Thermal microenvironments and the ecology of the alpine herb Aciphylla glacialis

Abstract

Alpine areas are often described as a gradient of warm air temperatures at low elevations to cold air temperatures at high. However, plant temperature does not usually follow the gradient of temperature often described for alpine areas; hence elevation per se becomes a less useful predictor of life conditions. To identify causal effects of climate on plant traits, microclimatic factors have to be taken into account. One of the most important determinants of plant microenvironment in alpine areas is snow duration. Plants living under snow remain at very stable and relatively warm temperatures even when air temperature is below zero. However, in patches where snow melts earlier, plants are exposed to very low temperatures and quite different thermal microenvironments. Because snow duration varies at fine spatial scales, species might occupy both early and late snowmelt microenvironments. I studied how contrasting microenvironments correlate with within-species differences in vegetative and physiological traits in Aciphylla glacialis, a widely distributed alpine plant. Seedlings and adult plants from early snowmelt sites had higher freezing resistance and seedlings exhibited greater acclimation capacity than late snowmelt sites, even when grown in common garden conditions and without prior exposure to field conditions. Variation in freezing resistance among adult plants was influenced by snow duration. The observed differences may reflect genetic or epigenetic differentiation, or other maternal environmental effects. To determine whether maternal environments influenced differences in A. glacialis progeny, I measured elements of the thermal microclimate during seed development in the field and correlated those conditions with seed and seedling traits. Seed germination, seed and seedling chemistry and seedling growth were all influenced by minimum and maximum temperatures and by the frequency of freezing events during seed development. Differences in seed germination and seedling nitrogen content among individuals were related to differences in carbon allocation to seeds, which themselves were correlated with frequency of exposure to freezing temperatures. On the other hand, maximum temperatures during seed development promoted seedling growth, perhaps via an epigenetic memory. My research reveals that temperature during seed development has important effects on progeny success and progeny cold tolerance. These findings are extremely important for alpine areas within the context of climate change. Warming and early snowmelt in alpine areas are paradoxically exposing plants to more frequent extreme freezing events. The existence of variation in cold tolerance among genotypes within species might help to maintain species diversity in a future climate. Temperature during seed development might prepare seeds to cope with unpredictable freezing and heating events predicted under future climate. Or, alternatively, if the maternal environmental cues are unreliable, maternal environments may prepare seeds for environmental conditions that no longer exist. Presence of more variation in freezing resistance within species and maternal environmental effects on freezing tolerance of plants, particularly seeds and seedlings, needs urgent consideration to evaluate effects of climate change on alpine flora.