Aim: “Where and why does a species exist” is a fundamental question in ecology. However, the actual range limits of alpine plant species are largely unexplored and unexplained. We aim at identifying the low temperature range limits of the two most abundant alpine graminoid species on acidic soils that intermingle in mosaics of high-elevation habitats across the European Alps.

Location: Alpine grasslands in the Swiss Alps.

Taxon: Carex curvula (Cyperaceae) and Nardus stricta (Poaceae), named by the genus name hereafter.

Results: Carex and Nardus clearly segregated across different microsites. Season length, growing degree hours and soil chemistry (pH, C/N-ratio, phosphorus) did not demarcate the two species’ ranges, while their distribution was strongly affected by soil minimum temperature in winter. Carex occurred at sites with and without protecting snow cover and resisted low soil temperatures (-13 °C). Nardus was absent at microsites with snow cover duration less than 5 months and soil minimum temperatures below -5 °C. During the growing season, leaves of Carex had a higher freezing resistance with LT50 of -16.1 °C than those of Nardus with LT50 of -13.3 °C (LT50: lethal temperature for 50% of the tissue). Tetrazolium staining in shoots also revealed a higher freezing resistance in Carex compared to Nardus, and shoot apices tolerated lowest temperatures: Carex -30 °C, Nardus -24 °C. Though, a vital shoot apex alone did not ensure regrowth after winter. Regrowth after severe frost events requires intact vessels and roots, all less freezing tolerant than apical meristems and young leaves.

Main conclusions: The cold range limits of these widespread alpine graminoid species are evidently set by thermal extremes in winter. Microtopography, thus snow distribution pattern, in concert with the species’ freezing resistance explains the cold edge of the fundamental niche of these two species.

We year-round assessed the temperature 3 cm below ground, closest to the plant meristems as well as snow cover duration, soil chemistry and vegetation characteristics at high spatio-temporal resolution, resulting in 115 well-characterized microsites. Field data were combined with the freezing resistance analyses (electrolyte leakage, tetrazolium vital staining, regrowth capability) in plant individuals at 38 microsites by employing mixed regression models.