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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.

Polyploid speciation is an ongoing, important source of angiosperm diversity. However, the barriers to polyploid speciation and mechanisms of establishment remain poorly understood for all but a few species. Several factors likely to have influenced tetraploid establishment, including barriers to triploid formation, consequences of mixed-ploidy pollen loads, and the reproductive success of the minority cytotype, were examined in snow buttercups (Ranunculus adoneus) through a series of pollination and transplant experiments. Tetraploid snow buttercups do not have stigmatic barriers to pollen from diploid plants, nor does pollen from tetraploid plants have an advantage over pollen from diploids when on tetraploid stigmas. Tetraploid plants transplanted into a diploid population produced 50% fewer seeds than tetraploid plants in a tetraploid population. Intrinsic barriers to triploid formation were relatively weak, but few triploid seeds formed when mixed-ploidy pollen was present. Fecundity of triploid plants was very low, and no tetraploid offspring resulted. These results indicate that in snow buttercups, a triploid plant will contribute 0.8% of the tetraploid seeds of a minority tetraploid plant making it a minor contributor to the demographics of tetraploid establishment. The reproductive costs facing minority cytotype plants may explain the previously observed spatial segregation in snow buttercups.

Newly formed tetraploid plants in sympatry with their diploid progenitors should face significant obstacles to persistence and population establishment because of low-fitness triploids formed by cross-ploidy pollinations. Prior models have found restrictive conditions for a minority tetraploid subpopulation to persist. A stochastic spatial model, parameterized using snow buttercups (Ranunculus adoneus), was used to examine the influence of limited seed and pollen dispersal distances on the success of minority tetraploids and the interaction of these factors with different rates of self-pollination and tetraploid advantage. Higher rates of self-pollination and increased tetraploid advantage increase the probability of tetraploid persistence. Limiting the dispersal of seeds and pollen further increases the positive impact of any given level of self-pollination and tetraploid advantage. Taxa with short-distance seed and pollen dispersal should face much less stringent barriers to sympatric polyploid speciation than taxa with long-distance dispersal patterns. With short-distance seed and pollen dispersal, polyploid speciation should be possible in the absence of ecological differentiation or recurrent polyploid formation through unreduced gametes.

Dark-septate endophytic (DSE) fungi are ubiquitous in the roots of Arctic and alpine plants, yet very little is known about their phylogenetic identities or effects on their host plants. Several such fungi were isolated from the alpine snowbed plant Ranunculus adoneus in the Front Range of Colorado, USA; one isolate was chosen for detailed study. The ability of this isolate to re-colonize plant roots in pot cultures was assessed, and phylogenetic analyses were performed using small-subunit (SSU), 5.8S and internal transcribed spacer (ITS) 2 ribosomal DNA sequences. This isolate had the ability to produce root endophytic structures in pot cultures similar to those reported from other sources and observed in R. adoneus roots. SSU phylogenetic analyses showed this isolate to be related to a clade within the Euascomycetes containing the Leotiales and Erysiphales. In addition, SSU and 5.8S-ITS2 sequences showed high phylogenetic similarity to a variety of isolates reported from other plants of diverse geographical origins. Although most of these isolates remain unidentified, one closely related isolate was the anamorphic taxon Phialophora gregata. The results suggest that this DSE isolate might belong to the fairly closely related group of plant endophytes that have varied effects on the plants that they inhabit.

We used experimental transplant studies to understand how dispersal and habitatspecific selection interact to influence plant populations occupying heterogeneous environments. The snow buttercup (Ranunculus adoneus) occupies a steep ecological and flowering time gradient caused by persistent snowmelt differences within its snow bed habitat. We transplanted seeds, seedlings, and adults to learn about the potential interactions between dispersal and selection. We found that adaptive differentiation is not occurring along the snowmelt gradient, despite striking differences in microhabitat conditions and reproductive phenology between early‐ and latemelting sites. Instead, our results imply that environmentally based differences in seed quality are contributing to directional gene flow from early‐melting locations toward latemelting locations. Emergence and early survival of seedlings is greater in late‐melting sites in some years, but the larger seeds produced by maternal plants in early‐melting locations consistently have a fitness advantage in all parts of the snow bed. Larger seeds survive longer in the soil and have a second peak of seedling emergence in their third year, but these late‐emerging seedlings are successful only if dispersed to less vegetated, late‐melting destinations. The longer growing season in earlymelting sites enhances vegetative growth at all life‐history stages and increases fecundity of seedling transplants but also limits the opportunity for establishment from seed. Our demographic analysis suggests that maternal environmental effects on propagule quality can lead to directional gene flow from benign to marginal sites in populations occupying heterogeneous habitats.

Solar tracking or heliotropism simultaneously raises organ temperature and light interception. For leaves and flowers carbon gain is maximized at the expense of water loss. In this study I explore how costs and benefits associated with water use by solar-tracking flowers of the alpine snow buttercup, Ranunculus adoneus change with ambient temperature. First, I test whether heliotropism increases the water cost of reproduction in the snow buttercup under extant alpine conditions. I then explore whether water use for evaporative cooling in solar-tracking flowers reduces the risk of over-heating as temperatures increase. Solar tracking, by elevating floral temperature and irradiance causes a 29% increase in water uptake by flowers. Gas exchange measurements suggest that the extra water taken up by solar-tracking flowers is released through transpiration. Transpirational cooling in turn allows solar-tracking flowers to gain advantages of enhanced light interception and warmth while reducing the risk of over-heating. Transpiration reduces excess temperature in solar-tracking flowers, but at a water cost. Results show that even in cool alpine habitats, flower heliotropism has water costs to balance its reproductive advantages. Plants with solar-tracking flowers may tolerate hotter conditions if soil moisture is plentiful, but not under drought.

Floral traits affect mating success via their influence on the microenvironment in which sexual reproduction occurs as well as their impact on pollinator attraction. Here we investigate the importance of flower heliotropism as a source of parental environmental effects on pollen quality and performance. Flowers of the snow buttercup. Ranunculus adoneus, closely track the sun's rays. We experimentally restrained flowers to test for effects of heliotropism on pollen quality and performance after pollination. When equivalent amounts of pollen were transferred to recipient pistils, pollen from solar-tracking donor flowers exhibited a 32% advantage in germination compared to pollen from stationary (tethered) donor flowers. By the end of anthesis, pistils of tracking flowers contained 40% more germinating pollen grains and 44% more pollen tubes midway down the style than pistils of stationary ones. Solar tracking had no direct effect on pollen tube growth. The greater amount of germinating pollen in tracking flowers accounted for the treatment effect on pollen tube density. A survey of pollen receipt and pollen germination in naturally tracking flowers indicated that solar tracking primarily affects pollen tube density by promoting pollen germination rather than pollen deposition. We conclude that flower heliotropism, by enhancing the paternal environment for pollen development and the maternal environment for pollen germination, represents a source of positive parental environmental effects on pollen performance in snow buttercups.

• Dark-septate endophytic (DSE) fungi are ubiquitous in the roots of Arctic and alpine plants, yet very little is known about their phylogenetic identities or effects on their host plants. • Several such fungi were isolated from the alpine snowbed plant Ranunculus adoneus in the Front Range of Colorado, USA; one isolate was chosen for detailed study. The ability of this isolate to re-colonize plant roots in pot cultures was assessed, and phylogenetic analyses were performed using small-subunit (SSU), 5.8S and internal transcribed spacer (ITS) 2 ribosomal DNA sequences. • This isolate had the ability to produce root endophytic structures in pot cultures similar to those reported from other sources and observed in R. adoneus roots. SSU phylogenetic analyses showed this isolate to be related to a clade within the Euascomycetes containing the Leotiales and Erysiphales. In addition, SSU and 5.8S-ITS2 sequences showed high phylogenetic similarity to a variety of isolates reported from other plants of diverse geographical origins. Although most of these isolates remain unidentified, one closely related isolate was the anamorphic taxon Phialophora gregata. • The results suggest that this DSE isolate might belong to the fairly closely related group of plant endophytes that have varied effects on the plants that they inhabit.

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.

We assessed the role of growing-season length in regulating absolute and relative cover of six coexisting dominant plant species in an alpine snowbed habitat. To help explain disparity in species-specific responses to growing-season length, we examined the developmental phenology and distribution of each species in relation to natural snow depth variation. Season length varies from @?50 d on early-melting edges of the snowbed to 35 d in the late-melting center, 100 m away. By experimentally altering snowpack, we uncoupled the relationship between spatial location and snowmelt schedule in three consecutive years, imposing the same early dates of snow release in a @'long growing-season@' treatment and the same late dates of snow release in a @'short growing-season@' treatment near the edge and center of the snowbed. Over the course of the experiment, growing-season length had significant effects on absolute and relative cover of the species studied (P < 0.025 and P < 0.005, respectively), and these effects were similar near both the edge and center of the snowbed. Yet, only for the snowbed specialist, Sibbaldia procumbens, were changes in absolute and relative cover under early and late snowmelt schedules predictable from the species' distribution along the historical snow depth gradient. S. procumbens increased in cover under a long growing-season and was more common in historically early-melting portions of the snowbed. Other species (e.g., Ranunculus adoneus, Artemisia scopulorum) were equally common in historically early- and late-melting locations within the snowbed, but showed discordant responses to experimentally imposed changes in snowmelt schedule. That the cover of many species under long-vs. short growing seasons was not predictable from their current distributional affinities in relation to snowmelt pattern likely reflects the disparity between the rates of processes exerting long-term control on species' abundances (colonization, soil development) and more immediate effects of growing-season length on plant growth. Consistent with this view, differences in developmental phenology better predicted species-specific responses to snowmelt schedule than distributional affinities. Species having leaf expansion schedules that are poorly synchronized with snowmelt typically had similar cover under early vs. late schedules of snow release (Geum rossii, Trifolium parryi, and Poa alpina). In contrast, species in which leaf expansion schedules are synchronized with snowmelt responded positively to early snow release (Ranunculus adoneus and Sibbaldia procumbens). We hypothesize that maintaining metabolic @'readiness@' under snowcover provides a mechanism for monopolizing nutrient flushes and competitor free intervals at snowmelt, and exploiting occasional long intervals for growth in years of little snow accumulation, but incurs a respiratory cost that is manifest as reduced growth and vegetative cover when snowmelt is delayed. Our results suggest that interspecific differences in growth phenology of coexisting species will promote shifts in snowbed plant communities with climate change within generations.