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

Abstract

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

Keywords: Niwot Ridge; distribution; elevation; flowering; photosynthesis; snowmelt gradient; spatio-temporal dynamics; water relations.

FIGURE 1

% slope, snowmelt timing (Julian day), richness, and % cover gradients fit as splines across our site. Points indicate locations of plots at our site and black contours indicate elevation. Splines were generated using generalized cross-validation scores to portray relationships between elevation and each variable along contours.

FIGURE 2

Non-metric multidimensional scaling (NMS) ordination of % cover measurements (i.e., vascular cover, bare ground, litter, gravel, and snowmelt) across the field site illustrating within community gradients. Numbers correspond to plot identifiers and colored triangles indicate snowmelt timing of each plot (warmer colors indicate later snowmelt).

FIGURE 3

Non-metric multidimensional scaling (NMS) ordination of species presence across the site with individual species (blue crosshairs) arrayed along cover gradients (VASC = vascular plant % cover, GRAV = gravel % cover, LITT = litter % cover, and MELT = snowmelt timing). Species abbreviation codes are in Supplementary Table S1.

FIGURE 4

Physiological measurements separated by alpine-restricted (closed circles and solid lines) and wide-ranging species (open circles and dotted lines) by sampling month (A–F) and snowmelt timing (G–L). Means and standard errors are presented for photosynthetic rates (A_net; μmols m^-2 s^-1), dark respiration (R_d; μmols CO₂ m^-2 s^-1), stomatal conductance (g; mol m^-2 s^-1), CO₂ assimilation (C_i; PPM), transpiration (E; mmols H₂O m^-2 s^-1), and water-use efficiency (WUE; μmol mmol^-1).

FIGURE 5

September photosynthesis (A_net; μmols m^-2 s^-1) measured on leaves exposed to 400 μmol mol^-1 (black lines) and 800 μmol mol^-1 (gray lines) of CO₂ on alpine-restricted (closed circles and soil lines) and wide-ranging species (open circles and dotted lines).

FIGURE 6

Pre-dawn leaf water potential (Ψ_pre-dawn; MPa) measurements for alpine-restricted (closed circles and solid lines) and wide-ranging species (open circles and dotted lines) by sampling month (left) and snowmelt timing (right).