Low winter temperatures and divergent freezing resistance set the cold range limit of widespread alpine graminoids

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 cold 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). Methods We assessed temperature 3 cm below ground, closest to the plant meristems, year-round as well as snow cover duration, soil chemistry and vegetation characteristics at high spatio-temporal resolution at 115 microsites. Field data were combined with various freezing resistance analyses in plant individuals at 38 microsites by employing mixed regression models. Results Carex and Nardus segregated across different microsites. Season length, growing degree hours and soil chemistry 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 degrees C). Nardus was absent at microsites with snow cover duration < 5 months and soil minimum temperatures below -5 degrees C. During the growing season Carex had a higher leaf/shoot freezing resistance (lethal temperature for 50% of the leaf tissue LT50: -16.1 degrees C) than Nardus (-13.3 degrees C). Shoot apices tolerated lowest temperatures: Carex -30 degrees C, Nardus -24 degrees C. Though, a vital shoot apex alone did not ensure regrowth after winter, but intact vessels and roots were required, all less freezing tolerant than apical meristems and young leaves. Main conclusions The cold range limits of these widespread alpine graminoids 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.