Snowmelt Gradient Research Articles

Local environmental context drives heterogeneity of early succession dynamics in alpine glacier forefields

Abstract. Glacier forefields have long provided ecologists with a model to study patterns of plant succession following glacier retreat. While plant-survey-based approaches applied along chronosequences provide invaluable information on plant communities, the “space-for-time” approach assumes environmental uniformity and equal ecological potential across sites and does not account for spatial variability in initial site conditions. Remote sensing provides a promising avenue for assessing plant colonization dynamics using a so-called “real-time” approach. Here, we combined 36 years of Landsat imagery with extensive field sampling along chronosequences of deglaciation for eight glacier forefields in the southwestern European Alps to investigate the heterogeneity of early plant succession dynamics. Based on the two complementary and independent approaches, we found strong variability in the time lag between deglaciation and colonization by plants and in subsequent growth rates and in the composition of early plant succession. All three parameters were highly dependent on the local environmental context, i.e., neighboring vegetation cover and energy availability linked to temperature and snowmelt gradients. Potential geomorphological disturbance did not emerge as a strong predictor of succession parameters, which is perhaps due to insufficient spatial resolution of predictor variables. Notably, the identity of pioneer plant species was highly variable, and initial plant community composition had a much stronger influence on plant assemblages than elapsed time since deglaciation. Overall, both approaches converged towards the conclusion that early plant succession is not stochastic as previous authors have suggested but rather determined by local ecological context. We discuss the importance of scale in deciphering the complexity of plant succession in glacier forefields and provide recommendations for improving botanical field surveys and using Landsat time series in glacier forefield systems. Our work demonstrates complementarity between remote sensing and field-based approaches for both understanding and predicting future patterns of plant succession in glacier forefields.

Temporal dynamics in alpine snowpatch plants along a snowmelt gradient explained by functional traits and strategies.

Alpine snowpatches are characterised by persistent snow cover, short growing seasons and periglacial processes, which has resulted in highly specialised plant communities. Hence, these snowpatch communities are among the most threatened from climate change. However, temporal dynamics in snowpatch microclimate and plant composition are rarely explored, especially in the marginal alpine environments of Australia. Seven snowpatches were categorised into early, mid and late snowmelt zones based on growing season length, with soil temperatures recorded from 2003 to 2020 and plant composition surveyed in 84 1m2 quadrats in 2007, 2013 and 2020. Microclimate, species diversity, plant cover and composition, along with community-weighted trait means and plant strategies were assessed to understand snowpatch dynamics in response to climate change. We found that growing season length and temperatures have increased in late melt zones, while changes were less consistent in early and mid melt zones. There were few changes in species diversity, but increases in graminoids and declines in snowpatch specialists in mid and late melt zones. Community-weighted plant height, leaf area and leaf weight also increased, particularly in mid and late melt zones, while plant strategies shifted from compositions of ruderal-tolerant to stress-tolerant. Here, we show that snowpatch communities are rapidly changing in response to longer growing seasons and warmer temperatures, with the greatest changes occurring where snow persists the longest. The results highlight the climate-induced loss of defining biotic and abiotic characteristics of snowpatches, as temporal convergence of compositions along snowmelt gradients threatens the distinctiveness of snowpatch plant communities.

Alpine plant communities differ in their seed germination requirements along a snowmelt gradient in the Caucasus

Mesoscale heterogeneity of alpine landscapes generates snowmelt gradients resulting in a distinct vegetation zonation from almost snow-free fellfields to long-lasting snowbeds. Although the vegetative trait variation along such gradients has been intensively studied, little is known about whether and how seed germination is adapted to the variable snowpack duration. Here, we compare the seed germination niches of 18 characteristic plant species occurring in four distinct plant communities (alpine lichen heath—Festuca varia grassland—Geranium-Hedysarum meadow—snowbed) along an alpine snowmelt gradient in the North Caucasus (Russia). In a fully factorial experiment, we tested seed germination responses to temperature (10/2, 14/6, 18/10, 22/14, 26/18 and 30/22 °C) and water potential (0, − 0.2, − 0.4, − 0.6 and − 0.8 MPa) gradients, reflecting the full spectrum of seedling establishment conditions in the study system. Community-specific germination niches were identified by generalised mixed model with Bayesian estimation. Our study revealed that the species from the four focal communities significantly differed in their temperature requirements for germination, whereas soil moisture was found to be a limiting factor for seedling establishment along the entire gradient. The snowbed germination was constrained to comparatively high temperatures above 10 °C, whereas seed germination in alpine lichen heaths occurred under a wide range of experimental conditions. The seed germination patterns of species from Festuca varia grasslands and Geranium-Hedysarum meadows reflected their intermediate position along the snowmelt gradient. We conclude that seed germination niche is affected by the environmental filtering along the snowmelt gradient thus departing from the general alpine germination syndrome.

Plant Traits and Community Metrics across a Snowmelt Gradient at Alpine Snowbank Sites on Mt. Washington, New Hampshire

In northeastern North America, alpine snowbank communities are rare plant assemblages that form in sheltered sites above treeline where late-lying snow provides insulation from late-season frosts and a longer-lasting source of water. We measured snowpack and studied community composition and plant traits at the species and community scales across the snowmelt gradient at snowbank sites on Mt. Washington, New Hampshire. We used nonmetric multidimensional scaling ordination and analysis of similarity to examine community composition across the snowmelt gradient and measured plant traits (height, leaf dry matter content, leaf area, and specific leaf area) of four focal species (Carex bigelowii, Chamaepericlymenum canadense, Clintonia borealis, and Maianthemum canadense). We assessed trait variability of the four focal species across the snowmelt gradient and evaluated community-weighted mean trait values and phenotypic plasticity between snowbank core and edge habitats. Analysis of similarity indicated that vascular plant diversity increased (p < 0.001), lichen and bryophyte cover decreased (p < 0.001; p < 0.025), and community-weighted mean leaf area increased (p < 0.001) in the core of snowbank communities where melting dates were later. Analysis of similarity indicated that vascular plant communities varied significantly across the snowmelt gradient. The transition in community composition and trait values across the snowmelt gradient are indicative of changing environmental conditions and ecosystem functions, though more research is needed to determine the extent to which phenotypic plasticity and ecotypic uniqueness influence trait expression of snowbank species. Genetic analysis may be necessary to evaluate population dynamics among isolated alpine communities, which may be vulnerable to climate change or displacement by exotic or lowland species.

Nitrous oxide surface fluxes in a low Arctic heath: Effects of experimental warming along a natural snowmelt gradient

Climate change is profound in the Arctic where increased snowfall during winter and warmer growing season temperatures may accelerate soil nitrogen (N) turnover and increase inorganic N availability. Nitrous oxide (N2O) is a potent greenhouse gas formed by soil microbes and in the Arctic, the production is seen as limited mainly by low inorganic N availability. Hence, it can be hypothesized that climate change in the Arctic may increase total N2O emissions, yet this topic remains understudied. We investigated the combined effects of variable snow depths and experimental warming on soil N cycling in a factorial field study established along a natural snowmelt gradient in a low Arctic heath ecosystem. The study assessed N2O surface fluxes, gross N mineralization and nitrification rates, potential denitrification activity, and the pools of soil microbial, soil organic and soil inorganic N, carbon (C) and phosphorus (P) during two growing seasons. The net fluxes of N2O averaged 1.7 μg N2O–N m−2 h−1 (range −3.6 to 10.5 μg N2O–N m−2 h−1), and generally increased from ambient (1 m) to moderate (2–3 m) snow depths. At the greatest snow depth (4 m) where snowmelt was profoundly later, N2O fluxes decreased, likely caused by combined negative effects of low summer temperatures and high soil moisture. Positive correlations between N2O and nitrate (NO3−) and dissolved organic N (DON) suggested that the availability of N was the main controlling variable along the snowmelt gradient. The maximum N2O fluxes were observed in the second half of August associated with high NO3− concentrations. The effect of growing season experimental warming on N2O surface flux varied along the snowmelt gradient and with time. Generally, the experimental warming stimulated N2O fluxes under conditions with increased concentrations of inorganic N. In contrast, warming reduced N2O fluxes when inorganic N was low. Experimental warming had no clear effects on soil inorganic N. The study suggests that if increased winter precipitation leads to a deeper snow cover and a later snowmelt, total emissions of N2O from low Arctic heath ecosystems may be enhanced in the future and, dependent on dissolved N availability, summer warming may stimulate or reduce total emissions.

Climate change shifts population structure and demographics of an alpine herb,Anemone narcissiflora ssp. sachalinensis(Ranunculaceae), along a snowmelt gradient

AbstractAlpine ecosystems, characterized by cold climates and short growing seasons, are thought to be most vulnerable to climate change. Warmer temperatures and earlier snowmelt extend the growing season length and increase drought stress for alpine plants, resulting in changes to their distribution.Anemone narcissiflora ssp. sachalinensisis a perennial herb that grows in the alpine snow‐meadows of northern Japan. In the last few decades, its distribution has shifted toward later snowmelt habitat in the Taisetsu Mountains of Hokkaido. We recorded demographic data for this species at early, middle and late snowmelt habitats over four years (2009–2012), and constructed transition matrix models to evaluate how demographic parameters and population growth rate vary between local habitats along a snowmelt gradient. The proportion of reproductive plants was low and seed production was limited in the early snowmelt habitat, with drier soil conditions, in comparison to the middle and late snowmelt habitats, with moist soil conditions. Evidence of the transition from small plants to those in the reproductive stage was limited in the early snowmelt habitat, suggesting that growth was inhibited; the local population in this habitat was estimated to be sustained by seed migration from later snowmelt habitats. These results indicate that advancing snowmelt under climate change may decrease the reproductive activity and population growth rate of snow‐meadow plants if seed migration from later snowmelt populations is limited, resulting in the extinction of local populations.

Soil invertebrate abundance, diversity, and community composition across steep high elevation snowmelt gradients in the European Alps

ABSTRACT We studied abundance, diversity, and composition of soil invertebrates along snowmelt gradients to generally understand how soil animal communities are responding to life conditions across snowbeds along a west–east transect of the European Alps and to create a reference inventory for future investigations of climate change effects on snowbed habitats. We extracted microarthropods (collembolans, oribatid mites) and macroinvertebrates (spiders, beetles, insect larvae) from soil cores taken from three sections along the snowmelt gradient: high (early snowmelt), middle, and low (late snowmelt) sections. Linear models showed no correlations between either soil conditions or time of snowmelt and densities of soil animals. A small, though statistically significant, variation in the generally high soil organic matter and sand contents and high porosity of snowbed soils seems to have no effect on soil invertebrates. Species found along the snowmelt gradient were in similar shares generalist and specialist species. Microarthropod community composition in general was driven by soil porosity and soil organic matter content; for macroinvertebrate community composition we found no specific driver. We conclude that invertebrate species assemblages in snowbeds are rather similar in the European Alps.

Temporal variations in the linkages between plants and flower visitors and the pollination success of Primula modesta along the snowmelt gradient

Although pollination networks between plants and flower visitors are diverse and flexible, seed production of many plant species is restricted by pollen limitation. Obligate outcrossers often suffer from low pollinator activity or severe interspecific competition for pollinator acquisition among co‐flowering species. This study focused on seasonal changes in plant–flower visitor linkages in an alpine ecosystem and examined whether and how this seasonality affected the seed‐set of Primula modesta, a self‐incompatible distylous herb having long‐tubed flowers. First, we recorded the linkages between plants and flower visitors along the snowmelt gradient. Then, pollination experiment was conducted to estimate the degree of pollen limitation over the course of flowering season of P. modesta. Flower visitors were classified by their tongue length based on the morphological matching with P. modesta flowers. As the season progressed, plant–visitor linkages became more diverse and generalized, and the visitation frequency to P. modesta flowers increased. In the later part of the season, however, the seed set of P. modesta was significantly reduced due to severe pollen limitation, presumably because of increased competition for long‐tongued pollinators among co‐flowering species. The present study revealed that pollinator availability for specialist species may be restricted even when plant–visitor linkages are diverse and generalized as a whole. In the case of P. modesta, morphological matching and competition for pollinators might be the main factors explaining this discrepancy.

Do patterns of intra-specific variability and community weighted-means of leaf traits correspond? An example from alpine plants

Intraspecific variability of the traits is usually less than interspecific, but directions of inter-and intraspecific variation along environmental gradients are not well studied. For 17 alpine species we test a hypothesis that the direction of intraspecific variation in leaf traits among different communities along an environmental gradient coincides consistently with community weighted mean (CWM) trait variation at the community level along the same gradient. We obtained two groups of leaf traits according to their response to CWM and topographic (snow depth and snow melt) gradients. For leaf mass and area intraspecific variation corresponded to CWM variation among communities. SLA, water content and leaf thickness patterns within species changed directly among communities according to the toposequence (snowmelt gradient). These results are well expressed for forbs, but mostly they were not significant for graminoids. For leaf area we obtained opposite response of forbs and graminoids to snowmelt gradient. Forbs increased, but graminoids decreased leaf area when snow depth increased. Intraspecific trait variation across natural gradients does not necessarily follow that for interspecific or community-level variation.

Arthropod community composition along snowmelt gradients in snowbeds in the Snowy Mountains of south‐eastern Australia

AbstractEnvironmental gradients drive variation in community composition across a range of spatial scales. In alpine regions, areas of long‐lasting snow (‘snow patches’) create snowmelt gradients that drive considerable change in vegetation structure and composition over small spatial scales. This study examined whether there is parallel variation in arthropod communities using snowmelt gradients in the Australian Alps. Mites (Acarina) were the most common arthropods in snow patches, followed by springtails while, among the insects, the orders Hymenoptera (primarily Formicidae), Diptera, Coleoptera (primarily Carabidae) and Hemiptera (primarily Cicadellidae) dominated. Along the snowmelt gradient, arthropod assemblages changed from having equal proportions of predators and herbivores in early‐melting zones to being predator‐dominated in late‐melting zones, particularly early in the growing season. This followed a transition in vegetation cover and composition and was driven by higher numbers of predacious carabid beetles in later‐melting zones. Overall, however, our results suggest that snowbed arthropod communities in the Australian alpine zone are more sensitive to short‐term effects, such as time since snowmelt, than to differences in vegetation structure and composition or long‐term patterns of snowmelt. Continued advancement of snowmelt timing due to warmer spring temperatures is therefore likely to have more impact on the seasonality of snowbed arthropod communities than on the overall community composition.

Snowmelt Timing Regulates Community Composition, Phenology, and Physiological Performance of Alpine Plants.

The spatial patterning of alpine plant communities is strongly influenced by the variation in physical factors such as temperature and moisture, which are strongly affected by snow depth and snowmelt patterns. Earlier snowmelt timing and greater soil-moisture limitations may favor wide-ranging species adapted to a broader set of ecohydrological conditions than alpine-restricted species. We asked how plant community composition, phenology, plant water relations, and photosynthetic gas exchange of alpine-restricted and wide-ranging species differ in their responses to a ca. 40-day snowmelt gradient in the Colorado Rocky Mountains (Lewisia pygmaea, Sibbaldia procumbens, and Hymenoxys grandiflora were alpine-restricted and Artemisia scopulorum, Carex rupestris, and Geum rossii were wide-ranging species). As hypothesized, species richness and foliar cover increased with earlier snowmelt, due to a greater abundance of wide-ranging species present in earlier melting plots. Flowering initiation occurred earlier with earlier snowmelt for 12 out of 19 species analyzed, while flowering duration was shortened with later snowmelt for six species (all but one were wide-ranging species). We observed >50% declines in net photosynthesis from July to September as soil moisture and plant water potentials declined. Early-season stomatal conductance was higher in wide-ranging species, indicating a more competitive strategy for water acquisition when soil moisture is high. Even so, there were no associated differences in photosynthesis or transpiration, suggesting no strong differences between these groups in physiology. Our findings reveal that plant species with different ranges (alpine-restricted vs. wide-ranging) could have differential phenological and physiological responses to snowmelt timing and associated soil moisture dry-down, and that alpine-restricted species’ performance is more sensitive to snowmelt. As a result, alpine-restricted species may serve as better indicator species than their wide-ranging heterospecifics. Overall, alpine community composition and peak % cover are strongly structured by spatio-temporal patterns in snowmelt timing. Thus, near-term, community-wide changes (or variation) in phenology and physiology in response to shifts in snowmelt timing or rates of soil dry down are likely to be contingent on the legacy of past climate on community structure.

Different temperature perception in high‐elevation plants: new insight into phenological development and implications for climate change in the alpine tundra

In alpine habitats, predicted warmer and longer growing seasons will influence plant phenology, with important implications for species adaptation and vegetation dynamics. However, little is known on the temperature sensitivity of different phenophases and on the characteristics allowing phenological variation among and within species.By integrating interannual micro‐climatic variability with experimental warming, we explored how the phenology of three alpine species is influenced by temperature and what mechanisms underlie intra‐ and inter‐specific phenological differences.The present study demonstrated that alpine plants have different temperature responses during their reproductive cycle, do not have constant thermal thresholds and heat‐use efficiencies to achieve the seed dispersal stage and can change their temperature sensitivity to flower along snowmelt gradients. In addition, the length of the reproductive cycle, which proved to be species‐specific under experimental warming, does not seem to be the only life‐history trait under selective pressure due to the short‐length of the snow‐free period.In a warming climate scenario, the phenology of sexual reproduction will be considerably altered, and alpine plants may be subjected to changes in population dynamics driven by altered perception of environmental cues appropriate for coordinating the timing of key life‐history events.

Phenological responses to small-scale spatial variation in snowmelt timing reveal compensatory and conservative strategies in subarctic-alpine plants

Background: In tundra ecosystems, the adjustment of phenological events, such as bud burst, to snowmelt timing is crucial to the climatic adaptation of plants. Natural small-scale variations in microclimate potentially enable plant populations to persist in a changing climate.Aims: To assess how plant phenology responds to natural differences in snowmelt timing.Methods: We observed the timing of eight vegetative and reproductive phenophases in seven dwarf-shrub species in relation to differences in snowmelt timing on a small spatial scale in an alpine environment in subarctic Finland.Results: Some species and phenophases showed accelerated development with later snowmelt, thus providing full or partial compensation for the shorter snow-free period. Full compensation resulted in synchronous occurrence of phenophases across the snowmelt gradient. In other species, there was no acceleration of development. The timing of phenophases varied between two consecutive years and two opposing mountain slope aspects.Conclusions: The results have shown three distinct patterns in the timing of phenophases in relation to snowmelt and suggest alternative strategies for adaptation to snowmelt timing. These strategies potentially apply to other species and tundra ecosystems and provide a framework, enabling one to compare and generalise phenological responses to snowmelt timing under different future climate scenarios.

Seasonal changes in pollen limitation and femaleness along the snowmelt gradient in a distylous alpine herb,Primula modesta

Flowering phenology of alpine plants is strongly determined by the timing of snowmelt, and the conditions of pollination of widely distributed plants vary greatly during their flowering season. We examined the reproductive success of the distylous alpine herb, Primula modesta, along the snowmelt gradient under natural conditions, and compared it with the result of artificial pollination experiments. In addition, the compositions and visit frequencies of pollinators to the flower of P. modesta were examined during the flowering period. The pin and thrum plants of P. modesta growing at the same site have an equal ability to produce seeds if a sufficient amount of legitimate pollen grains are deposited on the stigma surface. However, under natural conditions, their seed‐set success was often (even if not always) restricted by pollen limitation, and the functional gender of the pin and thrum plants biased to the female and male, respectively, associated with their growing sites. These variations were not ascribed to resource limitation nor biased morph ratio but to the seasonal changes in pollination situations, a replacement of pollinator types from long‐ to short‐tongued pollinators resulted in unidirectional pollen transfer from long stamens (thrum plants) to long styles (pin plants). The functional gender specialization may enhance the evolution of dioecy from heterostyly, but the severe pollen limitation may cause the breakdown of heterostyly into homostyly. To consider the evolutionary pathway of heterostylous plants, an accumulation of the empirical data is required demonstrating how phenological synchrony between plants and pollinators is decided and to what degree this relationship is stable over years, along with estimates of selection and gene flow in individual plants.

Variations in functional diversity in snowbed plant communities determining snowbed continuity

Snowbed habitats are home to plant species that have adapted to particular environmental conditions (i.e. long-lasting snow cover and short growing seasons). The presence of most of these species is dependent on a long period of snow cover and so their conservation may well depend in the future on their ability to adapt to the effects of climate change. The aim of this study was to assess the persistence of snowbed communities using functional trait and functional diversity indices. We used data for plant species abundances from 32 snowbeds in Andorra (Pyrenees) classified according to certain environmental variables (elevation, exposure, soil type and temperature) and snow cover duration. Nine functional traits were used to evaluate the functional diversity, which was characterised as consisting of functional richness, functional evenness, functional dispersion, functional divergence and the community-weighted mean trait values. In two snowbeds, plant traits were also recorded and variation analysed along a snowmelt gradient. We found that snowbed specialist species had functional traits that were well adapted to the particular abiotic conditions of snowbed habitats but that there was a predominance of the functional traits of grass species in species originating in neighbouring communities. We found less functional richness, fewer strategies and lower competitive ability in the adapted species as the severity of the abiotic conditions increased. Snowbed specialist species appear to be less sensitive to the length of the growing season than species from neighbouring communities. Our results suggest that non-specialist species will tend to appear more frequently in those snowbed habitats affected by the reduction in snow cover duration.

Small-scale drivers: the importance of nutrient availability and snowmelt timing on performance of the alpine shrub Salix herbacea.

Alpine plant communities are predicted to face range shifts and possibly extinctions with climate change. Fine-scale environmental variation such as nutrient availability or snowmelt timing may contribute to the ability of plant species to persist locally; however, variation in nutrient availability in alpine landscapes is largely unmeasured. On three mountains around Davos, Switzerland, we deployed Plant Root Simulator probes around 58 Salix herbacea plants along an elevational and microhabitat gradient to measure nutrient availability during the first 5 weeks of the summer growing season, and used in situ temperature loggers and observational data to determine date of spring snowmelt. We also visited the plants weekly to assess performance, as measured by stem number, fruiting, and herbivory damage. We found a wide snowmelt gradient which determined growing season length, as well as variations of an order of magnitude or more in the accumulation of 12 nutrients between different microhabitats. Higher nutrient availability had negative effects on most shrub performance metrics, for instance decreasing stem number and the proportion of stems producing fruits. High nutrient availability was associated with increased herbivory damage in early-melting microhabitats, but among late-emerging plants this pattern was reversed. We demonstrate that nutrient availability is highly variable in alpine settings, and that it strongly influences performance in an alpine dwarf shrub, sometimes modifying the response of shrubs to snowmelt timing. As the climate warms and human-induced nitrogen deposition continues in the Alps, these factors may contribute to patterns of local plants persistence.

With a little help from my friends: Community facilitation increases performance in the dwarf shrub Salix herbacea

Shifts between positive and negative plant interactions along environmental stress gradients can affect alpine plant performance. We removed neighbours around Salix herbacea, a common arctic and alpine dwarf shrub, along elevational and snowmelt gradients on three mountains in Switzerland. The objectives of our study were to determine the effect of neighbours on phenological, morphological, and fitness traits of S. herbacea, and to determine whether neighbour interactions shift from competition to facilitation along environmental stress gradients.Target plants without neighbours required less time for fruit production; however, they also were more likely to be damaged by caterpillar herbivory. Effects of neighbour removal changed along the environmental gradients: plants without neighbours had smaller leaves on earlier snowmelt sites, and increased fungal damage with increasing elevation. Without neighbour removal, damage generally led to reduced female flowering under later snowmelt conditions in the following summer.Our results indicate that the majority of neighbour interactions influencing S. herbacea are facilitative, particularly at stressful early snowmelt and high elevation sites. We suggest that neighbours moderate environmental conditions by protecting plants from temperature extremes, and reduce plant apparency to caterpillars. Neighbours also indirectly increase fitness by reducing damage. Facilitation by neighbours may become more important under climate change, as early snowmelt may increase stress.

Intrinsic and extrinsic factors acting on the reproductive process in alpine‐snowbed plants: roles of phenology, biological interaction, and breeding system

AbstractPollinator activity and competition for pollinators lead to quantitative and qualitative pollen limitations on seed production and affect the reproductive success of plant species, depending on their breeding system (e.g., self‐compatibility and heterospecific compatibility) and genetic load (e.g., inbreeding depression and hybrid inviability). In alpine ecosystems, snowmelt regimes determine the distribution and phenology of plant communities. Plant species growing widely along a snowmelt gradient often grow with different species among local populations. Their pollinators also vary in their abundance, activity, and behavior during the season. These variations may modify plant–pollinator and plant–plant interactions. We integrated a series of our studies on the alpine dwarf shrub, Phyllodoce aleutica (Ericaceae), to elucidate the full set of intrinsic (species‐specific breeding system) and extrinsic factors (snow condition, pollinator activity, and interspecific competition) acting on their reproductive process. Seasonality of pollinator activity led to quantitative pollen limitation in the early‐blooming populations, whereas in the late‐blooming populations, high pollinator activity ensured pollination service, but interspecific competition for pollinators led to qualitative and quantitative pollen limitation in less competitive species. However, negative effects of illegitimate pollen receipt on seed‐set success might be reduced when cryptic incompatibility systems (i.e., outcross pollen grains took priority over self‐ and heterospecific pollen grains) could effectively prevent ovule and seed discounting. Our studies highlight the importance of species‐specific responses of plant reproduction to changing pollinator availability along environmental gradients to understand the general features of pollination networks in alpine ecosystems.

Increased spring freezing vulnerability for alpine shrubs under early snowmelt

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.

A cost-effective monitoring method using digital time-lapse cameras for detecting temporal and spatial variations of snowmelt and vegetation phenology in alpine ecosystems

Alpine ecosystems are particularly vulnerable to the effects of climate change. Although long-term and detailed monitoring is required to conserve alpine ecosystems, field surveillance and satellite remote sensing have difficulties in providing wide coverage or frequent observation in mountain areas. In this study, a new method for monitoring alpine ecosystems by digital cameras was developed in order to detect both snow-cover areas and vegetation phenology at the plant community or species level. We used images from cameras that have been installed at mountain lodges in the northern Japanese Alps (at elevations around 2350–3100m). Red, green, and blue (RGB) digital numbers were derived from each pixel within the images. The snow-cover and snow-free pixels were statistically classified by analysis of variance of gray-level histograms. A flexible threshold was determined for each image to maximize the between-class variance. The temporal variations of the snowmelt rate showed site-specific characteristics and yearly variations. The snowmelt times reflected the local microtopography and differed among the habitats of various functional types of vegetation (i.e., evergreen dwarf pine, deciduous shrubs, evergreen Sasa, tall forbs, and snowbed plants). In addition, the vegetation phenology was quantified by using a vegetation index (green ratio) calculated from RGB digital numbers. An increase in the green ratio indicated the start of the growing period following snowmelt and a decrease indicated leaf senescence. By using pixel-based analysis of the temporal variations of the green ratio, local distributions of the start and end dates and length of the growing period were illustrated at the plant species level for the first time. The distribution of the start of the growing period strongly corresponded to the snowmelt gradient, whereas the end of the growing period was related to the vegetation type. Our results suggest that the length of the growing period mainly corresponded to the snowmelt gradient in relation to the local microtopography. Thus, commercially available digital time-lapse cameras enabled us to clarify the snow–vegetation relationships and the growing period at high temporal and spatial resolutions. This monitoring method should greatly improve our understanding of alpine ecosystems and help to assess the influence of future climate change.