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Unchanged risk of frost exposure for subalpine and alpine plants after snowmelt in Switzerland despite climate warming
Abstract
The length of the snow-free season is a key factor regulating plant phenology and shaping plant community composition in cold regions. While global warming has significantly advanced the time of snowmelt and the growth period at all elevations in the Swiss Alps, it remains unclear if it has altered the likelihood of frost risk for alpine plants. Here, we analyzed the influence of the snowmelt timing on the risk of frost exposure for subalpine and alpine plants shortly after snowmelt, i.e., during their most vulnerable period to frost at the beginning of their growth period. Furthermore, we tested whether recent climate warming has changed the risk of exposure of plants to frost after snowmelt. We analyzed snow and air temperature data in the Swiss Alps using six weather stations covering the period 1970–2016 and 77 weather stations covering the period 1998–2016, spanning elevations from 1418 to 2950 m asl. When analyzed across all years within each station, our results showed strong negative relationships between the time of snowmelt and the frequency and intensity of frost during the most vulnerable period to frost for subalpine and alpine plants, indicating a higher frost risk damage for plants during years with earlier snowmelt. However, over the last 46 years, the time of snowmelt and the last spring frost date have advanced at similar rates, so that the frequency and intensity of frost during the vulnerable period for plants remained unchanged.
... However, this may not be the case at high elevations where the influence of warming on the probability of freezing events is more complicated and uncertain. With increasing annual or seasonal mean temperature, the occurrence of growing-season freezing events was found to increase in central Chilean Andes (Sierra-Almeida and Cavieres, 2010) and southeast Tibet (Shen et al., 2018) or to remain unchanged in Swiss Alps (Rixen et al., 2012;Klein et al., 2018). Due to the low-temperature photoinhibition resulting from the combination of freezing temperature at night and high solar radiation in daytime (Johnson et al., 2004;Coop and Givnish, 2008), the increase of early-season freezing events may seriously hamper seed-based seedling establishment above treeline (Shen et al., 2014). ...
... Globally, the decrease of snow cover in mountainous areas has been observed in recent decades, in which the shorter snow cover duration is mainly due to the earlier snow melting in spring (Pederson et al., 2013;Klein et al., 2016;Xu et al., 2016). It has been reported that shallower snow pack and earlier snow melting are associated with earlier start of the growing season (Vaganov et al., 1999;Huang et al., 2019), which may result in higher frequency and intensity of frost in the early growing season (Klein et al., 2018). To our knowledge, however, there is lack of long-term and continuous measurements of microclimate factors to examine how the variations of growing season onset and snow cover drive the spatiotemporal variability of growing-season freezing events at alpine treelines. ...
... Over the past 40 years, the weakening of Indian summer monsoon and the reduction of cloud cover on the south side of the Himalayas have increased the incoming solar radiation in daytime but reduced the downward long-wave radiation at night (Yue et al., 2020), suggesting that the early-season freezing events may increase and the treelines may remain stable as observed in Sigdel et al. (2018). In the Swiss Alps, however, the maximum of global radiation occurs in summer (June and July) and the radiative cooling effect in the early growing season (April and May) can be mitigated by high cloud cover and low global radiation (Rottler et al., 2019), which explains why the early-season freezing events show no relation to increased temperature and advanced growing season in recent decades (Rixen et al., 2012;Klein et al., 2018). ...
Few data have demonstrated why early-season freezing events may increase under a warmer climate at alpine treelines, which is critical to understand the key limiting factors determining treeline dynamics under future climate warming. Here we test the hypothesis that the increase of early-season freezing events under a warmer climate is mainly associated with advanced onset of growing season and enhanced radiative cooling effect in the pre-monsoon season. We conducted 11-year observations of microclimate factors across north-facing and south-facing treelines in the Sergyemla Mountains, southeast Tibet. We found that the frequency, intensity, and duration of early-season freezing events were generally higher under a warmer climate on the south-facing slope or in the warmer years within a slope, in which the frequency of early-season freezing events significantly increased with increasing annual mean air-temperature. During 2006–2016, the frequency, intensity and duration of early-season freezing events typically showed a negative correlation with the onset date of growing season, while their frequency was positively correlated with the early-season global radiation. In each of both slopes, global radiation was significantly higher and long-wave radiation balance was much more negative on days with daily minimum air-temperature (Tmin) < 0 °C than with Tmin > 0 °C, indicating the generality of radiative cooling effect on early-season freezing events at high elevations. Our data support the hypothesis, revealing physical mechanisms for the increase of early-season freezing events with climate warming. The physical mechanisms should provide a general explanation for the topography-dependent pattern of tree species distribution and alpine treeline stability under climate warming in high mountainous regions like the Himalayas.
... Indeed, in the alpine environment, rising temperatures and sudden alterations in the snow dynamics, such as reductions in mass and duration of the snow layer, are the most likely drivers of changes that could affect plant species survival (Huelber et al. 2006). The high-elevation environment is likely to maintain, in the short term, a certain degree of protection against spring frost, due to a less dramatic reduction of the amount and duration of the snow cover (Klein et al. 2018) when compared to low or medium-elevation environments, which are more at risk of frostrelated damage (Vitasse et al. 2018). On the other hand, more competitive species of typical alpine grasslands could benefit from advantages such as the elongation of the growing season (Wang et al. 2016) or the increased nutrient input in the soil due to a higher bacterial activity at the expense of the strongly adapted snowbed specialists. ...
... Indeed, in the alpine environment, rising temperatures and sudden alterations in the snow dynamics, such as reductions in mass and duration of the snow layer, are the most likely drivers of changes that could affect plant species survival (Huelber et al. 2006). The high-elevation environment is likely to maintain, in the short term, a certain degree of protection against spring frost, due to a less dramatic reduction of the amount and duration of the snow cover (Klein et al. 2018) when compared to low or medium-elevation environments, which are more at risk of frostrelated damage (Vitasse et al. 2018). On the other hand, more competitive species of typical alpine grasslands could benefit from advantages such as the elongation of the growing season (Wang et al. 2016) or the increased nutrient input in the soil due to a higher bacterial activity at the expense of the strongly adapted snowbed specialists. ...
... The occurrence of different behaviors represents the practice of different strategies, each of which bearing benefits and disadvantages. In case of an early disappearance of the snow layer, an extremely fast onset could determine exposure to structural damages, especially for flower buds, linked to spring frosts (Inouye 2008;Klein et al. 2018;Vitasse et al. 2018). On the other hand, an exaggerated delay in the occurrence of late phenophases could lead to the failure to complete the reproductive cycle. ...
The study of plant phenology has frequently been used to link phenological events to various factors, such as temperature or photoperiod. In the high-alpine environment, proper timing of the phenological cycle has always been crucial to overcome harsh conditions and potential extreme events (i.e. spring frosts) but little is known about the response dynamics of the vegetation, which could shape the alpine landscape in a future of changing climate. Alpine tundra vegetation is composed by an array of species belonging to different phytosociological optima and with various survival strategies, and snowbed communities are a relevant expression of such an extreme-climate adapted flora. We set eight permanent plots with each one in a snowbed located on the Cimalegna plateau in Northwestern Italy and then we selected 10 most recurring species among our plots, all typical of the alpine tundra environment and classified in 3 different pools: snowbed specialists, grassland species and rocky debris species. For 3 years we registered the phenophases of each species during the whole growing season using an adaptation of the BBCH scale. We later focused on the three most biologically relevant phenophases, i.e., flower buds visible, full flowering, and beginning of seed dispersion. Three important season-related variables were chosen to investigate their relationship with the phenological cycle of the studied species: (i) the Day Of Year (DOY), the progressive number of days starting from the 1st of January, used as a proxy of photoperiod, (ii) Days From Snow Melt (DFSM), selected to include the relevance of the snow dynamics, and (iii) Growing Degree Days (GDD), computed as a thermal sum. Our analysis highlighted that phenological development correlated better with DFSM and GDD than with DOY. Indeed, models showed that DOY was always a worse predictor since it failed to overcome interannual variations, while DFSM and marginally GDD were better suited to predict the phenological development of most of the species, despite differences in temperature and snowmelt date among the three years. Even if the response pattern to the three variables was mainly consistent for all the species, the timing of their phenological response was different. Indeed, species such as Salix herbacea and Ranunculus glacialis were always earlier in the achievement of the phenophases, while Agrostis rupestris and Euphrasia minima developed later and the remaining species showed an intermediate behavior. However, we did not detect significant differences among the three functional pools of species.
... Par exemple, entre 1970 et 2015, la date de fonte des neiges a en moyenne avancé de près de 4 semaines dans les Alpes suisses à toutes les altitudes, contre un retard d'environ 2 semaines pour la mise en place du manteau neigeux à l'automne (Klein et al., 2016) (tableau 1 et figure 3). Cette diminution de près de 40 jours en 45 ans de la durée du manteau neigeux continu est particulièrement liée à un réchauffement important des températures de l'air au printemps et en été au cours de la même période, avec une augmentation de 0,4 à 0,6°C par décennie (Rixen et al., 2012, Klein et al., 2018 La durée du manteau neigeux est ainsi très sensible à l'évolution de la température de l'air (Hantel et Hirtl-Wielke, 2007), notamment au printemps (Hantel et al., 2000 ;Wielke et al., 2004), ce qui est donc aussi le cas pour sa date de démarrage à l'automne, sa date de fonte au printemps, ou encore à l'inverse, la durée sans présence de neige au sol en été (figure 3). A moyenne altitude, les études de Rebetez (1996), Bednorz (2004) et Schöner et al. (2019 indiquent que cette sensibilité du manteau neigeux existe également en hiver, puisque des températures de l'air plus élevées pendant cette saison ont pu être mises en relation avec un manteau neigeux moins épais. ...
... La plupart des dommages liés au gel et observés sur les communautés de plantes alpines surviennent dans les deux à trois semaines suivant la date de fonte des neiges, constituant alors une période de grande vulnérabilité face à ce risque de gel tardif (Rixen et al., 2012). La date de fonte des neiges constitue ainsi une période clé pour ces plantes, puisque dans le cas d'une occurrence plus précoce, elle peut significativement augmenter leur risque d'exposition au gel tardif pendant cette période de vulnérabilité (Choler, 2015 ;Klein et al., 2018) ainsi que les dommages liés à ces événements (Inouye, 2008 ;Sherwood et al., 2017), ou encore diminuer leur résistance face aux températures de l'air négatives (Wipf et al., 2009 ;Wheeler et al., 2014). Des variations d'épaisseur et de durée de la couverture neigeuse peuvent aussi affecter sensiblement la résistance au gel d'un certain nombre de communautés de plantes alpines au moment de la fonte des neiges, qui est par conséquent un moteur essentiel à la réponse de ces plantes face au changement climatique (Palacio et al., 2015). ...
... L'augmentation envisagée des températures de l'air devrait toutefois permettre de contrebalancer les effets néfastes d'une fonte des neiges plus précoce, en limitant les événements de gel tardif après la fonte des neiges et donc, en freinant le risque d'exposition et de dommages liés à ce gel pour les communautés de plantes alpines (Sherwood et al., 2017). Cette compensation a d'ailleurs déjà pu être observée dans les Alpes suisses, où le risque d'exposition au gel de ces plantes est resté inchangé depuis 1970 malgré une fonte des neiges progressivement plus précoce à toutes les altitudes, notamment grâce à une augmentation des températures minimales et maximales de l'air au cours de la même période (Klein et al., 2018). ...
Les régions de montagne, particulièrement vulnérables aux fluctuations du climat, ont subi d'importantes modifications environnementales au cours du XXe siècle. L'augmentation observée des températures de l'air depuis les années 1950 a notamment engendré un net recul des glaciers, mais aussi du manteau neigeux à toutes les altitudes. Cette réduction de l'enneigement en montagne est un facteur préoccupant dans de multiples domaines, qu'il s'agisse des écosystèmes, des cycles hydrologiques, ou encore du tourisme alpestre. De nombreux travaux de recherche à travers le monde témoignent du réchauffement de l'air et de la réduction observée du manteau neigeux dans les massifs montagneux depuis les années 1950, essentiellement à basse et moyenne altitude, mais peu de synthèses de ces différents changements ont été faites jusque-là. Cette revue bibliographique a pour but de faire l'état des lieux des principaux changements observés du manteau neigeux depuis le XXe siècle dans les Alpes européennes et leur mise en relation avec les changements climatiques relevés sur la même période. Un aperçu des différentes conséquences déjà observées sur les cycles hydrologiques, le tourisme hivernal et les écosystèmes est également présenté, ainsi que les projections futures d'évolution des paramètres du manteau neigeux d'ici la fin du XXIe siècle. Cette revue bibliographique fournit une source d'information utile pour les futures recherches se focalisant sur l'étude de la saisonnalité du manteau neigeux en région de montagne et ses implications directes, en particulier dans les zones alpines et subalpines.
Mots-clés : Alpes européennes, écosystèmes, manteau neigeux, changement climatique, réchauffement de l'air.
Abstract:
Snowpack variability from 1950 to 2016 in the European Alps under climate change: a review In the European Alps, air temperature warming has been particularly pronounced during the last decades, leading to glacier retreat and snowpack reduction as well as strong phenological shifts for numerous plant and animals. This observed decline of the snowpack is a major cause of concern, as mountains areas are humanly and economically of great importance (water supply, winter tourism), but also for ecosystems, as mountain regions are biodiversity hotspots. A large number of studies across the world already show climatic changes and snowpack reduction in mountain areas since the 1950s, particularly at low and mid-elevations, but only few overviews have so far synthetized all the observed changes in snowpack and their relation with climate change. This review aims to summarize snowpack variability since the XXth century in the European Alps, and its connection with climatic changes over the same period. Some of the impacts on hydrology, winter tourism and ecosystems that have been already reported will also be presented, as well as future projections of snowpack changes. Overall, this review provides a valuable source of information for future research focusing on snowpack seasonality and its implications in mountain regions, particularly in alpine and subalpine areas.
... Climate change shapes alpine vegetation and plant diversity in different ways. For instance, the increase in temperatures recorded in the last decades [21,22] has consistently altered alpine and nival species' distribution. In several European alpine plant communities, a general increase in species richness [23][24][25] was registered and it was explained as a "thermophilization process", which consists of the upward shifting of thermophilic plants towards higher altitudes [23,[26][27][28][29][30][31][32][33][34][35], or as a "range-filling process", performed by species dispersing from existing neighbor communities within the same elevation belt [36,37]. ...
... Considering the consistent and heterogeneous changes ongoing on alpine ecosystems, likely related with both the increase in temperatures [21,22] and the reduction in annual rainfalls [1,42,[48][49][50][51], the present work sets out to explore vegetation dynamics on alpine Mediterranean calcareous grasslands and swards in central Apennines (Italy). Through a re-visitation vegetation study (after 18 years), we explored the temporal changes in two alpine communities in the Maiella National Park (MNP) that are representative of the central Apennine's alpine belt: Apennine stripped grasslands, growing on steep slopes, and wind edge swards, both included in the 6170 EU Habitat "alpine and subalpine calcareous grasslands" [52,53]. ...
Global change threatens alpine biodiversity and its effects vary across habitat types and biogeographic regions. We explored vegetation changes over the last 20 years on two Mediterranean alpine calcareous grasslands in central Apennines (Italy): stripped grasslands (EUNIS code E4.436) with Sesleria juncifolia growing on steep slopes, and wind edge swards (EUNIS code E4.42) with Carex myosuroides. Based on a re-visitation of 25 vegetation plots of 4 × 4 m, we assessed changes in overall and endemic plant species cover and richness by nonparametric Kruskal-Wallis test. We explored changes in structure and ecology using growth forms and Landolt indicators for temperatures. We identified species' contribution to temporal changes using the similarity percentage procedure (SIMPER). The results evidenced a significant decline in all species cover and richness on both plant communities with a significant decline in alpine and endemic species and in hemicryptophytes with rosette and scapose ones on stripped grasslands, as well as a decline in subalpine and suffruticose chamaephytes species on wind edge swards. Such biodiversity loss, so far observed only in the warmest and Southern Mediterranean summits of Europe, is likely attributable to the combined effect of higher temperatures; the increase in the vegetative period; and the decrease in water availability, which is particularly severe in calcareous regions. Our study suggested the vulnerability of the analyzed alpine ecosystems to global change and the importance of monitoring activities to better understand vegetation trends and adaptation strategies in subalpine, alpine, and nival ecosystems.
... Par exemple, entre 1970 et 2015, la date de fonte des neiges a en moyenne avancé de près de 4 semaines dans les Alpes suisses à toutes les altitudes, contre un retard d'environ 2 semaines pour la mise en place du manteau neigeux à l'automne (Klein et al. 2016) (Tableau 1 et Figure 3). Cette diminution de près de 40 jours en 45 ans de la durée du manteau neigeux continu est particulièrement liée à un réchauffement important des températures de l'air au printemps et en été au cours de la même période, avec une augmentation de 0,4 à 0,6°C par décennie (Klein et al. 2018;Rixen et al. 2012) (Figure 3). ...
... Cette compensation a d'ailleurs déjà pu être observée dans les Alpes suisses, où le risque d'exposition au gel de ces plantes est resté inchangé depuis 1970 malgré une fonte des neiges progressivement plus précoce à toutes les altitudes, notamment grâce à une augmentation des températures minimales et maximales de l'air au cours de la même période (Klein et al. 2018). ...
RÉSUMÉ
Les régions de montagne sont des zones particulièrement exposées aux variations du climat. L’augmentation significative des températures de l’air observée au cours du XXe siècle dans les Alpes a eu des répercussions notables sur l’évolution spatiale et temporelle du manteau neigeux, engendrant d’importantes modifications au niveau des écosystèmes, des cycles hydrologiques ou encore des activités économiques humaines. Dans ce contexte de réchauffement de l’air, il est important de développer les connaissances actuelles sur la variabilité du manteau neigeux alpin, afin de mieux appréhender les conséquences de ces changements sur l’environnement direct. Cette thèse de doctorat a pour objectif d’étudier la relation entre le changement climatique de ces dernières décennies (1970-2016) et l’évolution temporelle du manteau neigeux continu dans les Alpes suisses au-delà de 1100 m d’altitude, ainsi que l’influence de cette couverture neigeuse sur le risque d’exposition au gel des plantes alpines au moment du démarrage de leur croissance après la fonte des neiges.
L’analyse portée sur l’évolution temporelle (1970-2015) des principales caractéristiques annuelles du manteau neigeux continu (épaisseur, durée, saisonnalité) entre 1100 et 2500 m d’altitude dans les Alpes suisses dévoile un recul généralisé de la couverture neigeuse au cours de cette période, que ce soit dans son épaisseur ou dans sa durée et quel que soit l’altitude, la zone géographique examinée ou les conditions climatiques locales. L’étude montre notamment que la durée du manteau neigeux continu s’est réduite en moyenne de 38 jours entre 1970 et 2015 et que cette réduction est plus particulièrement attribuable à une date de fonte des neiges de plus en plus précoce au printemps (-26 jours), plutôt qu’à un début d’enneigement continu plus tardif à l’automne (+12 jours).
La combinaison entre une date de fonte des neiges de plus en plus précoce avec une forte dépendance du démarrage de la croissance des plantes alpines à celle-ci, pose la question d’un éventuel risque accru d’exposition au gel de ces plantes, à une période où celles-ci y sont particulièrement vulnérables. L’analyse du risque d’exposition au gel de ces plantes lors de leur période de début de croissance illustre l’existence d’une solide relation entre la date de fonte des neiges et la fréquence ou l’intensité de gel lors des jours avoisinant cette période. En effet, il est observé en moyenne que plus la fonte des neiges est précoce, plus les fréquences et intensités de gel augmentent au cours de la période de démarrage de la croissance des plantes alpines et ce, quelle que soit l’altitude (1418-2950 m), la zone géographique ou encore la période temporelle analysée (1998-2016 ou 1970-2016) dans les Alpes suisses. Néanmoins, avec une augmentation moyenne des températures de l’air printanières de 0,6°C par décennie entre 1970 et 2016 dans les zones alpines et subalpines, aucun changement significatif n’a été observé dans le même temps quant à la fréquence ou à l’intensité de gel pendant cette période de début de croissance. Ce réchauffement a permis de contrebalancer les effets d’un déneigement du sol plus précoce, en décalant au même rythme les dernières occurrences de gel et le démarrage de la croissance des plantes alpines, limitant ainsi leur risque d’exposition au gel au cours de leur période de début de croissance.
L’ensemble des analyses menées dans cette thèse démontrent l’importance de la saisonnalité du manteau neigeux sur le démarrage de la croissance des plantes alpines, ainsi qu’une grande homogénéité spatiale des résultats à travers les Alpes suisses. En effet, qu’il s’agisse de l’évolution du manteau neigeux ou du risque d’exposition au gel tardif pour les plantes alpines, les résultats de ce travail se retrouvent sans distinction significative à travers l’ensemble du gradient d’altitude représentant les étages alpins et subalpins, au sein de zones géographiques diverses et éloignées ainsi que dans des conditions climatiques locales variées, indiquant qu’il s’agit de phénomènes d’ampleur supérieure à celle des Alpes suisses.
Mots-clés : Alpes suisses, Fonte des neiges, Gel tardif, Manteau neigeux, Plantes alpines, Réchauffement climatique
ABSTRACT
Mountain regions are particularly exposed to climate change. The significant increase of air temperatures observed during the XXth century in the Alps had strong impacts on the spatial and temporal variability of snow cover, causing major changes on ecosystems, hydrological cycles or human economic activities. In this context of global warming, it is important to improve knowledge on snowpack variability in order to better understand the consequences of these changes on the surrounding environment. This PhD thesis aims to explore the relationship between recent climate change (1970-2016) and the temporal evolution of continuous snowpack in the Swiss Alps over 1100 m asl, as well as the influence of this snowpack on the risk of frost exposure for alpine plants during their most vulnerable period to frost, i.e. at the beginning of their growth period shortly after the time of snowmelt.
The analysis of the main annual characteristics of the continuous snow cover (thickness, duration, seasonality) from 1100 to 2500 m asl in the Swiss Alps over the 1970-2015 period reveal a general decline of the snowpack, whether for its depth or its duration and irrespective of elevation, geographical location or local climatic conditions. This study also demonstrate that the snow cover duration has been shortened at all sites on average by 38 days between 1970 and 2015 and that this shortening is mainly driven by an earlier time of snowmelt (-26 days) rather than a later time of snow onset (+12 days).
The combination between an earlier time of snowmelt and a strong dependence of the beginning of growth of alpine plants to this snowmelt raises the question of a potential higher risk of frost exposure for these plants, during a period when they are particularly vulnerable to freezing events. The analysis of the risk of frost exposure for alpine plants during the beginning of their growth period illustrate the existence of a strong relationship between the time of snowmelt and the frequency or intensity of freezing events during the days surrounding this vulnerability period for plants. On average, an early time of snowmelt generally leads to an increasing frequency and intensity of frost during the vulnerable period for alpine plants, irrespective of elevation (1418-2950 m), geographical location or the temporal period analyzed (1998-2016 or 1970-2016) in the Swiss Alps. However, with an average spring air temperature increase of 0,6°C decade-1 between 1970 and 2016 in alpine and subalpine regions, the frequency and intensity of frost during the vulnerable period for alpine plants remained unchanged. This warming allowed a compensatory effect of an earlier time of snowmelt by shifting the last occurrence of frost and the beginning of alpine plants growth period to a same extent, thus limiting their exposure to late frost events during the beginning of their growth period.
All analyses conducted in this PhD thesis demonstrate the importance of snowpack seasonality on alpine plants growth period, as well as a strong spatial homogeneity of the results over the Swiss Alps. Whether for the snowpack evolution or the risk of exposure to late frost events for alpine plants, results may indeed be found without any significant distinction across all elevations, various geographical locations and a large panel of local climatic conditions, indicating that they could be extended beyond the Swiss Alps.
Keywords: Swiss Alps, Time of snowmelt, Late frost, Snow cover, Alpine plants, Global warming
... Britton et al. 2009;Carbognani et al. 2014) under climate change. Globally, the climatic changes recorded in the last decades have improved the conditions for plants in the alpine belt, with warmer summer temperatures and longer growing seasons (Rebetez and Reinhard 2008;Klein et al. 2016;CH2018 2018Klein et al. 2018). Generalist species or species from neighbouring alpine grasslands could colonise windy ridges and typical snowbeds. ...
... This corresponds to the increase of the indicator value for continentality, indicating more contrasted temperatures between nights and days. However, according to Klein et al. (2018), the risk of frost after snowmelt has not increased in the last decades. ...
The impacts of climate change on alpine summit floras have been widely investigated. However, only few studies included alpine grasslands and generally concluded that snowbeds, with a long snow cover duration and a short growing season, and windy ridges, with a short snow cover duration and strong winter frosts, are the most sensitive alpine grasslands. However, these habitats were mostly investigated in different regions, where local factors (e.g. nitrogen deposition, grazing) can co-vary with climate changes, potentially obscuring differences between habitats. Here, we focused on the Zermatt region (Swiss Alps) to investigate the impacts of climate change on snowbeds and windy ridges. Forty-three exhaustive historical plant inventories on windy ridges (acidophilic or basophilic) and 31 inventories in snowbeds (typical or wet) were repeated in quasi-permanent plots after approximately 23 years. Historical and recent records were compared with the Simpson index, Bray–Curtis dissimilarity, a PCA, ecological indicator values and the frequency and cover changes of species. There was a general increase in α-diversity and a decrease in β-diversity (homogenisation). Most of the new species in the plots were generalists from surrounding grasslands. The plant composition tended to be more thermophilous on acidophilic windy ridges and in typical snowbeds. The flora of acidophilic windy ridges became more similar to that of basophilic windy ridges and more eutrophic. We interpreted this as possibly arising from fertilisation by the aeolian dust deposition coming from the expanding glacial moraine in the valley. In snowbeds, the species indicated increasingly drier conditions, especially in wet snowbeds. Warming climate induces lower snowfall and earlier snowmelt, leading to a shorter snow cover duration. Hence, wet snowbeds are certainly among the most threatened plant communities by climate change in the Alps.
... Our results also corroborate previous studies showing detrimental effects of freezing events on plant growth (Klein et al., 2018;Liu et al., 2018) into multiple ways such as roots damages, limitations in roots uptakes of nitrogen and carbon allocation and storage (Pardee et al., 2018). Consistently, they support that an increase in frost days after the start of season have negative effects on productivity and promote longer growth period. ...
Growth responses of low mountain grasslands to Climate Change are poorly understood despite very large surfaces covered in Central Europe. They are characterized by still present agricultural exploitation and complex topographical features that limit species migration and increase differences in snow regimes. This study examined MODIS surface reflectances between 2000 and 2020 across the Vosges mountain grasslands to investigate trends and drivers of spatial patterns in annual maximum NDVI (Normalized Difference Vegetation Index). We found a majority of no significant trends indicating several environmental and ecological compensatory effects to warming in the Vosges Mountains. We also noted hotspots of browning grasslands (a decrease of annual maximum NDVI), largely overrepresented compared to the greening ones (an increase of annual maximum of NDVI), a pattern in contradiction with most productivity signals highlighted in European high mountain grasslands. Spatial patterns of browning are enhanced on north-facing slopes and at low elevations (<1100 m) where high producing grasslands with dominant herbaceous communities prevail. A low soil water recharge also appears pivotal to explain the probability of browning in the study site. Through the use of Winter Habitat Indices, we noted high responsiveness of low mountain grasslands to differences in intra seasonal snow regimes, partly modulated by topographic features. Prolonged and time-continuous snow cover promote higher productivity and shortened green-up period. High number of frost events result in lower productivity and prolonged green-up period. We hypothesize that observed growth responses in the Vosges Mountains are indicative of long term future responses to Climate Change in high mountain ranges. With shorter and more discontinuous snow cover, we expect higher diversity of growth responses in European low mountain grasslands due to strong contextual effects and high terrain complexity.
... Anticipated onset of the cambial activity can be associated to a higher risk of earlyfrost events in this species (Kern and Popa, 2008;Gruber et al., 2009). However, if snowmelt and frost date retreat at a similar rate, the risk of plant damage would not vary significantly (Klein et al., 2018). Moreover, we observed very few frost rings in the investigated period, and in general Pinus cembra is considered less prone to frost damage than other coexisting species, such as Larix decidua, due to delayed sprouting (Neuner, 2014). ...
... Spring phenology is advancing under climate change (Fu et al., 2014;Roberts et al., 2015;Thackeray et al., 2016). However, advances in the timing of key spring phenological events such as bud-burst can be greater than the advance in the date of the latest spring frost (Klein et al., 2018;Vitasse et al., 2018), andZohner et al. (2020) found that latespring frost risk has increased in Europe since 1959. Furthermore, some environmental factors, such as photoperiod, will not vary under climate change and interactions may be important. ...
... The effect of temperature was found to be a decisive factor in the change of phenological dates of SOS and POS, in agreement with Ganjurjav et al. [69] and Ren et al. [63], also influencing phenology spatially [70]. Consequently, the rise in temperatures recorded in the 2001-2021 time frame could also have had an indirect effect on the advancement of the SOS date by causing an advancement in the snowmelt dates recorded in recent years [71], without the risk of increasing frost exposure [72]. Snow melt and snow cover are indeed decisive in determining the length of the growing season and the phenological development of highaltitude and high-latitude grasslands [73,74], also influencing water availability or thermal conditions by soil insulation [75]. ...
The use of very long spatial datasets from satellites has opened up numerous opportunities, including the monitoring of vegetation phenology over the course of time. Considering the importance of grassland systems and the influence of climate change on their phenology, the specific objectives of this study are: (a) to identify a methodology for a reliable estimation of grassland phenological dates from a satellite vegetation index (i.e., kernel normalized difference vegetation index, kNDVI) and (b) to quantify the changes that have occurred over the period 2001–2021 in a representative dataset of European grasslands and assess the extent of climate change impacts. In order to identify the best methodological approach for estimating the start (SOS), peak (POS) and end (EOS) of the growing season from the satellite, we compared dates extracted from the MODIS-kNDVI annual trajectories with different combinations of fitting models (FMs) and extraction methods (EM), with those extracted from the gross primary productivity (GPP) measured from eddy covariance flux towers in specific grasslands. SOS and POS were effectively identified with various FM×EM approaches, whereas satellite-EOS did not obtain sufficiently reliable estimates and was excluded from the trend analysis. The methodological indications (i.e., FM×EM selection) were then used to calculate the SOS and POS for 31 grassland sites in Europe from MODIS-kNDVI during the period 2001–2021. SOS tended towards an anticipation at the majority of sites (83.9%), with an average advance at significant sites of 0.76 days year−1. For POS, the trend was also towards advancement, although the results are less homogeneous (67.7% of sites with advancement), and with a less marked advance at significant sites (0.56 days year−1). From the analyses carried out, the SOS and POS of several sites were influenced by the winter and spring temperatures, which recorded rises during the period 2001–2021. Contrasting results were recorded for the SOS-POS duration, which did not show a clear trend towards lengthening or shortening. Considering latitude and altitude, the results highlighted that the greatest changes in terms of SOS and POS anticipation were recorded for sites at higher latitudes and lower altitudes.
... At the alpine scale, this earlier snow removal-induced frost exposure is probably, for the moment, mostly visible in Southern regions and/or at low altitude. For instance, in the Swiss Alps (northernmost and innermost compared to the French Alps), Klein et al. (2018) have found that the frost risk to plants during their early development stage has been unchanged over the last decades, due to the fact that minimum and maximum air temperatures have risen at approximately the same rate, which has advanced in the same way the phenology of alpine plants and the latest frost events. ...
Summer mountain pastures (also called alpages ) are a central element for many agro-pastoral livestock systems in the alpine region, by providing the feedstock for herds during the summer transhumance. However, vegetation phenology and productivity in mountain pastures are increasingly affected by climate hazards exacerbated by climate change, such as early snow removal, late frost events, or droughts. Difficulties can then arise to match animal demand with forage resource on alpages and, in the long term, threaten the sustainable management of these highly multifunctional socio-ecological systems. To help agro-pastoral actors adapt, an essential step is to quantify the risk of impacts on the forage resource, due to an increased occurrence or intensity of climate hazards.
Exposure to climate hazards on alpages is defined locally by topographic aspects in combination with the broader influence of the regional climate. Our work therefore aimed at providing a tailored assessment of potential climate risk for the forage resource at the individual scale of each alpage in the French Alps. To this end, we developed agro-climatic indicators based on atmospheric and snow cover data accounting for geographic and topographic conditions, and applied them to a database providing unique spatially explicit information at the alpage level.
For the first time, we introduce a description of agro-climatic conditions and provide a classification of agro-climatic profiles of alpages in the French Alps, ranging from low to high potential risk for the forage resource, mainly following a North-South gradient combined with altitude. We also bring insights on the evolutions of the climate risk with climate change and discuss management implications for agro-pastoral livestock systems using alpages . We finally present a web-based visualization tool that aim at communicating agro-climatic profiles and their evolution to practitioners and at assisting decision makers in understanding climate-related risks on the alpages of the French Alps.
... We used climatic data recorded at the automatic weather station (AWS) Segl-Maria (Swiss National Basic Climatological Network), the closest station to our study sites providing a long climatic data series (1864-2018) including monthly air temperature, monthly total precipitation and total annual snow cover (Scherrer et al. 2013). Data from this AWS are representative of the climatic trends for the Swiss Alps (Klein et al. 2016(Klein et al. , 2018 and specifically of the climatic trends of upper Valtellina, as confirmed by statistically significant linear regressions against data from the four main AWSs in operation across our study area, which individually have shorter data series (Supporting information). ...
Shrub encroachment, a globally recognized response to climate warming, usually involves late successional species in mountain environments, without alteration to climax communities. We show that a major ecosystem change is occurring in the European Alps across a 1000 m elevation gradient, with pioneer hygrophilous Salix shrubs, previously typical of riparian forests, wetlands and avalanche ravines, encroaching into the climax communities of subalpine and alpine belts shrublands and grasslands, as well as snowbeds, pioneer vegetation and barren grounds in the nival belt.
We analyzed Salix recruitment through dendrochronological methods, and assessed its relationships with climate and atmospheric CO2 concentration. The dendrochronological data indicated that Salix encroachment commenced in the 1950s (based on the age of the oldest Salix individuals, recruited in 1957), and that it was correlated with increasing atmospheric CO2 concentration, spring warming and snow cover decrease. Hygrophilous Salix shrubs are expanding their distribution both through range filling and upwards migration, likely achieving competitive replacement of species of subalpine and alpine climax communities. They benefit from climate warming and CO2 fertilization and are not sensitive to spring frost damage and soil limitations, being observed across a gradient of soil conditions from loose glacial sediments in recently deglaciated areas (where soils had not had sufficient time to develop) to mature soils such as podzols (when colonizing late successional subalpine shrublands).
Salix encroachment may trigger ecosystem and landscape transformations, promoting the development of forests that replace pre‐existing subalpine shrublands, and of open woodlands invading alpine grasslands and snowbeds, making the alpine environment similar to sub‐Arctic and Arctic areas. This results in a new threat to the conservation of the plant species, communities and landscapes typical of the alpine biota, as mountain ranges such as the Alps provide limited opportunities for upward migration and range‐shift.
... In a comparison of arctic and alpine plant species, Oberbauer et al. [26] also found that many species are not just tracking temperature but rather perform a conservative phenology that suggests a modulating role of photoperiod. Comparing snowmelt date and the occurrence and severity of freezing events soon after snowmelt over the past 46 years, Klein et al. [27] found no enhanced risk by frost in the Swiss Alps. This conclusion rests on the assumption that plants are tracking snowmelt data so that they arrive at similar developmental stages soon after snowmelt irrespective of when snowmelt occurs. ...
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.
... Hence, with the documented earlier onset of snowmelt occurring in response to warmer winter and spring temperatures in documented in the European Alps (Beniston, 2012;Hall et al., 2015;Klein et al., 2016), surface pond embryos may be exposed to a greater risk of frost, which in turn would influence embryonic survival (Beattie, 1987;Frisbie et al., 2000;Muir et al., 2014) and population dynamics. This phenomenon has been observed for plants (Inouye, 2008), where the change in last spring frost timing is slower than the shift in plant phenology, leading to higher plant mortality Pardee et al., 2019; but see Klein et al., 2018 for evidence of consistent advances in snowmelt timing and last spring frost, leading to unchanged plant exposure to frost frequency and intensity). ...
The alarming decline of amphibians around the world calls for complementary studies to better understand their responses to climate change. In mountain environments, water resources linked to snowmelt play a major role in allowing amphibians to complete tadpole metamorphosis. As snow cover duration has significantly decreased since the 1970s, amphibian populations could be strongly impacted by climate warming, and even more in high elevation sites where air temperatures are increasing at a higher rate than at low elevation. In this context, we investigated common frog (Rana temporaria) breeding phenology at two different elevations and explored the threats that this species faces in a climate change context. Our objectives were to understand how environmental variables influence the timing of breeding phenology of the common frog, and explore the threats that amphibians face in the context of climate change in mountain areas. To address these questions, we collected 11 years (2009–2019) of data on egg-spawning date, tadpole development stages, snowmelt date, air temperature, rainfall and drying up of wetland pools at ∼1,300 and ∼1,900 m a.s.l. in the French Alps. We found an advancement of the egg-spawning date and snowmelt date at low elevation but a delay at high elevations for both variables. Our results demonstrated a strong positive relationship between egg-spawning date and snowmelt date at both elevations. We also observed that the risk of frost exposure increased faster at high elevation as egg-spawning date advanced than at low elevation, and that drying up of wetland pools led to tadpole mortality at the high elevation site. Within the context of climate change, egg-spawning date is expected to happen earlier in the future and eggs and tadpoles of common frogs may face higher risk of frost exposure, while wetland drying may lead to higher larval mortality. However, population dynamics studies are needed to test these hypotheses and to assess impacts at the population level. Our results highlight climate-related threats to common frog populations in mountain environments, but additional research should be conducted to forecast how climate change may benefit or harm amphibian populations, and inform conservation and land management plans in the future.
... Spring phenology is advancing under climate change (Fu et al., 2014;Roberts et al., 2015;Thackeray et al., 2016). However, advances in the timing of key spring phenological events such as bud-burst can be greater than the advance in the date of the latest spring frost (Klein et al., 2018;Vitasse et al., 2018), andZohner et al. (2020) found that latespring frost risk has increased in Europe since 1959. Furthermore, some environmental factors, such as photoperiod, will not vary under climate change and interactions may be important. ...
Genetic variation and phenotypic plasticity play a role in determining the performance of a tree provenance at a planting site. This paper explores their relative importance in determining growth, phenology and tree form in a broad geographic sample of 42 British provenances of common ash (Fraxinus excelsior L.) grown at two contrasting trial sites. We found significant genetic differences for tree height, timing of leaf flushing and leaf senescence, and stem forking among the provenances. These followed a clear latitudinal and climatic cline, where the northern provenances were shorter, their leaves flushed later and senesced earlier than the southern provenances. Provenance explained a much larger proportion of the variance for spring phenology (63 per cent) than for autumn phenology (15 per cent). The effect of the planting site was contrasting between spring and autumn: spring phenology showed very little plasticity, while autumn phenology presented higher levels of phenotypic plasticity. This could indicate that for ash spring phenology is under stronger selective pressure. We found a correlation between tree height, leaf phenology and forking, with early flushing provenances tending to be taller and more forked, which could reflect repeated frost damage. The findings underline the complexity of predicting performance in novel environments and demonstrate that small gains in tree growth may be counteracted by detrimental effects on stem form, a key contributor to timber value, due to susceptibility to the contemporary environment.
... During the 20th century Phenological and elevational shifts in the Alps in Western Europe, air temperatures warmed more in winter compared to summer (Moberg et al., 2006), but this ratio reversed by the end of the 20th and early 21st century . Hence, several studies have shown that summer and spring have warmed more than autumn and winter since the 1970s (Rebetez & Reinhard, 2008;Klein et al., 2018;Vitasse et al., 2018a). Since the end of the 1980s, maximum air temperatures have been increasing more than minimum temperatures in connection with the decrease in European air pollution and particulate matter, especially in spring and at mid and high elevations (Rebetez & Reinhard, 2008;Vitasse et al., 2018a). ...
Mountain areas are biodiversity hotspots and provide a multitude of ecosystem services of irreplaceable socio‐economic value. In the European Alps, air temperature has increased at a rate of about 0.36°C decade−1 since 1970, leading to glacier retreat and significant snowpack reduction. Due to these rapid environmental changes, this mountainous region is undergoing marked changes in spring phenology and elevational distribution of animals, plants and fungi. Long‐term monitoring in the European Alps offers an excellent natural laboratory to synthetize climate‐related changes in spring phenology and elevational distribution for a large array of taxonomic groups. This review assesses the climatic changes that have occurred across the European Alps during recent decades, spring phenological changes and upslope shifts of plants, animals and fungi from evidence in published papers and previously unpublished data. Our review provides evidence that spring phenology has been shifting earlier during the past four decades and distribution ranges show an upwards trend for most of the taxonomic groups for which there are sufficient data. The first observed activity of reptiles and terrestrial insects (e.g. butterflies) in spring has shifted significantly earlier, at an average rate of −5.7 and −6.0 days decade−1, respectively. By contrast, the first observed spring activity of semi‐aquatic insects (e.g. dragonflies and damselflies) and amphibians, as well as the singing activity or laying dates of resident birds, show smaller non‐significant trends ranging from −1.0 to +1.3 days decade−1. Leaf‐out and flowering of woody and herbaceous plants showed intermediate trends with mean values of −2.4 and −2.8 days decade−1, respectively. Regarding species distribution, plants, animals and fungi (N = 2133 species) shifted the elevation of maximum abundance (optimum elevation) upslope at a similar pace (on average between +18 and +25 m decade−1) but with substantial differences among taxa. For example, the optimum elevation shifted upward by +36.2 m decade−1 for terrestrial insects and +32.7 m decade−1 for woody plants, whereas it was estimated to range between −1.0 and +11 m decade−1 for semi‐aquatic insects, ferns, birds and wood‐decaying fungi. The upper range limit (leading edge) of most species also shifted upslope with a rate clearly higher for animals (from +47 to +91 m decade−1) than for plants (from +17 to +40 m decade−1), except for semi‐aquatic insects (−4.7 m decade−1). Although regional land‐use changes could partly explain some trends, the consistent upward shift found in almost all taxa all over the Alps is likely reflecting the strong warming and the receding of snow cover that has taken place across the European Alps over recent decades. However, with the possible exception of terrestrial insects, the upward shift of organisms seems currently too slow to track the pace of isotherm shifts induced by climate warming, estimated at about +62 to +71 m decade−1 since 1970. In the light of these results, species interactions are likely to change over multiple trophic levels through phenological and spatial mismatches. This nascent research field deserves greater attention to allow us to anticipate structural and functional changes better at the ecosystem level.
... In the snowbed habitat, where snow remains until midsummer, snowmelt time governed the flowering schedule. The time of snowmelt in alpine regions has been advancing at a global scale in response to global warming (Rixen et al. 2012;CaraDonna et al. 2014;Klein et al. 2018;Jennings and Molotch 2020). Snowmelt time at this study site has also advanced at a rate of 2.8 days per decade over the past 30 years (G. ...
Global warming tends to accelerate flowering phenology of alpine plants, and it may cause a decrease in fruit production due to lower pollinator activity and/or higher risk of frost damage earlier in the season. Because flowering period of alpine plants varies highly depending on snowmelt conditions, the effects of phenological variation on fruit-set success may vary among local populations. I observed the relationship between flowering time and fruit-set success in four populations of a bee-pollinated dwarf shrub, Rhododendron aureum, located in fellfield and snowbed habitats in northern Japan, for 12 or 13 years over the 25 years from 1995 to 2019. Flowering of the fellfield populations usually occurred in June, and flowering of the snowbed populations commonly started after mid-July, although there was considerable yearly variation in actual flowering time within individual populations. Generally, the fruit-set rates of the fellfield populations were low, with large yearly fluctuations, whereas those of the snowbed populations were stable and high. There was a clear trend toward a decrease in fruit-set rates with earlier flowering in the fellfield populations due to pollen limitation and occasional frost damage. The risk of frost damage increased with earlier flowering in the fellfield habitat. These results indicate that the effects of climate change on fruit-set success of alpine plants are strongly site-specific and are greatest early in the growing season.
... In temperate and boreal mountains, an earlier snowmelt extends the humid growing season, whereas in Mediterranean mountains, it would expand the dry summer period and thus the period of potential drought stress. Besides, positive effects of an extended growing season are hypothesised to be counteracted by the detrimental effects of an increasing frequency and intensity of frost (Choler 2018;Klein et al. 2018). ...
Climate change impacts are of a particular concern in small mountain ranges, where cold-adapted plant species have their optimum zone in the upper bioclimatic belts. This is commonly the case in Mediterranean mountains, which often harbour high numbers of endemic species, enhancing the risk of biodiversity losses. This study deals with shifts in vascular plant diversity in the upper zones of the Sierra Nevada, Spain, in relation with climatic parameters during the past two decades. We used vegetation data from permanent plots of three surveys of two GLORIA study regions, spanning a period of 18 years (2001–2019); ERA5 temperature and precipitation data; and snow cover durations, derived from on-site soil temperature data. Relationships between diversity patterns and climate factors were analysed using GLMMs. Species richness showed a decline between 2001 and 2008, and increased thereafter. Species cover increased slightly but significantly, although not for endemic species. While endemics underwent cover losses proportional to non-endemics, more widespread shrub species increased. Precipitation tended to increase during the last decade, after a downward trend since 1960. Precipitation was positively related to species richness, colonisation events, and cover, and negatively to disappearance events. Longer snow cover duration and rising temperatures were also related to increasing species numbers, but not to cover changes. The rapid biotic responses of Mediterranean alpine plants indicate a tight synchronisation with climate fluctuations, especially with water availability. Thus, it rather confirms concerns about biodiversity losses, if projections of increasing temperature in combination with decreasing precipitation hold true.
... During this time, freezing temperatures are frequent. Despite global warming, frost is anticipated to remain a predominant factor, as alpine plants seem to be exposed to an unchanged frost risk due to the earlier loss of snow cover [3]. At 3450 m a.s.l. on Mt. ...
Ranunculus glacialis grows and reproduces successfully, although the snow-free time period is short (2–3 months) and night frosts are frequent. At a nival site (3185 m a.s.l.), we disentangled the interplay between the atmospheric temperature, leaf temperatures, and leaf freezing frequency to assess the actual strain. For a comprehensive understanding, the freezing behavior from the whole plant to the leaf and cellular level and its physiological after-effects as well as cell wall chemistry were studied. The atmospheric temperatures did not mirror the leaf temperatures, which could be 9.3 °C lower. Leaf freezing occurred even when the air temperature was above 0 °C. Ice nucleation at on average −2.6 °C started usually independently in each leaf, as the shoot is deep-seated in unfrozen soil. All the mesophyll cells were subjected to freezing cytorrhysis. Huge ice masses formed in the intercellular spaces of the spongy parenchyma. After thawing, photosynthesis was unaffected regardless of whether ice had formed. The cell walls were pectin-rich and triglycerides occurred, particularly in the spongy parenchyma. At high elevations, atmospheric temperatures fail to predict plant freezing. Shoot burial prevents ice spreading, specific tissue architecture enables ice management, and the flexibility of cell walls allows recurrent freezing cytorrhysis. The peculiar patterning of triglycerides close to ice rewards further investigation.
... A decoupling of the seasonal association between different abiotic factors can lead to negative fitness consequences of phenological plasticity (Fig. 4c). For example, increasing mean temperatures and advancing time of snowmelt in spring are often decoupled from the risk of experiencing freezing temperatures (Klein et al. 2018), with the degree of coupling varying substantially among regions (Liu et al. 2018). Indeed, frost damage due to advanced phenology has been linked to lower performance in observational studies (Inouye 2008, Wheeler et al. 2015, in experiments manipulating temperature (Rixen et al. 2012) and snowmelt dates (Wipf et al. 2009), and in plants transplanted to lower elevations (Scheepens and Stöcklin 2013). ...
Phenological shifts, changes in the seasonal timing of life cycle events, are among the best documented responses of species to climate change. However, the consequences of these phenological shifts for population dynamics remain unclear. Population growth could be enhanced if species that advance their phenology benefit from longer growing seasons and gain a pre‐emptive advantage in resource competition. However, it might also be reduced if phenological advances increase exposure to stresses, such as herbivores and, in colder climates, harsh abiotic conditions early in the growing season. We exposed subalpine grasslands to ~ 3 K of warming by transplanting intact turfs from 2000 m to 1400 m elevation in the eastern Swiss Alps, with turfs transplanted within the 2000 m site acting as a control. In the first growing season after transplantation, we recorded species’ flowering phenology at both elevations. We also measured species’ cover change for three consecutive years as a measure of plant performance. We used models to estimate species’ phenological plasticity (the response of flowering time to the change in climate) and analysed its relationship with cover changes following climate change. The phenological plasticity of the 18 species in our study varied widely but was unrelated to their changes in cover. Moreover, early‐ and late‐flowering species did not differ in their cover response to warming, nor in the relationship between cover changes and phenological plasticity. These results were replicated in a similar transplant experiment within the same subalpine community, established one year earlier and using larger turfs. We discuss the various ecological processes that can be affected by phenological shifts, and argue why the population‐level consequences of these shifts are likely to be species‐ and context‐specific. Our results highlight the importance of testing assumptions about how warming‐induced changes in phenotypic traits, like phenology, impact population dynamics. This article is protected by copyright. All rights reserved.
... Frost damage impacts the ability of a plant to grow, take up nutrients and water, and compete with neighbouring species (Bokhorst et al., 2008;Rixen et al., 2012). Regarding climate warming, a recent survey of meteorological data in the Swiss Alps indicates that despite increased warming due to the earlier loss of snow cover, the risk of frost damage to alpine plants has remained unchanged (Klein et al., 2018). Consequently, this makes the summer freezing resistance of alpine plants an important aspect of plant distribution and alpine ecology. ...
The frequency and severity of night frosts increase during the summer in the alpine environment as elevation increases. Intra-specific, temporal, and elevational variability in the freezing resistance (FR) of alpine plants during the active growing period has not been documented, especially in regards to the intra-specific distribution of plants at their upper elevational limits of growth.
The FR of leaves of 13 species was investigated at regular intervals from June through August, 2017 at elevations between 600 and 3200 m. The examined species have different lower and upper elevational range limits, and either grow exclusively in alpine sites or extend into the subalpine life zone. Each species was sampled twice, close to its upper and lower range limit.
For most of the species, leaves exhibited a higher FR (−0,17 K/100 m) at the upper elevational range limit. The intra-specific FR was substantial, covering an amplitude of 7–15 K (>15 K in Pinus). Fully-expanded leaves had a lower level of FR in June. Evergreen leaves had a higher FR than deciduous leaves throughout the summer. Frost hardening in evergreen and a decrease in FR in deciduous leaves was observed in August; whereas, deciduous leaves at that time had already begun to senesce.
Ecotypes growing at their upper elevation limit were more frost hardy than individuals of the same species growing near the lower level of their elevation range. The most severe summer frosts occurred in June when leaves were fully expanded and their FR was lowest. Thus, the highest risk of frost damage to plants at alpine sites occurs in June, even though the most frost-susceptible phase of leaf expansion may have already passed.
... A study in the Swiss Alps, however, found that the time of snowmelt and the last spring frost date have advanced at similar rates, so that the frequency and intensity of frost during the vulnerable period for plants remained unchanged. 30 Although there are many similarities between alpine and arctic tundra communities, in the future they may diverge in their responses to climate change. Both areas are experiencing warming temperatures and alterations in precipitation patterns, and potentially longer growing seasons, but alpine tundra plants are more commonly responsive to daylength, which will limit the extent to which they can respond plastically to changes in the growing season; the earlier snowmelt that can result in decreased water availability can lead to earlier senescence, also limiting responses to the longer growing season. ...
Alpine environments are among the habitats most strongly affected by climate change, and consequently their unique plants and pollinators are faced with the challenge of adapting or going extinct. Changes in temperature and precipitation affect snowpack and snowmelt, resulting in changes in the growing season in this environment where plant growth and pollinator activity are constrained to the snow‐free season, which can vary significantly across the landscape if there is significant topographic complexity. As in other ecosystems, the resulting changes in phenology are not uniform among species, creating the potential for altered and new interspecific interactions. New plant and animal species are arriving as lower altitude species move up with warming temperatures, introducing new competitors and generating changes in plant–pollinator interactions. Repeating historical surveys, taking advantage of museum collections, and using new technology will facilitate our understanding of how plants and pollinators are responding to the changing alpine environment.
Zusammenfassung
Dieses Kapitel beschäftigt sich mit den klimatischen, ökologischen und sozioökonomischen treibenden Kräften, welche die Landnutzung in Österreich in der Vergangenheit und der Gegenwart maßgeblich bestimmt haben und die zukünftigen Entwicklungen beeinflussen werden. Es behandelt die in der Vergangenheit beobachteten und in der Zukunft erwarteten treibenden Kräfte von Landnutzungsänderungen in der Landwirtschaft (Abschn. 3.2), der Forstwirtschaft (Abschn. 3.3) und der Siedlungs- und Infrastrukturentwicklung (Abschn. 3.4). Abschließend werden die möglichen und erwarteten Auswirkungen dieser treibenden Kräfte auf die Bereitstellung der Ökosystemleistungen (ÖSL) beschrieben (Abschn. 3.5). Der Abbau von Mineralien wie Schotter oder Metalle wird aus Platzgründen nicht in diesem Kapitel behandelt, wenngleich es unumstritten ist, dass es sich auch dabei um landnutzungsrelevante Aktivitäten handelt. Kap. 3 unterscheidet zwischen natürlichen und anthropogenen Faktoren und wie sich diese auf die Landnutzung ausgewirkt haben und auswirken. Die sozioökonomischen Auswirkungen berücksichtigen dabei allerdings nicht die möglichen Anpassungs- oder Minderungsstrategien der einzelnen Sektoren, da diese in den Kap. 4 und 5 gesondert dargestellt werden.
Mountain meadows are an essential part of the alpine–subalpine ecosystem; they provide ecosystem services like pollination and are home to diverse plant communities. Changes in climate affect meadow ecology on multiple levels, for example, by altering growing season dynamics. Tracking the effects of climate change on meadow diversity through the impacts on individual species and overall growing season dynamics is critical to conservation efforts. Here, we explore how to combine crowd‐sourced camera images with machine learning to quantify flowering species richness across a range of elevations in alpine meadows located in Mt. Rainier National Park, Washington, USA. We employed three machine‐learning techniques (Mask R‐CNN, RetinaNet and YOLOv5) to detect wildflower species in images taken during two flowering seasons. We demonstrate that deep learning techniques can detect multiple species, providing information on flowering richness in photographed meadows. The results indicate higher richness just above the tree line for most of the species, which is comparable with patterns found using field studies. We found that the two‐stage detector Mask R‐CNN was more accurate than single‐stage detectors like RetinaNet and YOLO, with the Mask R‐CNN network performing best overall with mean average precision (mAP) of 0.67 followed by RetinaNet (0.5) and YOLO (0.4). We found that across the methods using anchor box variations in multiples of 16 led to enhanced accuracy. We also show that detection is possible even when pictures are interspersed with complex backgrounds and are not in focus. We found differential detection rates depending on species abundance, with additional challenges related to similarity in flower characteristics, labeling errors and occlusion issues. Despite these potential biases and limitations in capturing flowering abundance and location‐specific quantification, accuracy was notable considering the complexity of flower types and picture angles in this dataset. We, therefore, expect that this approach can be used to address many ecological questions that benefit from automated flower detection, including studies of flowering phenology and floral resources, and that this approach can, therefore, complement a wide range of ecological approaches (e.g., field observations, experiments, community science, etc.). In all, our study suggests that ecological metrics like floral richness can be efficiently monitored by combining machine learning with easily accessible publicly curated datasets (e.g., Flickr, iNaturalist).
Mountain meadows are an essential part of the alpine-subalpine ecosystem; they provide ecosystem services like pollination and nutrient recycling and are home to diverse plant communities. Changes in climate affect the meadow ecology on multiple levels, altering growing season dynamics and water availability, and tracking the effects of climate change on meadow diversity is critical to conservation efforts. Here, we explore how to combine crowd sourced camera images with machine learning to quantity flowering species richness across a range of elevations in alpine meadows located in Mt Rainier National Park, Washington, USA. We employed three machine learning techniques (Mask R-CNN, RetinaNet and YOLO) to detect wildflower species in images taken during two flowering seasons. We demonstrate that deep learning techniques can detect multiple species, providing information on flowering richness in photographed meadows. The results indicate higher richness just above the tree line for most of the species, which is comparable with patterns found using field studies. We found that the two-stage detector Mask R-CNN was more accurate than single-stage detectors like RetinaNet and YOLO, with the Mask R-CNN network performing best overall with mean average precision (mAP) of 0.67 followed by RetinaNet (0.5) and YOLO (0.4). We also show that detection is possible even when pictures are interspersed with complex backgrounds and are not in focus. We expect this approach can be used to address many ecological questions that benefit from automated flower detection, like studies of flowering phenology or floral resources, and that this approach can therefore complement a wide range of ecological approaches (e.g., field observations, experiments, community science, etc.). In all, our study suggests that ecological metrics like floral richness can be efficiently monitored by combining machine learning with easily accessible publicly curated datasets (e.g., Flickr, iNaturalist).
Snow is an important driver of ecosystem processes in cold biomes. Snow accumulation determines ground temperature, light conditions, and moisture availability during winter. It also affects the growing season’s start and end, and plant access to moisture and nutrients. Here, we review the current knowledge of the snow cover’s role for vegetation, plant-animal interactions, permafrost conditions, microbial processes, and biogeochemical cycling. We also compare studies of natural snow gradients with snow experimental manipulation studies to assess time scale difference of these approaches. The number of tundra snow studies has increased considerably in recent years, yet we still lack a comprehensive overview of how altered snow conditions will affect these ecosystems. Specifically, we found a mismatch in the timing of snowmelt when comparing studies of natural snow gradients with snow manipulations. We found that snowmelt timing achieved by snow addition and snow removal manipulations (average 7.9 days advance and 5.5 days delay, respectively) were substantially lower than the temporal variation over natural spatial gradients within a given year (mean range 56 days) or among years (mean range 32 days). Differences between snow study approaches need to be accounted for when projecting snow dynamics and their impact on ecosystems in future climates.
Snow is an important driver of ecosystem processes in cold biomes. Snow accumulation determines ground temperature, light conditions and moisture availability during winter. It also affects the growing season’s start and end, and plant access to moisture and nutrients. Here, we review the current knowledge of the snow cover’s role for vegetation, plant-animal interactions, permafrost conditions, microbial processes and biogeochemical cycling. We also compare studies of natural snow gradients with snow manipulation studies, altering snow depth and duration, to assess time scale difference of these approaches. The number of studies on snow in tundra ecosystems has increased considerably in recent years, yet we still lack a comprehensive overview of how altered snow conditions will affect these ecosystems. In specific, we found a mismatch in the timing of snowmelt when comparing studies of natural snow gradients with snow manipulations. We found that snowmelt timing achieved by manipulative studies (average 7.9 days advance, 5.5 days delay) were substantially lower than those observed over spatial gradients (mean range of 56 days) or due to interannual variation (mean range of 32 days). Differences between snow study approaches need to be accounted for when projecting snow dynamics and their impact on ecosystems in future climates.
Temperatures in mountain areas are increasing at a higher rate than the Northern Hemisphere land average, but how fauna may respond, in particular in terms of phenology, remains poorly understood. The aim of this study was to assess how elevation could modify the relationships between climate variability (air temperature and snow melt‐out date), the timing of plant phenology and egg‐laying date of the coal tit (Periparus ater). We collected 9 years (2011–2019) of data on egg‐laying date, spring air temperature, snow melt‐out date, and larch budburst date at two elevations (~1,300 m and ~1,900 m asl) on a slope located in the Mont‐Blanc Massif in the French Alps. We found that at low elevation, larch budburst date had a direct influence on egg‐laying date, while at high‐altitude snow melt‐out date was the limiting factor. At both elevations, air temperature had a similar effect on egg‐laying date, but was a poorer predictor than larch budburst or snowmelt date. Our results shed light on proximate drivers of breeding phenology responses to interannual climate variability in mountain areas and suggest that factors directly influencing species phenology vary at different elevations. Predicting the future responses of species in a climate change context will require testing the transferability of models and accounting for nonstationary relationships between environmental predictors and the timing of phenological events.
Sensitivity of grassland biomass production to climate is critical to impacts on multiple ecological processes and ecosystem services. Understanding its climate determinants is essential for climate change adaptation. This requires long-term monitoring, using robust methods that are appropriated by stakeholders. We tested the sensitivity of easily measured sward height to interannual climate variation in mountain grasslands. Using twelve consecutive years of measurements across 67 grassland plots representative of six land-use types associated with different landscape positions, we show that peak green biomass increased with mean summer months (June and July) maximum temperature. Different land-use types responded to specific combinations of climate parameters, but all except higher-elevation summer pastures were sensitive to summer months temperatures. We did not detect any effects of drought, with summer precipitation instead decreasing peak biomass of some grasslands due to cooling and cloudiness, also suggesting that soil water recharge from snowmelt was enough to sustain the first growth cycle. Summer pasture peak biomass decreased with number of frosts during the onset of growth in May. These result support the robustness and sensitivity of sward height as an indicator for climate response of peak fodder biomass. Differential responses across land-use types suggest some resource complementarity which can support tactical adaptation for farmers. During the three recent hottest summers (2015, 2017 and 2018) production was well below predicted values from actual temperatures, suggesting a potential regime shift when the vegetative growth period is shortened by temperature-driven acceleration in phenology and/or heat stress combined with high light intensity causing physiological damage. The baseline regime and the anomalies in hottest years need confirmation for longer time series and across a greater geographic extent. Further effects of drought and of an extended growing season are also likely for post-harvest or grazing regrowth.
For the first time there were collected data on vegetation of the hard-accessible areas
of highest position in mountains in Murmansk Region and Svalbard, and the diversity and species number there proved to be rather high. The species composition of ‘goltzy’ deserts in Murmansk Region differs essentially from this in sub-nival zone of Svalbard: Jaccard coefficient for lists of vascular plants is 0,08; that is due to different zonal and geographical position of areas. Difference in structure of plant cover, namely lower portion of fruticose lichens in sub-nival vegetation in Svalbard, is mainly due to effect of reindeer pasture
Globally accelerating trends in societal development and human environmental impacts since the mid-twentieth century1-7are known as the Great Acceleration and have been discussed as a key indicator of the onset of the Anthropocene epoch6. While reports on ecological responses (for example, changes in species range or local extinctions) to the Great Acceleration are multiplying8, 9, it is unknown whether such biotic responses are undergoing a similar acceleration over time. This knowledge gap stems from the limited availability of time series data on biodiversity changes across large temporal and geographical extents. Here we use a dataset of repeated plant surveys from 302 mountain summits across Europe, spanning 145 years of observation, to assess the temporal trajectory of mountain biodiversity changes as a globally coherent imprint of the Anthropocene. We find a continent-wide acceleration in the rate of increase in plant species richness, with five times as much species enrichment between 2007 and 2016 as fifty years ago, between 1957 and 1966. This acceleration is strikingly synchronized with accelerated global warming and is not linked to alternative global change drivers. The accelerating increases in species richness on mountain summits across this broad spatial extent demonstrate that acceleration in climate-induced biotic change is occurring even in remote places on Earth, with potentially far-ranging consequences not only for biodiversity, but also for ecosystem functioning and services.
One hundred years ago, Andrew D. Hopkins estimated the progressive delay in tree leaf-out with increasing latitude, longitude, and elevation, referred to as "Hopkins' bioclimatic law." What if global warming is altering this well-known law? Here, based on ∼20,000 observations of the leaf-out date of four common temperate tree species located in 128 sites at various elevations in the European Alps, we found that the elevation-induced phenological shift (EPS) has significantly declined from 34 d⋅1,000 m-1 conforming to Hopkins' bioclimatic law in 1960, to 22 d⋅1,000 m-1 in 2016, i.e., -35%. The stronger phenological advance at higher elevations, responsible for the reduction in EPS, is most likely to be connected to stronger warming during late spring as well as to warmer winter temperatures. Indeed, under similar spring temperatures, we found that the EPS was substantially reduced in years when the previous winter was warmer. Our results provide empirical evidence for a declining EPS over the last six decades. Future climate warming may further reduce the EPS with consequences for the structure and function of mountain forest ecosystems, in particular through changes in plant-animal interactions, but the actual impact of such ongoing change is today largely unknown.
We combined imagery from multiple sources (MODIS, Landsat-5, 7, 8) with land cover data to test for long-term (1984-2015) greening or browning trends of vegetation in a temperate alpine area, the Ecrins National Park, in the context of recent climate change and domestic grazing practices. We showed that over half (56%) of the Ecrins National Park displayed significant increases in peak normalized difference vegetation index (NDVI max) over the last 16 years (2000-2015). Importantly, the highest proportional increases in NDVI max occurred in rocky habitats at high elevations (> 2500 m a.s.l.). While spatial agreement in the direction of change in NDVI max as detected by MODIS and Landsat was high (76% overlap), correlations between log-response ratio values were of moderate strength (approx. 0.3). In the context of above treeline habitats, we found that proportional increases in NDVI max were higher between 1984 and 2000 than between 2000 and 2015, suggesting a slowing of greening dynamics during the recent decade. The timing of accelerated greening prior to 2000 coincided with a pronounced increase in the amount of snow-free growing degree-days that occurred during the 1980s and 1990s. In the case of grasslands and low-shrub habitats, we did not find evidence for a negative effect of grazing on greening trends, possibly due to the low grazing intensity typically found in the study area. We propose that the emergence of a longer and warmer growing season enabled high-elevation plant communities to produce more biomass, and also allowed for plant colonization of habitats previously characterized by long-lasting snow cover. Increasing plant productivity in an alpine context has potential implications for biodiversity trajectories and for ecosystem services in mountain landscapes. The presented evidence for long-term greening trends in a representative region of the European Alps provides the basis for further research on mechanisms of greening in alpine landscapes.
Climate change can have a broad range of effects on ecosystems and organisms, and early responses may include shifts in vegetation phenology and productivity that may not coincide with the energetics and forage timing of higher trophic levels. We evaluated phenology, annual height growth, and foliar frost responses of forbs to a factorial experiment of snow removal (SR) and warming in a high-elevation meadow over two years in the Rocky Mountains, United States. Species included arrowleaf balsamroot (Balsamorhiza sagittata, early-season emergence and flowering) and buckwheat (Eriogonum umbellatum, semi-woody and late-season flowering), key forbs for pollinator and nectar-using animal communities that are widely distributed and locally abundant in western North America. Snow removal exerted stronger effects than did warming, and advanced phenology differently for each species. Specifically, SR advanced green-up by a few days for B. sagittata to >2 wk in E. umbellatum, and led to 5- to 11-d advances in flowering of B. sagittata in one year and advances in bud break in 3 of 4 species/yr combinations. Snow removal increased height of E. umbellatum appreciably (~5 cm added to ~22.8 cm in control), but led to substantial increases in frost damage to flowers of B. sagittata. Whereas warming had no effects on E. umbellatum, it increased heights of B. sagittata by >6 cm (compared to 30.7 cm in control plots) and moreover led to appreciable reductions in frost damage to flowers. These data suggest that timing of snowmelt, which is highly variable from year to year but is advancing in recent decades, has a greater impact on these critical phenological, growth, and floral survival traits and floral/nectar resources than warming per se, although warming mitigated early effects of SR on frost kill of flowers. Given the short growing season of these species, the shifts could cause uncoupling in nectar availability and timing of foraging.
Global warming has strong impacts on snow cover, which in turn affects ecosystems, hydrological regimes and winter tourism. Only a few long-term snow series are available worldwide, especially at high elevation. Here, we analyzed several snowpack characteristics over the period 1970–2015 at eleven meteorological stations, spanning elevations from 1139 to 2540 m asl in the Swiss Alps. Snow cover duration has significantly shortened at all sites, on average by 8.9 days decade−1. This shortening was largely driven by earlier snowmelt (on average 5.8 days decade−1) and partly by later snow onset but the latter was significant in only ~30 % of the stations. On average, the snow season now starts 12 days later and ends 26 days earlier than in 1970. Overall, the annual maximum snow depth has declined from 3.9 to 10.6 % decade−1 and was reached 7.8 ± 0.4 to 12.0 ± 0.4 days decade−1 earlier, though these trends hide a high inter-annual and decadal variability. The number of days with snow on the ground has also significantly decreased at all elevations, in all regions and for all thresholds from 1 to 100 cm. Overall, our results demonstrate a marked decline in all snowpack parameters, irrespective of elevation and region, and whether for drier or wetter locations, with a pronounced shift of the snowmelt in spring, in connection with reinforced warming during this season.
Himalayan mountain glaciers and the snowpack over the Tibetan Plateau provide
the headwater of several major rivers in Asia. In situ observations of snow
cover extent since the 1960s suggest that the snowpack in the region have
retreated significantly, accompanied by a surface warming of
2–2.5 °C observed over the peak altitudes (5000 m). Using a
high-resolution ocean–atmosphere global climate model and an observationally
constrained black carbon (BC) aerosol forcing, we attribute the observed
altitude dependence of the warming trends as well as the spatial pattern of
reductions in snow depths and snow cover extent to various anthropogenic
factors. At the Tibetan Plateau altitudes, the increase in atmospheric
CO2 concentration exerted a warming of 1.7 °C, BC
1.3 °C where as cooling aerosols cause about 0.7 °C
cooling, bringing the net simulated warming consistent with the anomalously
large observed warming. We therefore conclude that BC together with CO2
has contributed to the snow retreat trends. In particular, BC increase is the
major factor in the strong elevation dependence of the observed surface
warming. The atmospheric warming by BC as well as its surface darkening of
snow is coupled with the positive snow albedo feedbacks to account for the
disproportionately large role of BC in high-elevation regions. These findings
reveal that BC impact needs to be properly accounted for in future regional
climate projections, in particular on high-altitude cryosphere.
In alpine environments, the growing season is severely constrained by low temperature and snow. Here, we aim at determining the climatic factors that best explain the interannual variation in spring growth onset of alpine plants, and at examining whether photoperiod might limit their phe- nological response during exceptionally warm springs and early snowmelts. We analysed 17 years of data (1998–2014) from 35 automatic weather stations located in subalpine and alpine zones ranging from 1560 to 2450 m asl in the Swiss Alps. These stations are equipped with ultrasonic sensors for snow depth measurements that are also able to detect plant growth in spring and summer, giving a unique opportunity to analyse snow and climate effects on alpine plant phenology. Our analysis showed high phenological variation among years, with one exceptionally early and late spring, namely 2011 and 2013. Overall, the timing of snowmelt and the beginning of plant growth were tightly linked irrespective of the elevation of the station. Snowmelt date was the best predictor of plant growth onset with air temperature after snowmelt modulating the plants’ development rate. This multiple series of alpine plant phenology suggests that currently alpine plants are directly tracking climate change with no major photoperiod limitation.
Alpine shrub- and grasslands are shaped by extreme climatic conditions such as a long-lasting snow cover and a short vegetation period. Such ecosystems are expected to be highly sensitive to global environmental change. Prolonged growing seasons and shifts in temperature and precipitation are likely to affect plant phenology and growth. In a unique experiment, climatology and plant growth was monitored for almost a decade at 17 snow meteorological stations in different alpine regions along the Swiss Alps. Regression analyses revealed highly significant correlations between mean air temperature in May/June and snow melt-out, onset of plant growth, and plant height. These correlations were used to project plant growth phenology for future climate conditions based on the gridded output of a set of regional climate models runs. Melt-out and onset of growth were projected to occur on average 17 days earlier by the end of the century than in the control period from 1971–2000 under the future climate conditions of the low resolution climate model ensemble. Plant height and biomass production were expected to increase by 77% and 45%, respectively. The earlier melt-out and onset of growth will probably cause a considerable shift towards higher growing plants and thus increased biomass. Our results represent the first quantitative and spatially explicit estimates of climate change impacts on future growing season length and the respective productivity of alpine plant communities in the Swiss Alps.
Alpine dwarf shrub communities are phenologically linked with snowmelt timing, so early spring exposure may increase risk of freezing damage during early development, and consequently reduce seasonal growth. We examined whether environmental factors (duration of snow cover, elevation) influenced size and the vulnerability of shrubs to spring freezing along elevational gradients and snow microhabitats by modelling the past frequency of spring freezing events. We sampled biomass and measured the size of Salix herbacea, Vaccinium myrtillus,
Vaccinium uliginosum and Loiseleuria procumbens in late spring. Leaves were exposed to freezing temperatures to determine the temperature at which 50 % of specimens are killed for each species and sampling site. By linking site snowmelt and temperatures to long-term climate measurements, we extrapolated the frequency of spring freezing events at each elevation, snow microhabitat and per species over 37 years. Snowmelt timing was significantly driven by microhabitat effects, but was independent of elevation. Shrub growth was neither enhanced nor reduced by earlier snowmelt, but decreased with elevation. Freezing resistance was strongly species dependent, and did not differ along the elevation or snowmelt gradient. Microclimate extrapolation suggested that potentially lethal freezing events (in May and June) occurred for three of the four species examined. Freezing events never occurred on late snow beds, and increased in frequency with earlier snowmelt and higher elevation. Extrapolated freezing events showed a slight, non-significant increase over the 37-year record. We suggest that earlier snowmelt does not enhance growth in four dominant alpine shrubs, but increases the risk of lethal spring freezing exposure for less freezing-resistant species.
Since the Industrial Revolution, CO 2 levels have been increasing with climate change. In this study, Analyze time-series changes in snow cover quantitatively and predict the vanishing point of snow cover statistically using remote sensing. The study area is Mt. Kilimanjaro, Tanzania. 23 image data of Landsat-5 TM and Landsat-7 ETM+, spanning the 27 years from June 1984 to July 2011, were acquired. For this study, first, atmospheric correction was performed on each image using the COST atmospheric correction model. Second, the snow cover area was extracted using the NDSI (Normalized Difference Snow Index) algorithm. Third, the minimum height of snow cover was determined using SRTM DEM. Finally, the vanishing point of snow cover was predicted using the trend line of a linear function. Analysis was divided using a total of 23 images and 17 images during the dry season. Results show that snow cover area decreased by approximately 6.47 km 2 from 9.01 km 2 to 2.54 km 2 , equivalent to a 73% reduction. The minimum height of snow cover increased by approximately 290 m, from 4,603 m to 4,893 m. Using the trend line result shows that the snow cover area decreased by approximately 0.342 km 2 in the dry season and 0.421 km 2 overall each year. In contrast, the annual increase in the minimum height of snow cover was approximately 9.848 m in the dry season and 11.251 m overall. Based on this analysis of vanishing point, there will be no snow cover 2020 at 95% confidence interval. This study can be used to monitor global climate change by providing the change in snow cover area and reference data when studying this area or similar areas in future research.
Upward migration of plant species due to climate change has become evident in several European mountain ranges. It is still, however, unclear whether certain plant traits increase the probability that a species will colonize mountain summits or vanish, and whether these traits differ with elevation. Here, we used data from a repeat survey of the occurrence of plant species on 120 summits, ranging from 2449 to 3418 m asl, in south-eastern Switzerland to identify plant traits that increase the probability of colonization or extinction in the 20th century. Species numbers increased across all plant traits considered. With some traits, however, numbers increased proportionally more. The most successful colonizers seemed to prefer warmer temperatures and well-developed soils. They produced achene fruits and/or seeds with pappus appendages. Conversely, cushion plants and species with capsule fruits were less efficient as colonizers. Observed changes in traits along the elevation gradient mainly corresponded to the natural distribution of traits. Extinctions did not seem to be clearly related to any trait.
Our study showed that plant traits varied along both temporal and elevational gradients. While seeds with pappus seemed to be advantageous for colonization, most of the trait changes also mirrored previous gradients of traits along elevation and hence illustrated the general upward migration of plant species. An understanding of the trait characteristics of colonizing species is crucial for predicting future changes in mountain vegetation under climate change.
We use output from global climate models available from the Coupled Model Intercomparison Project Phase 5 (CMIP5) for three different greenhouse gas emission scenarios to investigate whether the projected warming in mountains by the end of the 21st century is significantly different from that in low elevation regions. To remove the effects of latitudinal variation in warming rates, we focus on seasonal changes in the mid-latitude band of the northern hemisphere between 27.5° N and 40° N, where the two major mountain systems are the Tibetan Plateau/Himalayas in Asia and the Rocky Mountains in the United States. Results from the multi-model ensemble indicate that warming rates in mountains will be enhanced relative to non-mountain regions at the same latitude, particularly during the cold season. The strongest correlations of enhanced warming with elevation are obtained for the daily minimum temperature during winter, with the largest increases found for the Tibetan Plateau/Himalayas. The model projections indicate that this occurs, in part, because of proportionally greater increases in downward longwave radiation at higher elevations in response to increases in water vapor. The mechanisms for enhanced increases in winter and spring maximum temperatures in the Rockies appear to be influenced more by increases in surface absorption of solar radiation owing to a reduced snow cover. Furthermore, the amplification of warming with elevation is greater for a higher greenhouse gas emission scenario.
A better understanding of the impact of changing temperatures on snow amounts is very important for the ski industry, but it is difficult to measure, particularly at different times of the snow season and not only on an annual or seasonal basis. Here, we analyze the snow day vs precipitation day ratios on a monthly basis from November to April in Switzerland and at 52 meteorological stations located between 200 and 2,700 m above sea level over a 48-year time span. Our results show that the conditions measured in the 1960s in November and March correspond to the present ones in December, January, and February.
An analysis is presented of an ensemble of regional climate model (RCM) experiments from the ENSEMBLES project in terms of mean winter snow water equivalent (SWE), the seasonal evolution of snow cover, and the duration of the continuous snow cover season in the European Alps. Two sets of simulations are considered, one driven by GCMs assuming the SRES A1B greenhouse gas scenario for the period 1951–2099, and the other by the ERA-40 reanalysis for the recent past. The simulated SWE for Switzerland for the winters 1971–2000 is validated against an observational data set derived from daily snow depth measurements. Model validation shows that the RCMs are capable of simulating the general spatial and seasonal variability of Alpine snow cover, but generally underestimate snow at elevations below 1,000 m and overestimate snow above 1,500 m. Model biases in snow cover can partly be related to biases in the atmospheric forcing. The analysis of climate projections for the twenty first century reveals high inter-model agreement on the following points: The strongest relative reduction in winter mean SWE is found below 1,500 m, amounting to 40–80 % by mid century relative to 1971–2000 and depending upon the model considered. At these elevations, mean winter temperatures are close to the melting point. At higher elevations the decrease of mean winter SWE is less pronounced but still a robust feature. For instance, at elevations of 2,000–2,500 m, SWE reductions amount to 10–60 % by mid century and to 30–80 % by the end of the century. The duration of the continuous snow cover season shows an asymmetric reduction with strongest shortening in springtime when ablation is the dominant factor for changes in SWE. We also find a substantial ensemble-mean reduction of snow reliability relevant to winter tourism at elevations below about 1,800 m by mid century, and at elevations below about 2,000 m by the end of the century.
We assessed the freezing resistance of leaves ex situ of 25 Australian alpine plant species.We compared
the freezing resistance of forb, graminoid and shrub species from three alpine summits of different altitudes; from a low altitude site just above treeline, to a fully alpine tundra site. Foliar freezing resistance (LT50) in spring varied from -5.9°C to -18.7°C and standardized LT50 values within species were significantly related to site altitude. Additionally, when comparing all the species in the study, freezing resistance was significantly related to site; the LT50 of samples from a low-altitude summit (1696 m) were significantly lower than those of samples from mid- (1805 m) and high-altitude (1860 m) summits.The LT50 of juvenile foliage did not differ significantly from that of adult foliage. Shrubs were highly resistant to freezing. At the highest summit, we examined the course of seasonal freezing resistance from early summer to early autumn across three alpine plant communities that differed in the time of natural snowmelt; from sheltered (snowpatch) to exposed (open heath). No differences in freezing resistance over the growing season were detected for exposed or sheltered communities and there were no consistent trends indicating frost hardening over the growing season. Overall, the common Australian alpine species we investigated appear well adapted to freezing conditions throughout the snow-free growing season. We have no evidence to suggest that freezing temperatures soon after snowmelt in spring are especially damaging to the alpine plants at these summits.
Recent climate models predict future changes in temperature and precipitation in the Alps. To assess the potential response of alpine plant communities to climate change, we analyzed specific and combined effects of temperature, precipitation, and snow season timing on the growth of plants. This analysis is based on data from 17 snow meteorological stations and includes plant growth records from the same sites over 10 years. Using multiple regression and path analysis, we found that plant growth was primarily driven by climatic factors controlled by the timing of the snow season. Air temperature and precipitation before snow-up and after melt-out yielded the greatest direct impact on maximum plant height as well as growth rates. The variability of environmental drivers between sites versus between years had different effects on plant growth: e.g., sites with early melt-out dates hosted plant communities with tall, slow-growing vegetation. But interannual variations in melt-out dates at a given site did not produce measurable differences in plant growth performance. However, high temperatures after melt-out invariably resulted in a shortened growth period. We speculate that the plant growth patterns we observed in response to climate variation between sites are indicative of the long-term responses of alpine plant communities to persistent climate changes. With most climate models indicating shorter winters, we thus expect alpine grasslands in the Alps to display an enhanced biomass production in the future.
Snow properties such as snow density will likely change in a warmer climate. Changes in depth and extent of snow cover have been shown to affect soil nutrient dynamics and plant growth; however, effects of a changed snow density have so far not been explicitly tested. We altered snow properties (especially depth and density according to those found on ski runs) and investigated effects on soil temperatures, soil nitrogen mineralization, plant phenology, and productivity. A denser, thinner snow cover led to reduced soil insulation and lower soil temperatures, which consequently increased net N mineralization. A denser snow cover furthermore resulted in a delay in plant phenology of up to five weeks after melt-out. The results suggest that changes in snow density, which have been largely neglected in the global change discussion until now, can cause significant changes in soil and vegetation processes.
In temperate-zone mountains, summer frosts usually occur during unpredictable cold spells with snow-falls. Earlier studies have shown that vegetative aboveground organs of most high-mountain plants tolerate extracellular ice in the active state. However, little is known about the impact of frost on reproductive development and reproductive success. In common plant species from the European Alps (Cerastium uniflorum, Loiseleuria procumbens, Ranunculus glacialis, Rhododendron ferrugineum, Saxifraga bryoides, S. moschata, S. caesia), differing in growth form, altitudinal distribution and phenology, frost resistance of reproductive and vegetative shoots was assessed in different reproductive stages. Intact plants were exposed to simulated night frosts between -2 and -14 °C in temperature-controlled freezers. Nucleation temperatures, freezing damage and subsequent reproductive success (fruit and seed set, seed germination) were determined. During all reproductive stages, reproductive shoots were significantly less frost resistant than vegetative shoots (mean difference for LT(50) -4.2 ± 2.7 K). In most species, reproductive shoots were ice tolerant before bolting and during fruiting (mean LT(50) -7 and -5.7 °C), but were ice sensitive during bolting and anthesis (mean LT(50) around -4 °C). Only R. glacialis remained ice tolerant during all reproductive stages. Frost injury in reproductive shoots usually led to full fruit loss. Reproductive success of frost-treated but undamaged shoots did not differ significantly from control values. Assessing the frost damage risk on the basis of summer frost frequency and frost resistance shows that, in the alpine zone, low-statured species are rarely endangered as long as they are protected by snow. The situation is different in the subnival and nival zone, where frost-sensitive reproductive shoots may become frost damaged even when covered by snow. Unprotected individuals are at high risk of suffering from frost damage, particularly at higher elevations. It appears that ice tolerance in reproductive structures is an advantage but not an absolute precondition for colonizing high altitudes with frequent frost events.
Snow is an important environmental factor in alpine ecosystems, which influences plant phenology, growth and species composition in various ways. With current climate warming, the snow-to-rain ratio is decreasing, and the timing of snowmelt advancing. In a 2-year field experiment above treeline in the Swiss Alps, we investigated how a substantial decrease in snow depth and an earlier snowmelt affect plant phenology, growth, and reproduction of the four most abundant dwarf-shrub species in an alpine tundra community. By advancing the timing when plants started their growing season and thus lost their winter frost hardiness, earlier snowmelt also changed the number of low-temperature events they experienced while frost sensitive. This seemed to outweigh the positive effects of a longer growing season and hence, aboveground growth was reduced after advanced snowmelt in three of the four species studied. Only Loiseleuria procumbens, a specialist of wind exposed sites with little snow, benefited from an advanced snowmelt. We conclude that changes in the snow cover can have a wide range of species-specific effects on alpine tundra plants. Thus, changes in winter climate and snow cover characteristics should be taken into account when predicting climate change effects on alpine ecosystems.
We analysed long-term temperature trends based on 12 homogenised series of monthly temperature data in Switzerland at elevations between 316 m.a.s.l. and 2490 m.a.s.l for the 20 th century (1901–2000) and for the last thirty years (1975–2004). Comparisons were made between these two periods, with changes standardised to decadal trends. Our results show mean decadal trends of þ0.135 C during the 20 th century and þ0.57 C based on the last three decades only. These trends are more than twice as high as the averaged temperature trends in the Northern Hemisphere. Most stations behave quite similarly, indicating that the increasing trends are linked to large-scale rather than local processes. Seasonal analyses show that the greatest tempera-ture increase in the 1975–2004 period occurred during spring and summer whereas they were particularly weak in spring during the 20 th century. Recent temperature increases are as much related to increases in maximum temperatures as to increases in minimum temperature, a trend that was not apparent in the 1901–2000 period. The different seasonal warming rates may have important consequences for veg-etation, natural disasters, human health, and energy con-sumption, amongst others. The strong increase in summer temperatures helps to explain the accelerated glacier retreat in the Alps since 1980.
1] The number of days with a snow depth above a certain threshold is the key factor for winter tourism in an Alpine country like Switzerland. An investigation of 34 long-term stations between 200 and 1800 m asl (above sea level) going back for at least the last 60 years (1948 – 2007) shows an unprecedented series of low snow winters in the last 20 years. The signal is uniform despite high regional differences. A shift detection analysis revealed a significant step-like decrease in snow days at the end of the 1980's with no clear trend since then. This abrupt change resulted in a loss of 20% to 60% of the total snow days. The stepwise increase of the mean winter temperature at the end of the 1980's and its close correlation with the snow day anomalies corroborate the sensitivity of the mid-latitude winter to the climate change induced temperature increase.
Delphinium barbeyi is a common herbaceous wildflower in montane meadows at 2,900 m near the Rocky Mountain Biological Laboratory, and its flowers are important nectar resources for bumblebees and hummingbirds. During the period 1977–1999 flowering was highly variable in both timing (date of first flower ranged from 5 July to 6 August, mean=17 July) and abundance (maximum open flowers per 2×2-m plot ranged from 11.3 to 197.9, mean=82). Time and abundance of flowering are highly correlated with the previous winter's snowpack, as measured by the amount of snow remaining on the ground on 15 May (range 0–185 cm, mean=67.1). We used structural equation modeling to investigate relationships among snowpack, first date of bare ground, first date of flowering, number of inflorescences produced, and peak number of flowers, all of which are significantly correlated with each other. Snowpack depth on 15 May is a significant predictor of the first date of bare ground (R
2=0.872), which in turn is a significant predictor of the first date of flowering (R
2=0.858); snowpack depth is also significantly correlated with number of inflorescences produced (R
2=0.713). Both the number of inflorescences and mean date of first flowering are significant predictors of flowers produced (but with no residual effect of snowpack). Part of the effect of snowpack on flowering may be mediated through an increased probability of frost damage in years with lower snowpack – the frequency of early-season "frost events" explained a significant proportion of the variance in the number of flowers per stem. There is significant reduction of flower production in La Niña episodes. The variation in number of flowers we have observed is likely to affect the pollination, mating system, and demography of the species. Through its effect on snowpack, frost events, and their interaction, climate change may influence all of these variables.
Alpine shrub- and grasslands are shaped by extreme climatic conditions such as a long-lasting snow cover and a short vegetation period. Such ecosystems are expected to be highly sensitive to global environmental change. Prolonged growing seasons and shifts in temperature and precipitation are likely to affect plant phenology and growth. In a unique experiment, climatology and plant growth was monitored for almost a decade at 17 snow meteorological stations in different alpine regions along the Swiss Alps. Regression analyses revealed highly significant correlations between mean air temperature in May/June and snow melt-out, onset of plant growth, and plant height. These correlations were used to project plant growth phenology for future climate conditions based on the gridded output of a set of regional climate models runs. Melt-out and onset of growth were projected to occur on average 17 days earlier by the end of the century than in the control period from 1971-2000 under the future climate conditions of the low resolution climate model ensemble. Plant height and biomass production were expected to increase by 77% and 45%, respectively. The earlier melt-out and onset of growth will probably cause a considerable shift towards higher growing plants and thus increased biomass. Our results represent the first quantitative and spatially explicit estimates of climate change impacts on future growing season length and the respective productivity of alpine plant communities in the Swiss Alps.
Predicted increases in the length of the growing season as a result of climate change may more frequently expose high-elevation plants to severe frosts. Understanding the ability of these species to resist frosts during the growing season is essential for predicting how species may respond to changes in temperature regimes. Here, we assessed the freezing resistance of 24 species from the central Chilean Andes by determining their low temperature damage (LT(50)), ice nucleation temperature (NT), freezing point (FP) and freezing resistance mechanism (i.e. avoidance or tolerance). The Andean species were found to resist frosts from -8.2 to -19.5 degrees C during the growing season, and freezing tolerance was the most common resistance mechanism. Freezing resistance (LT(50)) varied within the growing season, decreasing towards the end of this period in most of the studied species. However, the FP showed the opposite trend. LT(50) increased with elevation, whilst FP was lower in plants from lower elevations, especially late in the growing season. Andean species have the potential to withstand severe freezing conditions during the growing season, and the aridity of this high-elevation environment seems to play an important role in determining this high freezing resistance.
If snow cover in alpine environments were reduced through climatic warming, plants that are normally protected by snow-lie in winter would become exposed to greater extremes of temperature and solar radiation. We examined the annual course of frost resistance of species of native alpine plants from southern New Zealand that are normally buried in snowbanks over winter (Celmisia haastii and Celmisia prorepens) or in sheltered areas that may accumulate snow (Hebe odora) and other species, typical of more exposed areas, that are relatively snow-free (Celmisia viscosa, Poa colensoi, Dracophyllum muscoides). The frost resistance of these principal species was in accord with habitat: those from snowbanks or sheltered areas showed the least frost resistance, whereas species from exposed areas had greater frost resistance throughout the year. P. colensoi had the greatest frost resistance (-32.5 degrees C). All the principal species showed a rapid increase in frost resistance from summer to early winter (February-June) and maximum frost resistance in winter (July-August). The loss of resistance in late winter to early summer (August-December) was most rapid in P. colensoi and D. muscoides. Seasonal frost resistance of the principal species was more strongly related to daylength than to temperature, although all species except C. viscosa were significantly related to temperature when the influence of daylength was accounted for. Measurements of chlorophyll fluorescence indicated that photosynthetic efficiency of the principal species declined with increasing daylength. Levels of frost resistance of the six principal alpine plant species, and others measured during the growing season, were similar to those measured in tropical alpine areas and somewhat more resistant than those recorded in alpine areas of Europe. The potential for frost damage was greatest in spring. The current relationship of frost resistance with daylength is sufficient to prevent damage at any time of year. While warmer temperatures might lower frost resistance, they would also reduce the incidence of frosts, and the incidence of frost damage is unlikely to be altered. The relationship of frost resistance with daylength and temperature potentially provides a means of predicting the responses of alpine plants in response to global warming.
Winters and early springs are predicted to become warmer in temperate climates under continued global warming, which in turn is expected to promote earlier plant development. By contrast, there is no consensus about the changes in the occurrence and severity of late spring frosts. If the frequency and severity of late spring frosts remain unchanged in the future or change less than spring phenology of plants does, vulnerable plant organs (dehardened buds, young leaves, flowers or young fruits) may be more exposed to frost damage. Here we analyzed long-term temperature data from the period 1975–2016 in 50 locations in Switzerland and used different phenological models calibrated with long-term series of the flowering and leaf-out timing of two fruit trees (apple and cherry) and two forest trees (Norway spruce and European beech) to test whether the risk of frost damage has increased during this period. Overall, despite the substantial increase in temperature during the study period, the risk of frost damage was not reduced because spring phenology has advanced at a faster rate than the date of the last spring frost. In contrast, we found that the risk of frost exposure and subsequent potential damage has increased for all four species at the vast majority of stations located at elevations higher than 800 m while remaining unchanged at lower elevations. The different trends between lower and higher elevations are due to the date of the last spring frost moving less at higher altitudes than at lower altitudes, combined with stronger phenological shifts at higher elevations. This latter trend likely results from a stronger warming during late compared to earlier spring and from the increasing role of other limiting factors at lower elevations (chilling and photoperiod). Our results suggest that frost risk needs to be considered carefully when promoting the introduction of new varieties of fruit trees or exotic forest tree species adapted to warmer and drier climates or when considering new plantations at higher elevations.
Despite the importance of spring freezing events for alpine species distribution, few studies have analysed the response of alpine shrub species to early spring freezes. It is also not known how snow cover gradients influence the process of de-hardening between individuals of the same species and their vulnerability to early spring frosts. We analysed early spring freezing resistance for the buds of eight alpine Ericaceae shrubs growing at 1 m snow depth at treeline in the Swiss Alps. Moreover, buds of Rhododendron ferrugineum and Loiseleuria procumbens were analysed for freezing resistance and sugar content along a snow depth gradient. The LT50 (lethal temperature for 50% of samples) of the eight species ranged from -25.1 ± 1.6 °C (Vaccinium vitis-idaea) to -11.1 ± 1.2 °C (Vaccinium uliginosum), with differences being related to the phenological stage in addition to shrub preferences for contrasting snow cover microsites. Although the effect of snow depth on the freezing resistance of plants was not significant, samples collected from 1 m and 1.5 m snow depth tended to be more vulnerable to freezing, particularly L. procumbens. Buds collected at shallower snow depths had higher sugar concentrations, indicative of their stronger physiological hardening. Consequently, we conclude that differences in snow cover may significantly affect the physiological hardening of plants during the onset of spring. Individuals growing at less than 0.5 m snow cover are hardier, i.e. show moderately higher freezing resistance than individuals from snow banks. Snow cover is a highly important aspect of climate change, and freezing resistance in alpine plants with respect to snow conditions can be a relevant driver of plant responses to climate change.
Twenty‐first century snow depth and snow water equivalent ( SWE ) changes are assessed for three time periods (2020–2049, 2045–2079 and 2070–2099) at 11 stations in Switzerland with the physics‐based snow model SNOWPACK and meteorological input data perturbed by the output from ten regional climate models ( RCMs ) through the delta change method. Unlike in previous studies, incoming long‐wave radiation has also been modified for future climatic conditions. We thus show the range of future snow simulations assuming different RCM projections.
Model validation yields satisfying results for simulating snow depth and SWE for the reference period with errors in the order of 9% and 15%, respectively. For the end of the century, the stations between 1000–1700 m a.s.l. show no pronounced elevation dependence but surprisingly react quite similarly in terms of the relative magnitude of snow cover decrease, which may reach 90%. The projected small increase in winter precipitation has almost no effect at these stations, but incoming long‐wave radiation has an important effect. At the high‐elevation station Weissfluhjoch (2540 m a.s.l.) however, the precipitation increase is partly able to compensate for the increased temperature. This would imply that the snow cover at mid‐elevation stations becomes temperature and radiation dominated and will react similarly to the spatially small differences in the projected temperature change. The low‐elevation stations already show a strong decrease in the near future, and the inclusion of modified incoming long‐wave radiation has almost no effect on the decrease of future snow depth and SWE because the temperatures are already close to the melting point in the reference period. At the end of the century, mean snow depth/ SWE are reduced by 35/32%, 83/86% and 96/97% at high‐, mid‐ and low‐elevations, respectively.
used a first-order, monthly snow model and observations to disentangle
seasonal influences on 20th century,regional snowpack anomalies in the
Rocky Mountains of western North America, where interannual variations
in cool-season (November-March) temperatures are broadly synchronous,
but precipitation is typically antiphased north to south and
uncorrelated with temperature. Over the previous eight centuries,
regional snowpack variability exhibits strong, decadally persistent
north-south (N-S) antiphasing of snowpack anomalies. Contrary to the
normal regional antiphasing, two intervals of spatially synchronized
snow deficits were identified. Snow deficits shown during the 1930s were
synchronized north-south by low cool-season precipitation, with spring
warming (February-March) since the 1980s driving the majority of the
recent synchronous snow declines, especially across the low to middle
elevations. Spring warming strongly influenced low snowpacks in the
north after 1958, but not in the south until after 1980. The post-1980,
synchronous snow decline reduced snow cover at low to middle elevations
by ~20% and partly explains earlier and reduced streamflow and both
longer and more active fire seasons. Climatologies of Rocky Mountain
snowpack are shown to be seasonally and regionally complex, with Pacific
decadal variability positively reinforcing the anthropogenic warming
trend.
We quantify the effect of the snow-albedo feedback on Swiss spring temperature trends using daily temperature and snow depth measurements from six station pairs for the period 1961–2011. We show that the daily mean 2-m temperature of a spring day without snow cover is on average 0.4 °C warmer than one with snow cover at the same location. This estimate is comparable with estimates from climate modelling studies. Caused by the decreases in snow pack, the snow-albedo feedback amplifies observed temperature trends in spring. The influence is small and confined to areas around the upward-moving snow line in spring and early summer. For the 1961–2011 period, the related temperature trend increases are in the order of 3–7 % of the total observed trend.
This review contrasts the frost resistance of plants from the Southern Hemisphere with that of the Northern Hemisphere and is principally concerned with plants from New Zealand, Australia, and South America. It gives a brief overview of methods for determining frost resistance in the field and in controlled environments with intact or excised plant parts. It considers various methods of determining frost resistance and the expression of critical temperatures causing damage, and discusses the problems of using excised plant parts and freezing of tissues. This review, however, is not principally concerned with physiological aspects of frost resistance, but more with biogeographic aspects of the environment and quantification of the relationships between frost resistance and temperature related factors such as altitude and latitude. It gives examples of differences in frost resistance between the two hemispheres and attributes these to the contrast between the climates of largely continental land masses in the Northern Hemisphere and the oceanic environment of the Southern Hemisphere. Furthermore, it also shows similarities between the frost resistance of plants from the Southern Hemisphere during the growing season and the maximum frost resistance of tropical alpine species and further similarities between species on oceanic islands in both hemispheres. Comprehensive lists of species’ frost resistance are included in tables and appendices.
Climate change and elevated atmospheric CO2 levels could increase the vulnerability of plants to freezing. We analyzed tissue damage resulting from naturally occurring freezing events in plants from a long–term in situ CO2 enrichment (+ 200 ppm, 2001–2009) and soil warming (+ 4°C since 2007) experiment at treeline in the Swiss Alps (Stillberg, Davos). Summer freezing events caused damage in several abundant subalpine and alpine plant species in four out of six years between 2005 and 2010. Most freezing damage occurred when temperatures dropped below –1.5°C two to three weeks after snow melt. The tree Larix decidua and the dwarf shrubs Vaccinium myrtillus and Empetrum hermaphroditum showed more freezing damage under experimentally elevated CO2 and/or temperatures than under control conditions. Soil warming induced a 50% die-back of E. hermaphroditum during a single freezing event due to melting of the protective snow cover. Although we could not identify a clear mechanism, we relate greater freezing susceptibility to a combination of advanced plant phenology in spring and changes in plant physiology. The climate record since 1975 at the treeline site indicated a summer warming by 0.58°C/decade and a 3.5 days/decade earlier snow melt, but no significant decrease in freezing events during the vegetation period. Therefore, in a warmer climate with higher CO2 levels but constant likelihood of extreme weather events, subalpine and alpine plants may be more susceptible to freezing events, which may partially offset expected enhanced growth with global change. Hence, freezing damage should be considered when predicting changes in growth of alpine plants or changes in community composition under future atmospheric and climate conditions.
In many biomes, global warming has resulted in advanced and longer growing seasons, which has often led to earlier flowering in plant taxa. Elevational gradients are ideal to study the effects of global warming as they allow transplantation of plants from their original cooler higher elevations down to elevations with a prospective climate. We transplanted plants from ten populations of the European alpine monocarpic herb species Campanula thyrsoides L. to three sites along a steep mountain slope (600, 1,235 and 1,850 m above sea level) in the Swiss Alps and asked whether reproductive phenology adjusts plastically to elevation and if these responses were adaptive, i.e. increased the fitness of plants. We further assessed current genetic differentiation in phenotypic traits and whether any such origin effects were due to adaptation to climatic conditions of origin. Our results showed that transplantation to lower elevations caused strong shifts in phenology, with plants starting growth and flowering earlier than plants placed at higher elevations. However, compared to flower production at high elevation, number of flowers per plant decreased 21 % at mid- and 61 % at low elevation. The shift in phenology thus came with a high cost in fitness, and we suggest that phenology is maladaptive when C. thyrsoides faces temperature conditions deviating from its natural amplitude. We conclude that the frequently reported phenological shift in plant species as a response to global warming may include heavy fitness costs that may hamper species survival.
In cold regions, snow cover duration is expected to decrease, especially in spring, as a consequence of climate warming. We investigated effects of changes in timing of snowmelt in relation to weather conditions on Vaccinium myrtillus, a dominant shrub in heath vegetation. We tested the hypothesis that advanced snowmelt will enhance shrub growth in years with few frosts, but will reduce shrub growth in years with frequent frosts.
A sub-alpine heath in the Northern Apennines (Italy).
We carried out two experiments. In the main experiment, snow was added to (+S) or removed from (−S) experimental plots in spring of three growing seasons (2004–2006), with a mean delay in snowmelt timing of about 2 wk from −S to +S. In a companion experiment, we simulated a freezing event in late spring 2004.
During the snowmelt period, the −S plants experienced 6–10 more frost events, compared with +S and unmanipulated controls (C) in 2004 and 2005, but not in 2006. In the first 2 yr leaf production, leaf expansion and flowering were all significantly reduced in the −S plants, while shoot elongation was unaffected. In the companion experiment with artificial frost V. myrtillus presented similar responses. Conversely, the manipulations of snow did not affect either the hydric or nutrient status of plants and soils.
The results overall support our hypothesis, as shown by the differing effects of snow depth and timing of melt on V. myrtillus in the 3 yr. Spring frost was the cause of reduced growth and reduced flower production in 2004 and 2005. However, advanced snowmelt will not decrease the cover of this dominant species. Therefore, the structure and species dominance patterns in sub-alpine heath are not expected to change significantly in response to reduced snow cover. Support for this conclusion is provided by the capacity of V. myrtillus to recover vegetatively from frost injury through stimulated shoot elongation, and by the low importance of sexual reproduction for propagating dominant ericaceous shrubs in closed heath communities.
The European phyto-phenological database of the EU 5th Framework project ‘POSITIVE’ facilitated an examination of the rate and spatial pattern of changes in spring phenology across Europe. This database was collected, evaluated and composed from different national databases of Eastern and Western Europe covering the time period 1951–1998. Results show that spring phases have advanced four weeks in Western and Central Europe, and have been delayed up to two weeks in Eastern Europe. Western European spring starts earlier because of the intensive flow of warmer Atlantic air masses; the Eastern part of Europe has a different phenological rhythm and trends, that can be explained by the influence of the Siberian high. The highest rate of significant (p < 0.05) phenological change (−0.3 to −0.4 days per year) occurs in the Western Europe and Baltic Sea regions for early spring phases of hazel and colts-foot. Spring phases of birch, apple and lilac, and summer phases, such as the flowering of linden, tend to occur earlier with an average rate of −0.1 to 0.3 days per year. Copyright © 2002 Royal Meteorological Society.
The historical record of snow duration (from 1950 to 2009) and of snowfall (from 1960 to 2009) collected in the Italian Alps are presented and analysed. A reduction of the snow cover duration and of the snowfall stronger in springtime was detected during the last 40 years with the greatest decreasing rate during the 1990s. The last decade is characterised by a recovery from the documented decreasing trend mainly evident between 800 m and 1500 m. Principal component trend analysis of the snow duration and of the snowfall showed a long term decreasing trend. The change point test showed the existence of breakpoints between 1984 and 1994 that characterise the snow duration and snowfall time series analysed by elevation range and by seasons. These breakpoints mark a drastic trend variability in the time series: a positive trend characterises the time series before the breakpoint and a decreasing trend characterises the historical record after the breakpoint. The described negative trends result from the documented decrease in winter and spring precipitation. This in turn may either relate to a change in fraction of liquid to solid precipitation, and/or be associated to an increase of the temperatures. Northern Hemisphere and Italian Alps snow cover trends strongly correlate in the frequency domain. Among the dominant frequencies the 11.2 period was detected that may relate to the 11-year solar activity cycle.
Summer frost resistance and ice nucleation temperatures for 33 alpine plant species were measured in situ to avoid the shortcomings of laboratory tests. Species were selected to investigate the relationship between plant stature and upper distribution boundary, and frost resistance and freezing patterns. The species tested in situ were on average 1.1 K (± 0.2, SE) frost hardier than in laboratory tests. Frost resistance (LT50) ranged from −4.5 to −14.6 °C and appeared insufficient to protect against air temperature minima, corroborating reports of natural frost damage. All species tolerated extracellular ice formation (recorded at −1.9 ± 0.2 °C; E1). Initial frost damage occurred at average temperatures 4.9 K below E1. In 64% of the species a second exotherm (E2) and frost damage were recorded between −3.7 and −9.4 °C. In the highest ranging species E2 was not detectable. Frost resistance increased with increasing upper distribution boundary (0.4 K per 100 m), corresponding well with the altitudinal decrease in air temperature minima. No relationship between plant stature and frost resistance was found. Graminoids were significantly frost hardier than other growth forms. Frost survival at high altitudes will depend not only on altitudinal increase in frost resistance but also on freezing avoidance strategies, snow cover protection and a high recuperation capacity.
Global climate change impacts can already be tracked in many physical and biological systems; in particular, terrestrial ecosystems provide a consistent picture of observed changes. One of the preferred indicators is phenology, the science of natural recurring events, as their recorded dates provide a high-temporal resolution of ongoing changes. Thus, numerous analyses have demonstrated an earlier onset of spring events for mid and higher latitudes and a lengthening of the growing season. However, published single-site or single-species studies are particularly open to suspicion of being biased towards predominantly reporting climate change-induced impacts. No comprehensive study or meta-analysis has so far examined the possible lack of evidence for changes or shifts at sites where no temperature change is observed. We used an enormous systematic phenological network data set of more than 125 000 observational series of 542 plant and 19 animal species in 21 European countries (1971–2000). Our results showed that 78% of all leafing, flowering and fruiting records advanced (30% significantly) and only 3% were significantly delayed, whereas the signal of leaf colouring/fall is ambiguous. We conclude that previously published results of phenological changes were not biased by reporting or publication predisposition: the average advance of spring/summer was 2.5 days decade−1 in Europe. Our analysis of 254 mean national time series undoubtedly demonstrates that species' phenology is responsive to temperature of the preceding months (mean advance of spring/summer by 2.5 days°C−1, delay of leaf colouring and fall by 1.0 day°C−1). The pattern of observed change in spring efficiently matches measured national warming across 19 European countries (correlation coefficient r=−0.69, P<0.001).
The frequency of freezing events during the early growing season and the vulnerability to freezing of plants in European high-altitude environments could increase under future atmospheric and climate change. We tested early growing season freezing sensitivity in 10 species, from four plant functional types (PFTs) spanning three plant growth forms (PGFs), from a long-term in situ CO2 enrichment (566 vs. 370 ppm) and 2-year soil warming (+4 K) experiment at treeline in the Swiss Alps (Stillberg, Davos). By additionally tracking plant phenology, we distinguished indirect phenology-driven CO2 and warming effects from direct physiology-related effects on freezing sensitivity. The freezing damage threshold (lethal temperature 50) under ambient conditions of the 10 treeline species spanned from −6.7±0.3 °C (Larix decidua) to −9.9±0.6 °C (Vaccinium gaultherioides). PFT, but not PGF, explained a significant amount of this interspecific variation. Long-term exposure to elevated CO2 led to greater freezing sensitivity in multiple species but did not influence phenology, implying that physiological changes caused by CO2 enrichment were responsible for the effect. The elevated CO2 effect on freezing resistance was significant in leaves of Larix, Vaccinium myrtillus, and Gentiana punctata and marginally significant in leaves of Homogyne alpina and Avenella flexuosa. No significant CO2 effect was found in new shoots of Empetrum hermaphroditum or in leaves of Pinus uncinata, Leontodon helveticus, Melampyrum pratense, and V. gaultherioides. Soil warming led to advanced leaf expansion and reduced freezing resistance in V. myrtillus only, whereas Avenella showed greater freezing resistance when exposed to warming. No effect of soil warming was found in any of the other species. Effects of elevated CO2 and soil warming on freezing sensitivity were not consistent within PFTs or PGFs, suggesting that any future shifts in plant community composition due to increased damage from freezing events will likely occur at the individual species level.
This book is unique in providing a global overview of alpine (high mountain) habitats that occur above the natural (cold-limited) tree line, describing the factors that have shaped them over both ecological and evolutionary timescales. The broad geographic coverage helps synthesise common features whilst revealing differences in the world’s major alpine systems from the Arctic to the Tropics. The words “barren” and “wasteland” have often been applied to describe landscapes beyond the treeline. However, a closer look reveals a large diversity of habitats, assemblages and individual taxa, largely connected to topographic diversity within individual alpine regions. The book considers habitat-forming factors (landforms, energy and climate, hydrology, soils, and vegetation) individually, as well as their composite impacts on habitat characteristics. Evolution and population processes are examined in the context of the responsiveness / resilience of alpine habitats to global change. Finally, a critical assessment of the potential impacts of climate change, atmospheric pollutants and land use is made and related to the management and conservation options available for these unique habitats.
The timing of life history traits is central to lifetime fitness and nowhere is this more evident or well studied as in the phenology of flowering in governing plant reproductive success. Recent changes in the timing of environmental events attributable to climate change, such as the date of snowmelt at high altitudes, which initiates the growing season, have had important repercussions for some common perennial herbaceous wildflower species. The phenology of flowering at the Rocky Mountain Biological Laboratory (Colorado, USA) is strongly influenced by date of snowmelt, which makes this site ideal for examining phenological responses to climate change. Flower buds of Delphinium barbeyi, Erigeron speciosus, and Helianthella quinquenervis are sensitive to frost, and the earlier beginning of the growing season in recent years has exposed them to more frequent mid-June frost kills. From 1992 to 1998, on average 36.1% of Helianthella buds were frosted, but for 1999-2006 the mean is 73.9%; in only one year since 1998 have plants escaped all frost damage. For all three of these perennial species, there is a significant relationship between the date of snowmelt and the abundance of flowering that summer. Greater snowpack results in later snowmelt, later beginning of the growing season, and less frost mortality of buds. Microhabitat differences in snow accumulation, snowmelt patterns, and cold air drainage during frost events can be significant; an elevation difference of only 12 m between two plots resulted in a temperature difference of almost 2 degrees C in 2006 and a difference of 37% in frost damage to buds. The loss of flowers and therefore seeds can reduce recruitment in these plant populations, and affect pollinators, herbivores, and seed predators that previously relied on them. Other plant species in this environment are similarly susceptible to frost damage so the negative effects for recruitment and for consumers dependent on flowers and seeds could be widespread. These findings point out the paradox of increased frost damage in the face of global warming, provide important insights into the adaptive significance of phenology, and have general implications for flowering plants throughout the region and anywhere climate change is having similar impacts.
Hearing' alpine plants growing after snowmelt: ultrasonic snow sensors provide long-term series of alpine plant phenology
- Y Vitasse
- M Rebetez
- G Filippa
- E Cremonese
- G Klein
- C Rixen
Vitasse Y, Rebetez M, Filippa G, Cremonese E, Klein G, Rixen C (2017)
'Hearing' alpine plants growing after snowmelt: ultrasonic snow
sensors provide long-term series of alpine plant phenology. Int J
Biometeorol 61:349-361