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What is addressed as growing season in 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. [ABSTRACT FROM AUTHOR]

During the past century atmospheric nitrogen deposition increased dramatically due to human activities worldwide. Currently, it exceeds the critical load for nitrogen (CLN) in over 90% of the Swiss forest area and raised bogs, in 80% of all fens and in 30% of species-rich grassland areas in Switzerland. Indirect gaseous nitrogen losses (HNO 2 , NO, N 2 O, N 2 ) from these soils induced by atmospheric nitrogen deposition are likely to be substantial. However, the approaches to estimate these indirect N emissions provided by the international organisations (UNFCCC, IPCC; UNECE, EMEP/EEA) are based on agricultural data only. They may not be suitable to estimate the indirect emissions from (semi-)natural ecosystems such as forests, extensively used grassland, and wetlands. The present study aims at calculating ecosystem-specific annual indirect N 2 O and NO emissions of (semi-)natural ecosystems in Switzerland for the years 1990, 2000, 2007 and 2010 using a simple linear model similar to the international guidelines. The approach here is based on empirical data for (semi-)natural ecosystems, derived from a literature survey, is driven by atmospheric nitrogen deposition and is ecosystem-specific with a high spatial resolution of 100 m × 100 m. Our results show that such ecosystems represent a strong source of indirect N emissions induced by atmospheric nitrogen deposition and emitted 1.61 ± 0.32 Gg N 2 O–N and 2.51 ± 0.53 Gg NO–N into the atmosphere in Switzerland in the year 2010, corresponding to 21% of the total Swiss N 2 O emissions and 10% of the NO x emissions. Thanks to the reduction of N emissions and thereby reduced atmospheric N deposition, the indirect N 2 O and NO emissions from (semi-)natural ecosystems are estimated to have been both reduced by c. 20% from 1990 to 2010. We conclude that the source strength for N 2 O and NO emissions of (semi-)natural ecosystems have been underestimated by the current approaches of IPCC and EMEP/EEA by a factor of 4.4 and 17, respectively. In regions and countries with a high fraction of (semi-)natural ecosystems, the approach offered here has the potential to improve the estimate of indirect N emissions substantially. [ABSTRACT FROM AUTHOR]

1. Elevated atmospheric carbon dioxide (CO2) concentrations have often been reported to increase carbon allocation below-ground, particularly to fine root production. However, for trees these responses have primarily been studied in young expanding systems while the evidence for late successional systems that have reached steady state above- and below-ground is very limited. 2. At the Swiss Canopy Crane (SCC) experimental site, we assessed whether elevated CO2 affects fine root biomass, fine root expansion and fine root C and N concentration under mature deciduous trees ( c. 100 years) exposed to 7 years of free air CO2 enrichment (FACE) in a typical near-natural central European forest. 3. After 5 and 6 years of CO2 enrichment, both, the soil core and ingrowth core method yielded similar reductions in biomass of c. –30% under elevated CO2 for live fine roots < 1 mm diameter. In year 7 of the experiment, when fine root biomass was re-assessed at peak season, there was no significant CO2-effect detectable. C and N concentrations in newly produced fine roots remained unaffected by elevated CO2. Soil moisture under CO2-exposed trees was significantly increased during rainless periods. 4. The isotopic label introduced into the system by canopy enrichment with 13C-depleted CO2 allowed us to trace the newly assimilated carbon. After 6 years of growth at 550 ppm CO2, recent fine roots (< 1 mm, ingrowth cores) of CO2-enriched trees consisted of 51% new carbon, suggesting a rather slow root turnover and/or slow mixing of old and new carbon in these trees. 5. Reduced tree water consumption under elevated CO2 and resultant soil water savings might cause these trees to reduce their fine root investments in a future CO2-enriched atmosphere. 6. Our findings and those from other multi-year experiments indicate that fine root mass in late successional systems may also be unaffected or even suppressed instead of being stimulated by elevated CO2. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: In the alpine regions, more than half of the year''s precipitation is conserved as snow. We investigated inorganic ion storage in snow pack at 2500m (a.s.l.) in the Central Alps, Switzerland. Chemical composition of snow was dominated by nitrogenous and sulphate ions. Ion pools showed high local and inter-annual variability (2002, 2003), but differences in ion pools were not simply due to varying snow depth. The highest soluble nitrogen (N) pools were found in March (1.1–1.7kgNha−1). Later, in spring (April and May), N pools in snow were significantly lower (0.5–0.8kgNha−1). Ion storage in snow was compared with N fluxes in rain, which ranged between 2.0kg and 3.3kgNha−1 summer−1. Both forms of wet precipitation yielded a total wet N input into alpine grasslands of maximal 5.0kgNha−1 a−1. Atmospheric N deposition data for alpine ecosystems in the European Alps are rare and the monitored data here deviate by a factor of 2 from modelled N deposition. Nitrogen deposition in the alpine zone of the Central Alps is much smaller than previously assumed, but N pools stored in snow correspond to, at the most, 34% of N released by N mineralisation in alpine soils. Net N mineralisation accounts for only a fraction of the annual N uptake of alpine plant; thus, the additional N source by melting snow contributes substantially to alpine plants N uptake, particularly during periods when N demands are highest. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Increased atmospheric nitrogen (N) deposition is driving nutrient imbalances, soil acidification, biodiversity losses and the long-term reduction in stability of sensitive ecosystems which previously had limited N. In this study, we analysed the concentrations of seven different N compounds in precipitation and in the air at 34 sites across Switzerland. We calculated the N deposition by precipitation (bulk deposition) and applied the inferential method to derive dry deposition (gases, aerosols) from air concentrations. We then quantified the total inorganic N deposition by adding together the bulk and dry deposition. Finally, the total inorganic N input into the sensitive ecosystems of the 34 sites was compared to the critical loads of these ecosystems. N deposition by precipitation was the main contributor to the total N load in 16 out of 34 sites, especially into open ecosystems such as alpine/subalpine grassland, mountain hay meadows, and raised bogs. Dry deposition of ammonia (NH 3) was the second most important pathway, in particular for forests close to agricultural activities, due to high NH 3 concentrations and the higher deposition velocity. The N deposition exceeded the lower limit of the Critical Load of Nitrogen (CLN) range at most sites, and at many sites even surpassed the upper limit of the CLN range. No, or minor, exceedances of the critical loads for N were found only at remote sites at higher elevation in the Central Alps. Annual inorganic N deposition between 2000 and 2017 revealed a significant decline in oxidised N compounds at four of five sites (−1.6–1.8% per year), but reduced compounds only decreased at two sites (−1% and −1.4% per year) and even increased at one site (+1.2% per year), despite adopted abatement strategies for agricultural practices. This emphasises that most sensitive ecosystems in Switzerland continue to be exposed to excessive N loads through atmospheric deposition, with detrimental consequences for the biodiversity and stability of these ecosystems. • Current inorganic nitrogen (N) input to sensitive Swiss ecosystems exceeds the critical loads. • N deposition by precipitation is the main contributor to N loading in non-forested ecosystems. • Dry deposition of NH 3 from agricultural activities adds substantially to the N load in forests. [ABSTRACT FROM AUTHOR]

Alpine ecosystems are, similar to arctic ecosystems, characterized by a very long snow season. Previous studies investigating arctic or alpine ecosystems have shown that winter CO2 effluxes can dominate the annual balance and that the timing and duration of the snow cover plays a crucial role for plant growth and phenology and might also influence the growing season ecosystem CO2 strength and dynamics. The objective of this study was to analyze seasonal and annual CO2 balances of a grassland site at an elevation of 2440 m a.s.l in the Swiss central Alps. We continuously measured the NEP using the eddy covariance method from June 2013 to October 2014, covering two growing seasons and one winter. We analyzed the influence of snow melt date on the CO2 exchange dynamics at this site, because snow melt differed about 24 days between the 2 years. To this end, we employed a process-based ecosystem carbon cycling model to disentangle the co-occurring effects of growing season length, environmental conditions during the growing season, and physiological/structural properties of the canopy on the ecosystem carbon balance. During the measurement period, the site was a net sink for CO2 although winter efflux contributed significantly to the total balance. The cumulative growing season NEP as well as mean and maximum daily CO2 uptake rates was lower during the year with the later snow melt, and the results indicated that the differences were mainly due to differing growing season lengths. [ABSTRACT FROM AUTHOR]

The N-fixing shrub Alnus viridis is currently encroaching on montane grasslands in the Alps as a result of reduced land management and complete abandonment. Alnus introduces large amounts of nitrogen (N) into these formerly N-poor grasslands and restricts the succession to montane forests. We studied pools and fluxes of N and the associated C pools in pastures (controls) and adjacent Alnus shrublands at two elevations (1650 versus 1950 m a.s.l.) in three valleys in the Swiss central Alps. The total N and C pools stored in 50-year-old Alnus shrubland did not exceed those in adjacent pastures with a total of approximately 610 g N m in phytomass plus soil (down to 30 cm) at both elevations. In Alnus stands, reduced soil N pools balanced the gain in phytomass N pools, a likely result of a faster turnover of soil N. The soil solution under Alnus was continuously enriched with nitrate, with a total N leaching of 0.79 g N m season (June-October) under 50-year-old stands at both elevations and the highest flux of 1.76 g N m season in 25-year-old shrubland at low elevation, clearly indicating an excess of available N in Alnus shrubland. In contrast, N leaching across all pastures was close to zero (0.08 g N m) throughout the season. At the catchment scale, streamlet water showed increased nitrate concentrations with typical flushing peaks in spring and autumn, provided more than one fifth of the catchment area was covered by Alnus shrubs. We conclude that the expansion of Alnus rapidly converts centuries-old, N-poor grassland into N saturated shrubland, irrespective of elevation, and it reduces the C storage potential of the landscape because the Alnus dominance constrains re-establishment of a natural montane forest. [ABSTRACT FROM AUTHOR]

While exerting no obvious function under 'average' environmental conditions, the presence of certain plant specialists becomes crucial in the event of a complete failure of a community due to severe disturbance such as landslides. Plants capable of growing at erosion edges may act as potential edge-engineers by coping with unstable ground and stabilizing the soil with their roots. We hypothesized that life conditions at erosion edges select for a particular set of specialists or species with specific traits, the identification of which was the aim of the study. Across 17 small-scale transects (0.40 × 1.60 m) from intact meadows to landslide edges (Ursern Valley, Swiss Alps, c. 1,600 m a.s.l.), we quantified plant species abundance by the point intercept method and characterized growth conditions based on Landolt's indicator values, leaf δC, and volumetric soil moisture in the uppermost soil layers. We observed a clear change of plant species composition and relative abundance from the meadow to the edge, presumably induced by the 25 % lower soil moisture and microclimatic exposure. Species richness at the edge was two-thirds of that in the meadow, but was positively correlated with species richness of the adjacent meadow. Species with 'edge-preference' had either (1) rolled or festucoid leaves like Festuca spp., Avenella flexuosa and Nardus stricta, or (2) small, scleromorphic leaves like Vaccinium vitis- idaea, Calluna vulgaris and Thymus ssp. Graminoids with rolled/festucoid leaves were found to be the most dominant edge-specialists. The grass Festuca valesiaca s.l. emerged as the most dominant plant species at the edge, having an 11-times higher cover at the edge than in the meadow. In this montane grassland, a single species contributes to the stabilization of erosion edges and may be regarded as a potential keystone species for slope stability and regeneration after landslides even its role has not so far been established. [ABSTRACT FROM AUTHOR]

Large areas of alpine pastures and meadows currently face declining land use or abandonment, which leads to tall-grass transition ecosystems with higher leaf area index ( LAI), potentially increased evapotranspiration ( ET) and thus, reduced water yield. Elevated atmospheric CO2, on the other hand, is known to reduce stomata opening and hence, leaf-level transpiration, which may translate into higher soil moisture and enhanced total runoff. Here, we quantify these opposing effects of global change on the water balance of alpine grassland in a field experiment in the Swiss Alps (2440 m a.s.l.)., Rates of ET and deep seepage (percolation water) of four alpine grassland types (dominated by Agrostis, Nardus, Carex or forbs) were measured using intact monoliths in 51 weighing lysimeters. A part of the monoliths was clipped to simulate sheep grazing during three seasons (2008-2010). Another set was exposed to elevated CO2 (580 ppm) using free-air CO2 enrichment ( FACE) during the 2009 growing season., Simulated grazing reduced bright day ET by on average −12% across all years, with the most pronounced effects in the high-stature swards. Correspondingly, the higher biomass and LAI in unclipped grassland lowered the seasonal sum of deep seepage by −13% in a drier summer (2009) and by −5% in a rather wet summer (2010) compared to clipped swards., CO2 enrichment reduced ET in all grassland types by −3 to −7%, increased δ18 O in foliage and enhanced soil moisture, but not deep seepage. Hence, future CO2 slightly counteracts the land use effects at canopy level, however, not in terms of water yield., Synthesis. Our results indicate that both grazing and elevated CO2 are mitigating the effects of dry spells on alpine vegetation. The net effect of the continuous decline in the land use and of elevated CO2 is negative for catchment water yield and thus, for potential hydroelectric power production. Although these economic 'costs' are rather moderate per hectare of alpine grassland, sums are substantial when scaled to the vast areas potentially affected in the Alps. These calculated 'costs' attribute economic value to the eco-hydrological benefits of land care at these high elevations. [ABSTRACT FROM AUTHOR]

At high elevations, many vital processes (growth, reproduction and nutrient cycling) are shaped by the presence of snow throughout most of the year. Alpine plants therefore have to pass through their phenological cycle within the short growing season of just about three months. While the synchronization of flowering via photoperiod enhances successful reproduction and reduces the risk of early season freezing damage, it also prevents plants from profiting from earlier snowmelt and longer growing seasons. For the future, it is not clear whether snowmelt dates at elevations < 2500 m a.s.l. are going to be advanced due to higher temperatures or whether they are going to be delayed due to more late winter precipitation and a higher snowpack. In a unique snow and summer precipitation manipulation experiment in the Swiss central Alps (2500 m a.s.l.), we explore the consequences of altered onset of the growing season (earlier and later) on the flowering phenology of common species in a typical late successional grassland. The results of three years (2016 - 2018) clearly show a dominance of opportunistic flowering behavior. Most of the commonly occurring species closely follow snowmelt and the onset of flowering is most likely determined by species-specific thermal sums. While relatively low temperature sums trigger flowering in species like Carex curvula (sedge) and Geum montanum (herb), other species as Trifolium alpinum (N-fixing herb) and Helictotrichon versicolor (grass) start flowering at higher thermal sums. Leontodon helveticus (herb) pursues a completely different strategy and starts to flower during a certain time period of the year. The exceptionally early start of the growing season in 2017 (ca. three weeks earlier) reaffirms the existence of these distinct strategies. In late successional alpine grasslands, future timing of snowmelt will therefore evoke a clear response in the timing of the onset of flowering with consequences for plant reproduction in the longer term. [ABSTRACT FROM AUTHOR]

• The Alpine region is warming fast, and concurrently, the frequency and intensity of climate extremes are increasing. It is currently unclear whether alpine ecosystems are sensitive or resistant to such extremes. • We subjected Swiss alpine grassland communities to heat waves with varying intensity by transplanting monoliths to four different elevations (2440-660m above sea level) for 17 d. Half of these were regularly irrigated while the other half were deprived of irrigation to additionally induce a drought at each site. • Heat waves had no significant impacts on fluorescence (Fv/Fm, a stress indicator), senescence and aboveground productivity if irrigation was provided. However, when heat waves coincided with drought, the plants showed clear signs of stress, resulting in vegetation browning and reduced phytomass production. This likely resulted from direct drought effects, but also, as measurements of stomatal conductance and canopy temperatures suggest, from increased high-temperature stress as water scarcity decreased heat mitigation through transpiration. • The immediate responses to heat waves (with or without droughts) recorded in these alpine grasslands were similar to those observed in the more extensively studied grasslands from temperate climates. Responses following climate extremes may differ in alpine environments, however, because the short growing season likely constrains recovery. [ABSTRACT FROM AUTHOR]