Comparative seed germination traits in alpine and subalpine grasslands: higher elevations are associated with warmer germination temperatures

Abstract

Seed germination traits in alpine grasslands are poorly understood, despite the sensitivity of these communities to climate change. This article tests the hypothesis that germination traits predict plant species occurrence along the alpine-subalpine elevation gradient. A phylogenetic comparative analysis was performed using fresh seeds from twenty-two plant species typically found in alpine and subalpine grasslands (1600 – 2400 m) of the Cantabrian Mountains, northern Spain (43º N, 5º W). Laboratory experiments were conducted with seeds from each species to characterize germinability, optimum germination temperature and the effect of cold and warm stratification on dormancy breaking. Variability in the estimated traits was reduced by Phylogenetic Principal Component Analysis (phyl.PCA). Phylogenetic Generalised Least Squares regression (PGLS) was used to fit a model in which species average elevation was predicted from the species position in the PCA axes. Most subalpine species germinated in snow-like conditions, whereas most alpine species needed an accumulation of warm temperatures. Phylogenetic signal was low. The first PCA axis ordered germination traits according to overall germinability, whilst the second axis ordered them according to the preference for warm or cold germination treatments. The second axis significantly predicted species occurrence in the alpine-subalpine elevation gradient, as higher elevation species tended to have warmer germination preferences. Our results show that germination traits in high-mountain grasslands are closely linked to the alpine-subalpine gradient. Alpine species, especially those from stripped and wind-edge communities, have a preference for warmer germination niches, suggesting that summer emergence prevents frost damage during seedling establishment. In contrast, alpine snowfield and subalpine grassland plants have cold germination niches, indicating that winter emergence may occur under snow to avoid drought stress.