Snowmelt Gradient Research Articles

No evidence for adaptation of two Polygonum viviparum morphotypes of different bulbil characteristics to length of growing season: abundance, biomass and germination

The high degree of habitat heterogeneity and habitat fragmentation in arctic ecosystems may support a high genotypic and ecotypic variability. This may buffer the survival of plant species threatened by global climate change, which affects the Arctic more strongly than other ecosystems. Here, we assessed if two morphotypes of Polygonum viviparum (characterised by different colours of their bulbils) differ in their abundance along a snowmelt gradient, if their biomass allocation patterns are influenced differentially by environmental variables, and if the temperature dependency of bulbil germination differs between morphotypes. We found slight differences in the effect of timing of snowmelt on abundance of the morphotypes, which seem to have little ecological relevance. Total biomass and biomass allocation were similar between morphotypes and were negatively correlated with soil-water content. Bulbil germination (the onset of growth of the bulbil) was assessed over a temperature range from 2 to 25°C and results indicate an earlier (maximum of 5 days) "germination" of one morphotype, but final bulbil germination (>80%) and bulbil-germination rate were similar for both types. Bulbil germination was weakly temperature dependent, with faster emergence at higher temperatures. Overall, our results could provide no convincing evidence for differences between the two examined morphotypes that could be of ecological relevance with respect to anticipated climate change in the Arctic.

Effects of snowmelt timing on leaf traits, leaf production, and shoot growth of alpine plants: Comparisons along a snowmelt gradient in northern Sweden

Effects of snow-melt timing on leaf traits (for five deciduous and five evergreen species), shoot growth, and leaf production (for five evergreen species) of tundra plants were studied along a snow-melt gradient in an alpine snowbed in northern Sweden. In deciduous plants, leaf life-span and leaf mass per area (LMA) decreased, and nitrogen concentration (leaf N) increased with decreasing growing season, whereas in evergreen plants, both leaf life-span and leaf N increased with decreasing growing season. By extending leaf life-span, evergreen plants are able to have a large leaf mass, which may contribute to maintain net annual carbon gain in short snow-free seasons. In two predominantly boreal evergreen species, Empetrum hermaphroditum and Vaccinium vitis-idaea, leaf life-span was negatively correlated with both annual leaf production and shoot growth, but there were no similar significant correlations for the other three, strictly arctic-alpine evergreen species (Cassiope tetragona, Loiseleuria procumbens, and Diapensia lapponica). Based on these results, we predict that extension of season length will decrease leaf N of both deciduous and evergreen species, and will accelerate leaf turnover of evergreen plants. Although annual leaf production and shoot growth of boreal species may increase with an extension of season length, they will remain unchanged in strictly arctic-alpine species.

Life on The Edge: Adaptation Versus Environmentally Mediated Gene Flow in The Snow Buttercup,Ranunculus Adoneus

We used experimental transplant studies to understand how dispersal and habitat-specific 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.

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.

Intraspecific variation of leaf traits in several deciduous species in relation to length of growing season

Effects of growing season length on leaf traits were studied in three deciduous species found along an alpine snowmelt gradient (snow-free period ranging from 69 to 117 days), and in four deciduous...

Changes in Vegetation and Soil Fertility along a Predictable Snowmelt Gradient in the Mosquito Range, Colorado, U. S. A.

Changes in edaphic conditions and vegetation along snowmelt gradients are well known in many alpine areas, but very few studies have used multivariate techniques to document these changes at a microgeographic scale and to measure predictability of snowmelt patterns. This paper describes concordant changes in microenvironmental factors and plant species abundances among >200 evenly spaced microsites in a large alpine snowbed in the Mosquito Range, Colorado. The snowmelt gradient was highly consistent: averaged over all microsites, the mean date of snowmelt varied by only 1 wk from 1989 to 1992, and relative date of snowmelt among microsites was highly predictable (R2 > 90% among years). Soil in later melting microsites had reduced organic content, water content, nitrogen, phosphorous, and acidity. Total soil nitrogen and water content decreased through the growing season, while pH increased significantly. Species richness and total vegetation cover were significantly greater in early melting microsites, and soil disturbance by pocket gophers and erosion peaked in moderately latemelting areas. Canonical correspondence analysis (CCA) showed that microgeographic changes in the plant community were associated with four environmental factors: snowmelt date, rock cover, soil disturbance, and soil organic content. Plant species that are common in the vegetation surrounding the snowbed tended to decrease in abundance in later melting snowbed sites. Other species attained peak abundance in the less productive late melting sites, and a few appeared to be associated with soil disturbance. Plant community differences between paired sampling units (quadrats) were determined more by differences in their snowmelt date than by their spatial separation, suggesting that plant species distributions along this alpine gradient are determined principally by ecological interactions and physiological tolerances, rather than by historical accident and limited dispersal.

Relationship between flowering time and fruit set of the entomophilous alpine shrub, Rhododendron aureum (Ericaceae), inhabiting snow patches

Flowering phenology, fruit set, and pollinator frequency of the ericaceous alpine dwarf shrub, Rhododendron aureum, were compared among three quadrats arranged along a snowmelt gradient on a slope. Bagging and self‐pollination experiments showed that R. aureum was physiologically self‐compatible, but pollinator visitation highly enhanced fruit set. Depending on the lateness of snowmelt, flowering time varied from mid‐June to late July among quadrats. Pollinator visitation increased as the flowering season progressed, and fruit set was significantly higher in the quadrat with latest snowmelt. Emasculation experiments suggested that later flowering might promote cross‐pollination. Thus, later flowering was advantageous for effective pollination. On the other hand, later‐flowering plants often failed to set fruits because of the onset of autumn frost and snow before fruit maturation.

Effects of Snow-Free Period on the Phenology of Alpine Plants Inhabiting Snow Patches

Phenology of alpine plants was studied with reference to the time span of snow-free period by setting six quadrats along a snowmelt gradient. Of the 56 species, 17 inhabited four or more quadrats and their phenology was observed for 3 yr, 1988 to 1990. Contraction of the snow-free period reduced flowering and seeding rates. Phenology of the alpine plants was controlled primarily by (1) growth form, (2) the span of preflowering, and (3) the term for the maturity of seeds. Shrub species, especially evergreen shrubs, took longer for maturing their seeds. They abandoned sexual reproduction and reproduced vegetatively by layering in the short snow-free habitats. Forb and graminoid species opened their flowers even in the

La végétation des combes à neige du golfe de Richmond, Québec nordique

An exhaustive survey of snow patches, along an altitudinal gradient situated in an area of about 10 km2 along the Hudson Bay highlands, underlines the role of altitude, exposition, and topography in maintaining these habitats. The principal component analysis from a stratified sampling of the snow patches in the three cuesta lithological zones (arkose, dolomite, and basalt) stresses the effect of the snow melt gradient on species distribution within a snow patch. The ordination of abiotic variables performed on the ordination frame of snow patches shows that lithology and substratum are the principal factors responsible for the vegetation characteristics, allowing the determination of two main types of snow patches in the arkose and dolomite zones and three types in the basalt zone.