No increase in alpine snowbed productivity in response to experimental lengthening of the growing season

Climate change effects on snow cover and thermic regime in alpine tundra might lead to a longer growing season, but could also increase risks to plants from spring frost events. Alpine snowbeds, i.e. alpine tundra from late snowmelt sites, might be particularly susceptible to such climatic changes. Snowbed communities were grown in large monoliths for two consecutive years, under different manipulated snow cover treatments, to test for effects of early (E) and late (L) snowmelt on dominant species growth, plant functional traits, leaf area index (LAI) and aboveground productivity. Spring snow cover was reduced to assess the sensitivity of snowbed alpine species to severe early frost events, and dominant species freezing temperatures were measured. Aboveground biomass, productivity, LAI and dominant species growth did not increase significantly in E compared to L treatments, indicating inability to respond to an extended growing season. Edapho‐climatic conditions could not account for these results, suggesting that developmental constraints are important in controlling snowbed plant growth. Impaired productivity was only detected when harsher and more frequent frost events were experimentally induced by early snowmelt. These conditions exposed plants to spring frosts, reaching temperatures consistent with the estimated freezing points of the dominant species (～?10 °C). We conclude that weak plasticity in phenological response and potential detrimental effects of early frosts explain why alpine tundra from snowbeds is not expected to benefit from increased growing season length.     

Small-scale plant species distribution in snowbeds and its sensitivity to climate change

Alpine snowbeds are characterized by a long-lasting snow cover and low soil temperature during the growing season. Both these key abiotic factors controlling plant life in snowbeds are sensitive to anthropogenic climate change and will alter the environmental conditions in snowbeds to a considerable extent until the end of this century. In order to name winners and losers of climate change among the plant species inhabiting snowbeds, we analyzed the small-scale species distribution along the snowmelt and soil temperature gradients within alpine snowbeds in the Swiss Alps. The results show that the date of snowmelt and soil temperature were relevant abiotic factors for small-scale vegetation patterns within alpine snowbed communities. Species richness in snowbeds was reduced to about 50% along the environmental gradients towards later snowmelt date or lower daily maximum temperature. Furthermore, the occurrence pattern of the species along the snowmelt gradient allowed the establishment of five species categories with different predictions of their distribution in a warmer world. The dominants increased their relative cover with later snowmelt date and will, therefore, lose abundance due to climate change, but resist complete disappearance from the snowbeds. The indifferents and the transients increased in species number and relative cover with higher temperature and will profit from climate warming. The snowbed specialists will be the most suffering species due to the loss of their habitats as a consequence of earlier snowmelt dates in the future and will be replaced by the avoiders of late-snowmelt sites. These forthcoming profiteers will take advantage from an increasing number of suitable habitats due to an earlier start of the growing season and increased temperature. Therefore, the characteristic snowbed vegetation will change to a vegetation unit dominated by alpine grassland species. The study highlights the vulnerability of the established snowbed vegetation to climate change and requires further studies particularly about the role of biotic interactions in the predicted invasion and replacement process.     

Demography of an endangered endemic rupicolous cactus

The snow cover extent is an important factor for the structure and composition of arctic and alpine tundra communities. Over the last few decades, snowmelt in many arctic and alpine regions has advanced, causing the growing season to start earlier and last longer. In a field experiment in subarctic tundra in Interior Alaska, I manipulated the timing of snowmelt and measured the response in mortality, phenology, growth, and reproduction of the eight dominant plant species. I then tested whether the phenological development of these species was controlled by snowmelt date or by temperature (in particular growing degree days, GDD). In order to expand our understanding of plant sensitivity to snowmelt timing, I explored whether the response patterns can be generalized with regard to the temporal niche of each species. Differences in the phenology between treatments were only found for the first stages of the phenological development (=phenophases). The earlier the temporal niche (i.e., the sooner after snowmelt a species develops) the more its phenology was sensitive to snowmelt. Later phenophases were mostly controlled by GDD, especially in late-developing species. In no species did an earlier snowmelt and a longer growing season directly enhance plant fitness or fecundity, in spite of the changes in the timing of plant development. In conclusion, the temporal niche of a species’ phenological development could be a predictor of its response to snowmelt timing. However, only the first phenophases were susceptible to changes in snowmelt, and no short-term effects on plant fitness were found.     

Nonlinear flowering responses to climate: are species approaching their limits of phenological change?

Many alpine and subalpine plant species exhibit phenological advancements in association with earlier snowmelt. While the phenology of some plant species does not advance beyond a threshold snowmelt date, the prevalence of such threshold phenological responses within plant communities is largely unknown. We therefore examined the shape of flowering phenology responses (linear versus nonlinear) to climate using two long-term datasets from plant communities in snow-dominated environments: Gothic, CO, USA (1974–2011) and Zackenberg, Greenland (1996–2011). For a total of 64 species, we determined whether a linear or nonlinear regression model best explained interannual variation in flowering phenology in response to increasing temperatures and advancing snowmelt dates. The most common nonlinear trend was for species to flower earlier as snowmelt advanced, with either no change or a slower rate of change when snowmelt was early (average 20% of cases). By contrast, some species advanced their flowering at a faster rate over the warmest temperatures relative to cooler temperatures (average 5% of cases). Thus, some species seem to be approaching their limits of phenological change in response to snowmelt but not temperature. Such phenological thresholds could either be a result of minimum springtime photoperiod cues for flowering or a slower rate of adaptive change in flowering time relative to changing climatic conditions.     

Habitat-specific responses in the flowering phenology and seed set of alpine plants to climate variation: implications for global-change impacts

The timing of the snowmelt is a crucial factor in determining the phenological schedule of alpine plants. A long-term monitoring of snowmelt regimes in a Japanese alpine area revealed that the onset of the snowmelt season has been accelerated during the last 17 years in early snowmelt sites but that such a trend has not been detected in late snowmelt sites. This indicates that the global warming effect on the snowmelt pattern may be site-specific. The flowering phenology of fellfield plants in an exposed wind-blown habitat was consistent between an unusually warm year (1998) and a normal year (2001). In contrast, the flowering occurrence of snowbed plants varied greatly between the years depending on the snowmelt time. There was a large number of flowering species in the fellfield community from mid- to late to late June and from mid- to late July. The flowering peak of an early-melt snowbed plant community was in the middle of the flowering season and that of a late-melt snowbed community was in the early flowering season. These habitat-specific phenological patterns were consistent between 1998 and 2001. The effects of the variation in flowering timing on seed-set success were evaluated for an entomophilous snowbed herb, Peucedanum multivittatum, along the snowmelt gradient during a 5-year period. When flowering occurred prior to early August, mean temperature during the flowering season positively influenced the seed set. When flowering occurred later than early August, however, the plants enjoyed high seed-set success irrespective of temperature conditions if frost damage was absent. These observations are probably explained based on the availability of pollinators, which depends not only on ambient temperature but also on seasonal progress. These results suggest that the effects of climate change on biological interaction may vary depending on the specific habitat in the alpine ecosystem in which diverse snowmelt patterns create complicated seasonality for plants within a very localized area.     

The Response of the Alpine Dwarf Shrub Salix herbacea to Altered Snowmelt Timing: Lessons from a Multi-Site Transplant Experiment

Climate change is altering spring snowmelt patterns in alpine and arctic ecosystems, and these changes may alter plant phenology, growth and reproduction. To predict how alpine plants respond to shifts in snowmelt timing, we need to understand trait plasticity, its effects on growth and reproduction, and the degree to which plants experience a home-site advantage. We tested how the common, long-lived dwarf shrub Salix herbacea responded to changing spring snowmelt time by reciprocally transplanting turfs of S. herbacea between early-exposure ridge and late-exposure snowbed microhabitats. After the transplant, we monitored phenological, morphological and fitness traits, as well as leaf damage, during two growing seasons. Salix herbacea leafed out earlier, but had a longer development time and produced smaller leaves on ridges relative to snowbeds. Longer phenological development times and smaller leaves were associated with reduced sexual reproduction on ridges. On snowbeds, larger leaves and intermediate development times were associated with increased clonal reproduction. Clonal and sexual reproduction showed no response to altered snowmelt time. We found no home-site advantage in terms of sexual and clonal reproduction. Leaf damage probability depended on snowmelt and thus exposure period, but had no short-term effect on fitness traits. We conclude that the studied populations of S. herbacea can respond to shifts in snowmelt by plastic changes in phenology and leaf size, while maintaining levels of clonal and sexual reproduction. The lack of a home-site advantage suggests that S. herbacea may not be adapted to different microhabitats. The studied populations are thus unlikely to react to climate change by rapid adaptation, but their responses will also not be constrained by small-scale local adaptation. In the short term, snowbed plants may persist due to high stem densities. However, in the long term, reduction in leaf size and flowering, a longer phenological development time and increased exposure to damage may decrease overall performance of S. herbacea under earlier snowmelt.     

3种高山植物的物候和种群分布格局在融雪梯度上的变化

在青藏高原东部的一个高山雪床,沿着融雪梯度分别设置早融、中间和晚融3个融雪部位,然后测定川西小黄菊（Pyrethrum tatsienense）、长叶火绒草（Leontopodium longifolium）和圆穗蓼（Polygonum macrophyllum）在3个融雪部位上的物候差异以及种群分布格局的变化。结果表明：从早融到晚融的梯度上,3个物种的物候期都不同程度地有所推迟。其中,开始生长的时间推迟12~14 d,始花期推迟6~8 d,盛花期推迟6 d左右,但同一种植物在不同的融雪部位上的衰老枯黄期趋于一致,这标志着在晚融部位同一植物的生长期要缩短。在种群层次上,长叶火绒草和圆穗蓼的分布格局随着融雪的推迟都发生了一定的变化,基本上表现为从早融部位的集群分布到中间或晚融部位的随机分布。川西小黄菊在各个融雪部位上都表现为集群分布,但集群的强度随融雪的推迟逐渐减弱。     

Effects of temperature and date of snowmelt on growth, reproduction, and flowering phenology in the arctic/alpine herb, Ranunculus glacialis

Changes in growing season temperature and duration may have profound effects on the population dynamics of arctic and alpine plant species in snow-bed and fell-field habitats. We examined how a typical herbaceous pioneer species, Ranunculus glacialis, responded to experimental climate change in open-top chambers for three seasons at an alpine site in southern Norway. Warming had no significant effect on any reproductive, growth or phenological variables, except for seed weight, which increased significantly during the first 2 ears. Despite large differences in average date of snowmelt among years, average reproductive output and ramet size differed little among years. Within-year variation in date of snowmelt had no impact on seed number or weight in either control or warmed plots. Leaf width and ramet leaf number decreased significantly with later snowmelt within a year. Experimental warming reduced the negative effect on ramet size of late snowmelt within a year to some extent. In general, R. glacialis reacts contrary to most other arctic/alpine species to experimental warming. Species with such low responsiveness to environmental conditions may be particularly vulnerable to climatic change, especially if their habitat is invaded by other species with higher phenotypic plasticity and a better competitive ability.     

Responses of flowering phenology of snowbed plants to an experimentally imposed extreme advanced snowmelt

In snowbed habitats, characterized by a long-lasting snow cover, the timing of snowmelt can be included among the major factors controlling plant phenology. Nevertheless, only a few ecological studies have tested the responses of flowering phenology of species growing in very late snow-free habitats to an advanced snowmelt (AS) date. The aim of this study was to determine the impacts of an extremely earlier melt-out of snow on flowering phenology of vascular plant species inhabiting an alpine snowbed. The study was conducted in the high Gavia Valley (Italy, 2,700 m a.s.l.). On 30th May 2012, we removed manually the snow cover and set up an experiment with 5 AS and 5 control plots. Phenological observations of the most abundant vascular species were conducted every 4–6 days. Moreover, we calculated cumulative soil temperature and recorded the mortality of reproductive structures of three species. For several species flowering occurred earlier, and the prefloration period was extended in the AS treatment in comparison with the control. For the majority of species, cumulative soil temperatures in the AS treatment and the control were comparable, confirming that temperature exerts the main control on the flowering of the species inhabiting snowbeds. Earlier flowering species resulted more affected by an AS date in comparison with later flowering species. The mortality of reproductive structures did not increase in the AS treatments in comparison with the control suggesting that few and weak frost events in late spring do not affect the survival of reproductive structures of the species studied.     

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 2350–3100 m). 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.     

Contrasting effects of warming and increased snowfall on Arctic tundra plant phenology over the past two decades

Recent changes in climate have led to significant shifts in phenology, with many studies demonstrating advanced phenology in response to warming temperatures. The rate of temperature change is especially high in the Arctic, but this is also where we have relatively little data on phenological changes and the processes driving these changes. In order to understand how Arctic plant species are likely to respond to future changes in climate, we monitored flowering phenology in response to both experimental and ambient warming for four widespread species in two habitat types over 21 years. We additionally used long‐term environmental records to disentangle the effects of temperature increase and changes in snowmelt date on phenological patterns. While flowering occurred earlier in response to experimental warming, plants in unmanipulated plots showed no change or a delay in flowering over the 21‐year period, despite more than 1 °C of ambient warming during that time. This counterintuitive result was likely due to significantly delayed snowmelt over the study period (0.05–0.2 days/yr) due to increased winter snowfall. The timing of snowmelt was a strong driver of flowering phenology for all species – especially for early‐flowering species – while spring temperature was significantly related to flowering time only for later‐flowering species. Despite significantly delayed flowering phenology, the timing of seed maturation showed no significant change over time, suggesting that warmer temperatures may promote more rapid seed development. The results of this study highlight the importance of understanding the specific environmental cues that drive species’ phenological responses as well as the complex interactions between temperature and precipitation when forecasting phenology over the coming decades. As demonstrated here, the effects of altered snowmelt patterns can counter the effects of warmer temperatures, even to the point of generating phenological responses opposite to those predicted by warming alone.     

Species coexistence, intransitivity, and topological variation in competitive tournaments

Competitive intransitivity occurs when species’ competitive abilities cannot be listed in a strict hierarchy, but rather form competitive loops, as in the game ‘Rock-Paper-Scissors’. Indices are useful for summarizing intransitivity in communities; however, as with most indices, a great deal of information is compressed into single number. So while recent ecological theory, experiments, and natural history observations demonstrate that competitive intransitivity can promote species coexistence, the consequence of variation in the ‘topology’ of competitive interactions that is not accounted for by intransitivity indices is much less well understood. We use a continuous analytical model and two complementary discrete lattice models (one spatially explicit, the other aspatial) to demonstrate that such variation does indeed greatly affect species coexistence. Specifically, we show that although intransitivity indices are good at capturing broad patterns of coexistence, communities with different levels of intransitivity can have equal coexistence, and communities with equal intransitivity can have different coexistence, due to underlying variation in competitive network topology.     

POPULATION STRUCTURE ALONG A STEEP ENVIRONMENTAL GRADIENT: CONSEQUENCES OF FLOWERING TIME AND HABITAT VARIATION IN THE SNOW BUTTERCUP,RANUNCULUS ADONEUS

Few studies have determined how gene flow and selection interact to generate population genetic structure in heterogeneous environments. One way to identify the potential role played by natural selection is to compare patterns of spatial genetic structure between different life cycle stages and among microenvironments. We examined patterns of spatial structure in a population of the snow buttercup (Ranunculus adoneus), using both adult plants and newly emerged seedlings. The study population spans a steep environmental gradient caused by gradual melting of snow within a permanent snowbed. Early-melting sites are characterized by denser vegetation, more fertile soils, and a longer growing season than late-melting sites tens of meters away. The flowering time of R. adoneus is controlled entirely by time of snowmelt, so the contiguous population is phenologically substructured into a series of successively flowering cohorts, reducing the opportunity for direct pollen transfer between early- and late-melting sites. For four highly polymorphic enzyme loci in this tetraploid species, there was subtle, but statistically significant, genetic differentiation between early, middle, and late-melting cohorts; adults usually showed greater differentiation among snowmelt zones than did seedlings. At two loci in adults and one locus in seedlings, homozygotes were more common than predicted at Hardy-Weinberg equilibrium, even when assuming maximum levels of double reduction during meiosis. This pattern suggests the occurrence of self-fertilization and/or population substructure. To determine how spatial isolation and phenological separation each contribute to genetic substructure, we used bivariate regression models to predict the numbers of allele differences between randomly paired individuals as a function of meters separation in space and days separation in flowering time. For newly emerged seedlings, we found that spatial separation was positively associated with genetic difference, but that the additional contribution of phenological separation to genetic difference was not significant. This implies that seeds and/or pollen move effectively across the snowmelt gradient, despite differences in flowering time. As was true for seedlings, spatial separation between paired adults contributed to greater genetic difference, but for a given spatial separation, the genetic difference between adult plants was reduced by phenological separation. This result implies that postemergence selection is favoring at least some seeds that migrate across the snowmelt gradient. Directional gene flow across the snowmelt gradient probably results from a genetic source-sink interaction, that is, the colonization of ecologically marginal late-melting sites by high quality seeds produced by the larger subpopulation in early-melting sites. Effective gene flow from high to low quality microenvironments is likely to impede adaptation to late-melting locations.     

Site-specific factors influence the richness and phenology of snowbed plants in the Pyrenees

Although the timing of snowmelt and growth temperatures appear to be the main factors that influence the species richness and phenology of snowbed plants, site-specific characteristics may also play a role in modifying the effects of the timing of snowmelt and temperature. In this study, the effects of site-specific factors (microtopography and snow origin) on species richness and plant phenology were evaluated in 72 plots in two snowbeds in the Andorran Pyrenees. Snowmelt patterns influenced the spatial distribution of species richness and abundance. Site-specific factors had significant effects on the responses of species (shortening or lengthening the duration of the phenophase) and on the extent to which the timing of snowmelt influenced leaf expansion and flowering. Notably, the highest rates of leaf expansion occurred on late snowmelt isoclines, where, nevertheless, the time taken to reach peak flowering was significantly longer than on the early snowmelt isoclines. The results of this study highlight the fact that, in addition to the effects of interannual variability in climate, site-specific factors have a significant effect on the phenology and reproductive success of the commonest plants in the snowbed communities of the Pyrenees.     

Effects of nitrogen and phosphorus supply on growth and flowering phenology of the snowbed forb Gnaphalium supinum L.

The warming-induced increase in nutrient mineralization and the further increase in atmospheric nitrogen depositions raise the topic of whether and how alpine plants will react to enhanced nutrient availability. Despite several studies have shown the effects of fertilization on primary production of alpine plants, only few studies have considered the influences of nutrients on reproduction. Here, we investigated the effects of nitrogen (N) and phosphorus (P) amendments on cover, number of ramets, flowering effort and phenological timing of Gnaphalium supinum, an arctic-alpine widespread snowbed species. We set up an experimental design with four fertilization treatments (low N, P without additional N, low N + P, and high N + P) and an unfertilized control for three years (2003–2005), within a late snowbed located in the Italian Alps (Gavia Pass, 2700 m a.s.l.). The cover of Gnaphalium supinum was recorded at the peak of the aboveground biomass development in the three years, while the temporal dynamic of ramet density and reproductive phenophases were monitored during the 2005 growing season. The clonal growth of G. supinum resulted to be co-limited by N and P, while the flowering effort was stimulated by P. Flowering date was advanced by P supply, while N alone did not show any significant effect on phenology. In a warming scenario, with a predicted increase in N and P availability by nutrient mineralization and atmospheric deposition, this species should probably experience some benefits for its growth and reproduction if not limited by other factors such as the length of the growing season or interspecific competition.     

Climate change affects the outcome of competitive interactions—an application of principal response curves

It has been hypothesised that climate change may affect vegetation by changing the outcome of competitive interactions. We use a space-for-time approach to evaluate this hypothesis in the context of alpine time-of-snowmelt gradients. Principal response curves, a multivariate repeated-measurement analysis technique, are used to analyse for compositional differences in local ridge-to-snowbed gradients among 100 m altitudinal bands from 1,140 to 1,550 m a.s.l., corresponding to a temperature gradient of 2.5°C (local lapse rate is 0.6°C). The interaction between time-of-snowmelt and altitude is strongly significant statistically, indicating that the altitudinal gradient cannot be explained simply by the physiological responses of the species, but that there are also changes in the outcome of competitive interactions. At higher altitudes, there is a decrease in the time-of-snowmelt ranges of species which have intermediate times-of-snowmelt optima, whereas snowbed (chinophilous) species have wider time-of-snowmelt ranges. As snowbed species can survive, grow and reproduce at very early snow-free sites at high altitudes, the most likely explanation for their absence from all but the latest time-of-snowmelt habitats at lower altitudes is competitive exclusion by more vigorous lee-side species. This suggests that with future climate change snowbed species will experience, in addition to habitat fragmentation and reduced size of habitats due to increased temperature and snowmelt, an indirect effect due to competitive exclusion from late-snowmelt sites by species that have their optima outside snowbeds.     

Influence of Snowmelt Timing on the Diet Quality of Pyrenean Rock Ptarmigan (Lagopus muta pyrenaica): Implications for Reproductive Success

The Pyrenean rock ptarmigan (Lagopus muta pyrenaica) is the southernmost subspecies of the species in Europe and is considered threatened as a consequence of changes in landscape, human pressure, climate change, and low genetic diversity. Previous studies have shown a relationship between the date of snowmelt and reproductive success in the Pyrenean ptarmigan. It is well established that birds laying early in the breeding season have higher reproductive success, but the specific mechanism for this relationship is debated. We present an explicative model of the relationship between snowmelt date and breeding success mediated by food quality for grouse in alpine environments. From microhistological analyses of 121 faecal samples collected during three years in the Canigou Massif (Eastern Pyrenees), and the assessment of the chemical composition of the main dietary components, we estimated the potential quality of individual diets. Potential dietary quality was correlated with free-urate faecal N, a proxy of the digestible protein content ingested by ptarmigan, and both were correlated with phenological stage of consumed plants, which in turn depends on snowmelt date. Our findings suggest that the average snowmelt date is subject to a strong interannual variability influencing laying date. In years of early snowmelt, hens benefit from a longer period of high quality food resources potentially leading to a higher breeding success. On the contrary, in years of late snowmelt, hens begin their breeding period in poorer nutrient condition because the peaks of protein content of their main food items are delayed with respect to laying date, hence reducing breeding performance. We discuss the possible mismatch between breeding and snowmelt timing.     

Do Arctic-nesting birds respond to earlier snowmelt? A multi-species study in north Yukon, Canada

Climate change has altered the timing of many ecological processes, especially in the Arctic. The initiation of nesting is a key signal of phenological changes in Arctic-nesting birds, and is possibly connected to the circumpolar trend of earlier snowmelt. We collected data on lay dates of 7 bird species, representing shorebirds, passerines, a bird of prey, and seabirds, nesting on Herschel Island, Yukon, Canada, in the years 1984–1986 and 2007–2009. Snowmelt was significantly earlier in the 2007–2009 period. Shorebirds and passerines showed trends to earlier lay dates in conjunction with earlier snowmelt; the other species did not. The strength of response in lay date was correlated with the general categories of foods known to be used by study species. However, six species showed a longer time interval between snowmelt and egg-laying in early compared to late springs, suggesting the need for further monitoring of how robust their responses to snowmelt are in the future.     

Counterbalancing effects of competition for resources and facilitation against grazing in alpine snowbed communities

Alpine snowbeds are habitats where the major limiting factors for plant growth are herbivory and a small time window for growth due to late snowmelt. Despite these limitations, snowbed vegetation usually forms a dense carpet of palatable plants due to favourable abiotic conditions for plant growth within the short growing season. These environmental characteristics make snowbeds particularly interesting to study the interplay of facilitation and competition. We hypothesised an interplay between resource competition and facilitation against herbivory. Further, we investigated whether these predicted neighbour effects were species‐specific and/or dependent on ontogeny, and whether the balance of positive and negative plant–plant interactions shifted along a snowmelt gradient. We determined the neighbour effects by means of neighbour removal experiments along the snowmelt gradient, and linear mixed model analyses. The results showed that the effects of neighbour removal were weak but generally consistent among species and snowmelt dates, and depended on whether biomass production or survival was considered. Higher total biomass and increased fruiting in removal plots indicated that plants competed for nutrients, water, and light, thereby supporting the hypothesis of prevailing competition for resources in snowbeds. However, the presence of neighbours reduced herbivory and thereby also facilitated survival. For plant growth the facilitative effects against herbivores in snowbeds counterbalanced competition for resources, leading to a weak negative net effect. Overall the neighbour effects were not species‐specific and did not change with snowmelt date. Our finding of counterbalancing effects of competition and facilitation within a plant community is of special theoretical value for species distribution models and can explain the success of models that give primary importance to abiotic factors and tend to overlook interrelations between biotic and abiotic effects on plants.