Silene Acaulis Research Articles

Local trampling disturbance effects on alpine plant populations and communities: Negative implications for climate change vulnerability.

Global change is modifying species communities from local to landscape scales, with alterations in the abiotic and biotic determinants of geographic range limits causing species range shifts along both latitudinal and elevational gradients. An important but often overlooked component of global change is the effect of anthropogenic disturbance, and how it interacts with the effects of climate to affect both species and communities, as well as interspecies interactions, such as facilitation and competition. We examined the effects of frequent human trampling disturbances on alpine plant communities in Switzerland, focusing on the elevational range of the widely distributed cushion plant Silene acaulis and the interactions of this facilitator species with other plants. Examining size distributions and densities, we found that disturbance appears to favor individual Silene growth at middle elevations. However, it has negative effects at the population level, as evidenced by a reduction in population density and reproductive indices. Disturbance synergistically interacts with the effects of elevation to reduce species richness at low and high elevations, an effect not mitigated by Silene. In fact, we find predominantly competitive interactions, both by Silene on its hosted and neighboring species and by neighboring (but not hosted) species on Silene. Our results indicate that disturbance can be beneficial for Silene individual performance, potentially through changes in its neighboring species community. However, possible reduced recruitment in disturbed areas could eventually lead to population declines. While other studies have shown that light to moderate disturbances can maintain high species diversity, our results emphasize that heavier disturbance reduces species richness, diversity, as well as percent cover, and adversely affects cushion plants and that these effects are not substantially reduced by plant–plant interactions. Heavily disturbed alpine systems could therefore be at greater risk for upward encroachment of lower elevation species in a warming world.

Improving niche projections of plant species under climate change: Silene acaulis on the British Isles as a case study

Empirical works to assist in choosing climatically relevant variables in the attempt to predict climate change impacts on plant species are limited. Further uncertainties arise in choice of an appropriate niche model. In this study we devised and tested a sharp methodological framework, based on stringent variable ranking and filtering and flexible model selection, to minimize uncertainty in both niche modelling and successive projection of plant species distributions. We used our approach to develop an accurate, parsimonious model of Silene acaulis (L.) presence/absence on the British Isles and to project its presence/absence under climate change. The approach suggests the importance of (a) defining a reduced set of climate variables, actually relevant to species presence/absence, from an extensive list of climate predictors, and (b) considering climate extremes instead of, or together with, climate averages in projections of plant species presence/absence under future climate scenarios. Our methodological approach reduced the number of relevant climate predictors by 95.23% (from 84 to only 4), while simultaneously achieving high cross-validated accuracy (97.84%) confirming enhanced model performance. Projections produced under different climate scenarios suggest that S. acaulis will likely face climate-driven fast decline in suitable areas on the British Isles, and that upward and northward shifts to occupy new climatically suitable areas are improbable in the future. Our results also imply that conservation measures for S. acaulis based upon assisted colonization are unlikely to succeed on the British Isles due to the absence of climatically suitable habitat, so different conservation actions (seed banks and/or botanical gardens) are needed.

Multivariate Relationships between Snowmelt and Plant Distributions in the High Arctic Tundra

We investigated multivariate relationships among snowmelt, soil physicochemical properties and the distribution patterns of Arctic tundra vegetation. Seven dominant species were placed in three groups (Veg-1, 2, 3) based on niche overlap (Pianka’s Index) and ordination method, and a partial least squares path model was applied to estimate complex multivariate relationships of four latent variables on the abundance and richness of plant species. The abundance of Veg-1 (Luzula confusa and Salix polaris) was positively correlated with early snowmelt time, high soil nutrients and dense moss cover, but the abundance of Veg-2 (Saxifraga oppositifolia, Bistorta vivipara and Silene acaulis) was negatively correlated with these three variables. Plant richness was positively associated with early snowmelt and hydrological properties. Our results indicate that the duration of the snowpack can directly influence soil chemical properties and plant distribution. Furthermore, plant species richness was significantly affected by snow melt time in addition to soil moisture and moss cover. We predict that L. confusa and S. polaris may increase in abundance in response to early snowmelt and increased soil moisture-nutrient availability, which may be facilitated by climate change. Other forb species in dry and infertile soil may decrease in abundance in response to climate change, due to increasingly unfavourable environmental conditions and competition with mosses.

Climate and synchrony with conspecifics determine the effects of flowering phenology on reproductive success in Silene acaulis

ABSTRACT Changes in flowering phenology resulting from climate change could impact individual plant fitness and population viability. Flowering phenology can mediate plant reproductive success in several ways, including pollinator interactions, flowering synchrony with conspecifics, and timing of suitable abiotic conditions. We explored factors that control phenology and reproductive success for an alpine cushion plant, Silene acaulis, across two years and four sites, totaling 1,123 plants, in Colorado, USA. We investigated relationships between flowering time, flowering synchrony, and reproductive success with local abiotic conditions and pollinator behavior. Mean flowering phenology was strongly correlated with the timing of snowmelt across sites and years. Relative to mean flowering times, earlier flowering plants generally produced more flowers and experienced greater soil moisture during flowering but reduced synchrony with conspecifics. Fruit set tended to increase with greater soil moisture, synchrony during flowering, and earlier flowering times. Pollinator visitation increased with local Silene flower density. Earlier snowmelt and drier conditions later in the season favor earlier flowering, but these effects are partially counteracted by the positive effects of synchrony, perhaps because of changes in pollinator visitation. Overall, while both biotic and abiotic effects influence reproductive success, late-season drought may outweigh the benefits of flowering synchrony to increasingly favor earlier flowering.

Зональная растительность равнинных восточноевропейских тундр

Зональная растительность равнинных восточноевропейских тундр

Both life-history plasticity and local adaptation will shape range-wide responses to climate warming in the tundra plant Silene acaulis.

Many predictions of how climate change will impact biodiversity have focused on range shifts using species-wide climate tolerances, an approach that ignores the demographic mechanisms that enable species to attain broad geographic distributions. But these mechanisms matter, as responses to climate change could fundamentally differ depending on the contributions of life-history plasticity vs. local adaptation to species-wide climate tolerances. In particular, if local adaptation to climate is strong, populations across a species' range-not only those at the trailing range edge-could decline sharply with global climate change. Indeed, faster rates of climate change in many high latitude regions could combine with local adaptation to generate sharper declines well away from trailing edges. Combining 15years of demographic data from field populations across North America with growth chamber warming experiments, we show that growth and survival in a widespread tundra plant show compensatory responses to warming throughout the species' latitudinal range, buffering overall performance across a range of temperatures. However, populations also differ in their temperature responses, consistent with adaptation to local climate, especially growing season temperature. In particular, warming begins to negatively impact plant growth at cooler temperatures for plants from colder, northern populations than for those from warmer, southern populations, both in the field and in growth chambers. Furthermore, the individuals and maternal families with the fastest growth also have the lowest water use efficiency at all temperatures, suggesting that a trade-off between growth and water use efficiency could further constrain responses to forecasted warming and drying. Taken together, these results suggest that populations throughout species' ranges could be at risk of decline with continued climate change, and that the focus on trailing edge populations risks overlooking the largest potential impacts of climate change on species' abundance and distribution.

Shift from facilitative to neutral interactions by the cushion plant Silene acaulis along a primary succession gradient

AbstractBackgroundThe stress‐gradient hypothesis predicts a shift from facilitative to competitive plant interactions with decreasing abiotic stress. This has been supported by studies along elevation and temperature gradients, but also challenged by the hypothesis of a facilitation collapse at extremely harsh sites. Although facilitation is known to be important in primary succession, few studies have examined these hypotheses along primary succession gradients.AimTo examine whether there is a relationship between the presence of the circumpolar cushion plant Silene acaulis and other species, and if so, whether there is a shift between positive and negative interactions along a primary succession gradient in a glacier foreland.LocationFinse, southern Norway.MethodsWe examined the performance of the common alpine forb Bistorta vivipara, species richness of vascular plants, bryophytes and lichens, and the number of seedlings and fertile vascular plants in S. acaulis cushions, and control plots without S. acaulis, along a succession gradient with increasing distance from a glacier front, and thus decreasing abiotic stress. To examine if S. acaulis cushions modify the abiotic environment, we recorded soil temperature, moisture, organic content and pH in cushions and control plots.ResultsBistorta vivipara performed better, as shown by bigger leaves in S. acaulis cushions compared to control plots in the harshest part of the gradient close to the glacier. There were few differences in B. vivipara performance between cushion and control plots in the more benign environment further away from the glacier. This suggests a shift from facilitative to mainly neutral interactions by S. acaulis on the performance of B. vivipara with decreasing abiotic stress. A trend, although not significant, of higher vascular species richness and fertility inside S. acaulis cushions along the whole gradient, suggests that S. acaulis also facilitates community‐level species richness. The causal mechanism of this facilitation is likely that the cushions buffer extreme temperatures.ConclusionsOur results support the stress‐gradient hypothesis for the relationship between the cushion plant S. acaulis and the performance of a single species along a primary succession gradient in a glacier foreland. S. acaulis also tended to increase vascular plant species richness and fertility regardless of stress level along the gradient, suggesting facilitation at the community level. We found no collapse of facilitation at the most stressful end of the gradient in this alpine glacier foreland.

Environmental stress effects on reproduction and sexual dimorphism in the gynodioecious species Silene acaulis

In gynodioecious species, hermaphrodite plants invest both in seed and pollen production, whereas female plants only produce fruits. For both sexes to coexist, such unbalanced investment is expected to translate in some kind of reproductive compensation, particularly under severe nutrient shortage or environmental stress. This study investigates 1) whether females of the gynodioecious boreo-alpine plant Silene acaulis show any overall compensation in fitness components; 2) the existence of sexual dimorphism in flowers that favor different modes in each sex; 3) whether environmental severity, characterized by means of a Species Distribution Model, strength these patterns. Flower size, fruit and seed production, and germination success were recorded in six populations across the Pyrenees and the Alps. Fruit set was significantly higher in female (59%) than in hermaphrodite plants (17%), and female organs (ovary and style) were larger in female than in hermaphrodite plants, but female flowers were smaller. Contrary to expectations, environment severity was not related with most of the biological traits studied and only germination was associated to environmental gradients. These results confirm that fruit production plays an important role as a compensatory strategy between sexes and, consequently, may favor the presence of both sexes in a gynodioecious species.

Anther smuts of Silene acaulis and S. uniflora in the Outer Hebrides, including an assessment of ITS genotypes of Microbotryum silenes-acaulis

Anther smuts on Silene acaulis and S. uniflora from the Outer Hebrides, Scotland, UK), are analysed using morphological and molecular techniques, and found to represent Microbotryum silenes-acaulis and M. silenes-inflatae, respectively. This is the frst identifcation of caryophyllaceous anther smuts in the Outer Hebrides according to modern species concepts and the frst report of Microbotryum silenes-acaulis confrmed by molecular analysis from the British Isles. Additionally, the genetic structure of Microbotryum silenes- acaulis, based on all currently available ITS sequences, is analysed and discussed. Seven ITS genotypes are determined for Microbotryum silenes-acaulis, including three genotypes in North America and four genotypes in Europe. Compared to European accessions, all North American accessions share specifc nucleotides and are genetically divergent.

Contrasting Population Dynamics in the Boreo-AlpineSilene acaulis(Caryophyllaceae) at Its Southern Distribution Limit

Classical theory predicts a high demographic vulnerability and diminished performance of peripheral plant populations, but recent studies highlight the fact that environmental factors may override that geographical pattern. In this study, we compare the density, population structure and population dynamics of the boreo-alpine Silene acaulis (Caryophyllaceae) in two locations at its southern European distribution limit, using matrix projection models. One population was close to the species' lower altitudinal limit, and thus exposed to more ecologically marginal conditions (higher intraspecific competition and temperature) than the population higher up. The low-altitude population was sparser and its members were older as a result of lower recruitment and survival of new individuals. Additionally, this population's growth rate was lower, mainly due to a shorter permanence of large plants. The contrasting demographic differences between two closely located, southern peripheral populations highlight the impo...

Genetic variation in Silene acaulis increases with population age

This study examined the relationship between genetic variation and the mean age of individuals within populations of the alpine cushion plant Silene acaulis (L.) Jacq. along a chronosequence on a glacier foreland in the Central European Alps. We studied 67 individuals of S. acaulis from four populations located at 2390, 2530, 2680, and 2820 m a.s.l. on the glacier foreland and applied a combined approach of growth-ring analysis and individual size measurements to determine the mean age of individuals within populations. Genetic variation was studied using amplified fragment length polymorphism analysis. The mean age of individuals within populations ranged from 11 to 20 years and increased with decreasing altitude toward the older stages of the chronosequence, reaching a maximum age of 50 years. According to regression analysis, genetic variation increased slightly from early to late successional stages and was associated with the mean age of individuals within populations. The results suggest that genetic variation in S. acaulis depends on the mean age of individuals within populations, and emphasize that both species diversity and genetic diversity in plant species can increase during succession on glacier forelands. Integration of age determination methods proves to be a promising approach in the analysis of genetic variation in plant species.

Distribution and population structure of the anther smut Microbotryum silenes-acaulis parasitizing an arctic-alpine plant.

Cold-adapted organisms with current arctic-alpine distributions have persisted during the last glaciation in multiple ice-free refugia, leaving footprints in their population structure that contrast with temperate plants and animals. However, pathogens that live within hosts having arctic-alpine distributions have been little studied. Here, we therefore investigated the geographical range and population structure of a fungus parasitizing an arctic-alpine plant. A total of 1437 herbarium specimens of the plant Silene acaulis were examined, and the anther smut pathogen Microbotryum silenes-acaulis was present throughout the host's geographical range. There was significantly greater incidence of anther smut disease in more northern latitudes and where the host locations were less dense, indicating a major influence of environmental factors and/or host demographic structure on the pathogen distribution. Genetic analyses with seven microsatellite markers on recent collections of 195 M.silenes-acaulis individuals revealed three main genetic clusters, in North America, northern Europe and southern Europe, likely corresponding to differentiation in distinct refugia during the last glaciation. The lower genetic diversity in northern Europe indicates postglacial recolonization northwards from southern refugia. This study combining herbarium surveys and population genetics thus uniquely reveals the effects of climate and environmental factors on a plant pathogen species with an arctic-alpine distribution.

Cushion plant morphology controls biogenic capability and facilitation effects of Silene acaulis along an elevation gradient

Summary The stress‐gradient hypothesis (SGH) predicts that the balance of plant–plant interactions shifts along abiotic environmental gradients, with facilitation becoming more frequent under stressful conditions. However, recent studies have challenged this perspective, reporting that positive interactions are, in some cases, more common at the intermediate level of environmental severity gradients. Here, we test whether and how neighbour effects by Silene acaulis cushions vary along a 700 m wide altitudinal transect, in relation to cushion morphological traits and environmental severity. Field measurements along the gradient, within and outside cushions, included (i) species richness and cover of coexisting vascular plants; (ii) cushion morphology; (iii) above‐ and below‐ground microclimate; and (iv) soil quality. We used the relative interaction index to decouple neighbour trait effects and environmental severity effects on plant diversity at different elevations. The ability of the cushion plant to facilitate heterospecifics shifts considerably along the elevation gradient, being greatest at the intermediate level. On the other hand, Silene morphological traits steadily change along the gradient, from lax, soft and flat‐shaped cushion habits at low elevation to tightly knit and dome‐shaped habits at high elevation. Cushion morphological changes are associated with mitigating effects on microclimate, indicating that cushions effectively act as a heat‐trap at medium and high elevations, while at low elevations the soft and flat cushions avoid excessive heat accumulation by tight coupling with the surrounding atmosphere. At the upper end of the gradient, cushion cespitose–pulvinate compactness and high stem density appear to be critical traits in modulating the net effect of plant–plant interaction, since the space available for hosting other vascular species is considerably reduced. In conclusion, this work provides a mechanistic link between plant morphological traits, associated biogenic microclimate changes and variation in net plant–plant interactions along the explored severity gradient. Our findings support an alternative conceptual model to SGH, with plant facilitation collapsing at the upper extreme of the abiotic stress gradient.

Vicariance, long-distance dispersal, and regional extinction-recolonization dynamics explain the disjunct circumpolar distribution of the arctic-alpine plant Silene acaulis.

Many arctic-alpine species have vast geographic ranges, but these may encompass substantial gaps whose origins are poorly understood. Here we address the phylogeographic history of Silene acaulis, a perennial cushion plant with a circumpolar distribution except for a large gap in Siberia. We assessed genetic variation in a range-wide sample of 103 populations using plastid DNA (pDNA) sequences and AFLPs (amplified fragment length polymorphisms). We constructed a haplotype network and performed Bayesian phylogenetic analyses based on plastid sequences. We visualized AFLP patterns using principal coordinate analysis, identified genetic groups using the program structure, and estimated genetic diversity and rarity indices by geographic region. The history of the main pDNA lineages was estimated to span several glaciations. AFLP data revealed a distinct division between Beringia/North America and Europe/East Greenland. These two regions shared only one of 17 pDNA haplotypes. Populations on opposite sides of the Siberian range gap (Ural Mountains and Chukotka) were genetically distinct and appear to have resulted from postglacial leading-edge colonizations. We inferred two refugia in North America (Beringia and the southern Rocky Mountains) and two in Europe (central-southern Europe and northern Europe/East Greenland). Patterns in the East Atlantic region suggested transoceanic long-distance dispersal events. Silene acaulis has a highly dynamic history characterized by vicariance, regional extinction, and recolonization, with persistence in at least four refugia. Long-distance dispersal explains patterns across the Atlantic Ocean, but we found no evidence of dispersal across the Siberian range gap.

Characterization of 14 microsatellite markers for Silene acaulis (Caryophyllaceae)(1).

Premise of the study:Fifty candidate microsatellite markers, generated using 454 shotgun sequencing, were tested for the widespread arctic/alpine herb Silene acaulis (Caryophyllaceae).Methods and Results:Fourteen out of 50 markers resulted in polymorphic products with profiles that enabled interpretation. The numbers of alleles per locus ranged from two to six, and the expected heterozygosity per locus ranged from 0.06 to 0.68. Analysis of F0 and F1 samples proved that one allele was always inherited maternally. Four multiplex mixes have been developed.Conclusions:Microsatellite markers for this species will be a valuable tool to study detailed small-scale genetic patterns in an arctic/alpine herb and to relate them to demographic parameters.

Rhizosphere effect of three plant species of environment under periglacial conditions (Majella Massif, central Italy)

The chemical, physical and biological processes occurring in the rhizosphere can influence plant growth by modifying root associated microorganisms and nutrient cycles. Although rhizosphere has been widely investigated, little is known about the rhizosphere effect of pioneer plants in soils of periglacial environments. The knowledge of the processes controlling soil–plant relationships in these severe environments may help understanding the ecological evolution of newly deglaciated surfaces. We selected three plants [Helianthemum nummularium (L.) Mill. subsp. grandiflorum (Scop.), Dryas octopetala (L.), and Silene acaulis (L.) Jacq. subsp. cenisia (Vierh.) P. Fourn.] that sparsely occupy deglaciated areas of central Apennines (Italy), with the aim to assess changes between rhizosphere and bulk soil in terms of physical, chemical, and biological properties. The three plants considered showed to have different rhizosphere effect. Helianthemum induced a strong rhizosphere effect through a synergistic effect between root activity and a well adapted rhizosphere microbial community. Dryas did not foster a microbial community structure specifically designed for its rhizosphere, but consumes most of the energetic resources supplied by the plant to make nutrients available. Conversely to the other two species, Silene produced slight soil changes in the rhizosphere, where the microbial community had a structure, abundance and activity similar to those of the bulk soil. The ability to colonize harsh environments of Silene is probably linked to the shape and functions of its canopy rather than to a functional rhizosphere effect.This study showed that the rhizosphere effect differed by species also under high environmental pressure (periglacial conditions, poorly developed soil), and the activity of roots and associated microbial community is decisive in modifying the soil properties, so to create a suitable environment where plants are able to grow.

Lake Store Finnsjøen – a key for understanding Lateglacial/early Holocene vegetation and ice sheet dynamics in the central Scandes Mountains

The Lateglacial (LG) deglaciation and vegetation development in the Scandes Mountains has been debated for a century. Here we present new evidence from microfossils, radiocarbon dated plant macrofossils and sedimentary ancient DNA from laminated sediments in Lake Store Finnsjøen (1260 m a.s.l.) at Dovre, Central Norway. Combined with previous results from three other Dovre lakes, this allows for new interpretations of events during and immediately after the LG deglaciation. The Finnsjøen sediments present the first uninterrupted record of local vegetation development in the Scandes Mountains from the late Younger Dryas (YD), ca 12,000 cal years BP, to the early Holocene around 9700 cal years BP. The local vegetation in late YD/early Holocene was extremely sparse with pioneer herbs (e.g. Artemisia norvegica, Beckwithia, Campanula cf. uniflora, Koenigia, Oxyria, Papaver, Saxifraga spp.) and dwarf-shrubs (Betula nana, Salix including Salix polaris). From 11,300 cal years BP, local vegetation rapidly closed with dominant Dryas, Saxifraga spp., and Silene acaulis. From ca 10,700 cal years BP, open birch-forests with juniper, Empetrum nigrum and other dwarf-shrubs developed. Pine forests established within the area from 10,300 cal years BP. We identified the cold Preboreal Oscillation (PBO), not earlier described from pollen data in South Norway, around 11,400 cal years BP by a regional pollen signal. Distinct local vegetation changes were not detected until the post-PBO warming around 11,300 cal years BP. Apparently, the earlier warming at the YD/Holocene transition at 11,650 cal years BP was too weak and short-lived for vegetation closure at high altitudes at Dovre. For the first time, we demonstrate a regional glacier readvance and local ice cap formations during the YD in the Scandes Mountains. In two of the deep lakes with small catchments, YD glaciation blocked sedimentation without removing old sediments and caused a hiatus separating sediments of the ice-free LG interstadial (LGI) from those of the ice-free Holocene period. Both regional glaciers and local ice caps caused hiati. Ice-free pre-YD conditions at Dovre followed by a YD readvance point to a scenario that is intermediate between the maximum ice model postulating a thick glacier during the entire LG, and the minimum ice model postulating thin and multi-domed early LG ice.

Ecological implications of reduced pollen supply in the alpine: a case study using a dominant cushion plant species

The reproductive assurance hypothesis states that self-incompatible female plants must produce twice the number of seeds relative to their self-compatible hermaphroditic counterparts to persist in gynodioecious populations. This is a viable life-history strategy, provided that pollination rates are sufficiently high. However, reduced pollination rates in alpine plants are likely due to climate induced plant-pollinator mismatches and general declines in pollinators. Using a gynodioecious population of the dominant plant Silene acaulis (Caryophyllaceae), we tested the reproductive assurance hypothesis and also the stress gradient hypothesis with a series of pollinator exclusion trials and extensive measurements of subsequent reproductive output (gender ratio, plant size, percent fruit-set, fruit weight, seeds per fruit, total seeds, seed weight, and seed germination). The reproductive assurance hypothesis was supported with female plants being more sensitive to and less likely to be viable under reductions in pollination rates. These findings are the first to show that the stress gradient hypothesis is also supported under a gradient of pollen supply instead of environmental limitations. Beneficiary abundance was negatively correlated to percent fruit-set under current pollen supply, but became positive under reduced pollen supply suggesting that there are important plant-plant-pollinator interactions related to reproduction in these alpine plant species.

Ecological implications of reduced pollen deposition in alpine plants: a case study using a dominant cushion plant species

The reproductive assurance hypothesis states that self-incompatible female plants must produce twice the number of seeds relative to their self-compatible hermaphroditic counterparts to persist in gynodioecious populations. This is a viable life-history strategy, provided that pollination rates are sufficiently high. However, reduced pollination rates in alpine plants are likely due to climate induced plant-pollinator mismatches and general declines in pollinators. Using a gynodioecious population of the dominant plant Silene acaulis (Caryophyllaceae), we tested the reproductive assurance hypothesis and also the stress gradient hypothesis with a series of pollinator exclusion trials and extensive measurements of subsequent reproductive output (gender ratio, plant size, percent fruit-set, fruit weight, seeds per fruit, total seeds, seed weight, and seed germination). The reproductive assurance hypothesis was supported with female plants being more sensitive to and less likely to be viable under reductions in pollination rates. These findings are the first to show that the stress gradient hypothesis is also supported under a gradient of pollen supply instead of environmental limitations. Beneficiary abundance was negatively correlated to percent fruit-set under current pollen supply, but became positive under reduced pollen supply suggesting that there are important plant-plant-pollinator interactions related to reproduction in these alpine plant species.

Ecological implications of reduced pollen deposition in alpine plants: a case study using a dominant cushion plant species.

The reproductive assurance hypothesis states that self-incompatible female plants must produce twice the number of seeds relative to their self-compatible hermaphroditic counterparts to persist in gynodioecious populations. This is a viable life-history strategy, provided that pollination rates are sufficiently high. However, reduced pollination rates in alpine plants are likely due to climate induced plant-pollinator mismatches and general declines in pollinators. Using a gynodioecious population of the dominant plant Silene acaulis (Caryophyllaceae), we tested the reproductive assurance hypothesis and also the stress gradient hypothesis with a series of pollinator exclusion trials and extensive measurements of subsequent reproductive output (gender ratio, plant size, percent fruit-set, fruit weight, seeds per fruit, total seeds, seed weight, and seed germination). The reproductive assurance hypothesis was supported with female plants being more sensitive to and less likely to be viable under reductions in pollination rates. These findings are the first to show that the stress gradient hypothesis is also supported under a gradient of pollen supply instead of environmental limitations. Beneficiary abundance was negatively correlated to percent fruit-set under current pollen supply, but became positive under reduced pollen supply suggesting that there are important plant-plant-pollinator interactions related to reproduction in these alpine plant species.