can be fully compensated by snowmaking, but by the end of the century, the results become more nuanced. While snowmaking can provide a continuous 100-day season throughout the twenty-first century, the economically important Christmas holidays are increasingly at risk under the high-emission scenario in the late twenty-first century. The overall high snow reliability of the resort comes at the cost of an increased water demand. The total water consumption of the resort will rise by 79% by the end of the century (2070–2099 compared to 1981–2010; scenario RCP8.5), implying that new water sources will have to be exploited. Future water management plans at the catchment level, embracing the stakeholders, could help to solve future claims for water in the region.

Alpine plants have evolved a tight seasonal cycle of growth and senescence to cope with a short growing season. The potential growing season length (GSL) is increasing because of climate warming, possibly prolonging plant growth above- and belowground. We tested whether growth dynamics in typical alpine grassland are altered when the natural GSL (2–3 months) is experimentally advanced and thus, prolonged by 2–4 months. Additional summer months did not extend the growing period, as canopy browning started 34–41 days after the start of the season, even when GSL was more than doubled. Less than 10% of roots were produced during the added months, suggesting that root growth was as conservative as leaf growth. Few species showed a weak second greening under prolonged GSL, but not the dominant sedge. A longer growing season under future climate may therefore not extend growth in this widespread alpine community, but will foster species that follow a less strict phenology.

Aim: “Where and why does a species exist” is a fundamental question in ecology. However, the actual range limits of alpine plant species are largely unexplored and unexplained. We aim at identifying the cold range limits of the two most abundant alpine graminoid species on acidic soils that intermingle in mosaics of high‐elevation habitats across the European Alps.Location: Alpine grasslands in the Swiss Alps.Taxon: Carex curvula (Cyperaceae) and Nardus stricta (Poaceae).Methods: We assessed temperature 3 cm below ground, closest to the plant meristems, year‐round as well as snow cover duration, soil chemistry and vegetation characteristics at high spatio‐temporal resolution at 115 microsites. Field data were combined with various freezing resistance analyses in plant individuals at 38 microsites by employing mixed regression models.Results: Carex and Nardus segregated across different microsites. Season length, growing degree hours and soil chemistry did not demarcate the two species' ranges, while their distribution was strongly affected by soil minimum temperature in winter. Carex occurred at sites with and without protecting snow cover and resisted low soil temperatures (−13°C). Nardus was absent at microsites with snow cover duration < 5 months and soil minimum temperatures below −5°C. During the growing season Carex had a higher leaf/shoot freezing resistance (lethal temperature for 50% of the leaf tissue LT50: −16.1°C) than Nardus (−13.3°C). Shoot apices tolerated lowest temperatures: Carex −30°C, Nardus −24°C. Though, a vital shoot apex alone did not ensure regrowth after winter, but intact vessels and roots were required, all less freezing tolerant than apical meristems and young leaves.Main conclusions: The cold range limits of these widespread alpine graminoids are evidently set by thermal extremes in winter. Microtopography, thus snow distribution pattern, in concert with the species' freezing resistance explains the cold edge of the fundamental niche of these two species.

The alpine belt hosts the treeless vegetation above the high elevation climatic treeline. The way alpine plants manage to thrive in a climate that prevents tree growth is through small stature, apt seasonal development, and ‘managing’ the microclimate near the ground surface. Nested in a mosaic of micro-environmental conditions, these plants are in a unique position by a close-by neighborhood of strongly diverging microhabitats. The range of adjacent thermal niches that the alpine environment provides is exceeding the worst climate warming scenarios. The provided mountains are high and large enough, these are conditions that cause alpine plant species diversity to be robust against climatic change. However, the areal extent of certain habitat types will shrink as isotherms move upslope, with the potential areal loss by the advance of the treeline by far outranging the gain in new land by glacier retreat globally.

The majority of alpine plants are of small stature. Through their small size alpine plants are decoupled from the free atmospheric circulation and accumulate solar heat. However, a few alpine species do not follow that “rule” and protrude with their aboveground structures from the microclimatic shelter of the main canopy boundary layer. We aim at explaining the phenomenon of being tall by exploring the biomass production and carbon relations of four pairs of small and tall phylogenetically related taxa in alpine grassland. We compared species and stature-specific biomass allocation, shifts in non-structural carbohydrate (NSC) concentrations in different tissues throughout the season, and we used ¹³C labels to track carbon transfer from leaves to belowground structures. Small and tall herbs did not differ in their above- to belowground biomass allocation. The NSC composition (starch, fructan, simple sugars) and allocation did not show a stature-specific pattern, except for higher concentrations of simple sugars in tall species during their extended shoot growth. In relative terms, tall species had higher NSC pools in rhizomes, whereas small species had higher NSC pools in roots. Our findings do not place tall alpine forbs in an exceptional category in terms of biomass allocation and carbohydrate storage. The tall versus small stature of the examined herbs does not seem to be associated with specific adjustments in carbon relations. ¹³C pulse labelling revealed early C autonomy in young, unfolding leaves of the tall species, which are thus independent of the carbon reserves in the massive belowground organs.

The start of the growing season for alpine plants is primarily determined by the date of snowmelt. We analysed time series of snow depth at 23 manually operated and 15 automatic (IMIS) stations between 1055 and 2555 m asl in the Swiss Central Alps. Between 1958 and 2019, snowmelt dates occurred 2.8 ± 1.3 days earlier in the year per decade, with a strong shift towards earlier snowmelt dates during the late 1980s and early 1990s, but non-significant trends thereafter. Snowmelt dates at high-elevation automatic stations strongly correlated with snowmelt dates at lower-elevation manual stations. At all elevations, snowmelt dates strongly depended on spring air temperatures. More specifically, 44% of the variance in snowmelt dates was explained by the first day when a three-week running mean of daily air temperatures passed a 5 °C threshold. The mean winter snow depth accounted for 30% of the variance. We adopted the effects of air temperature and snowpack height to Swiss climate change scenarios to explore likely snowmelt trends throughout the twenty-first century. Under a high-emission scenario (RCP8.5), we simulated snowmelt dates to advance by 6 days per decade by the end of the century. By then, snowmelt dates could occur one month earlier than during the reference periods (1990–2019 and 2000–2019). Such early snowmelt may extend the alpine growing season by one third of its current duration while exposing alpine plants to shorter daylengths and adding a higher risk of freezing damage.