A common soil temperature threshold for the upper limit of alpine grasslands in European mountains

Abstract

While climatic research about treeline has a long history, the climatic conditions corresponding to the upper limit of closed alpine grasslands remain poorly understood. Here, we propose a climatic definition for this limit, the ‘grassline’, in analogy to the treeline, which is based on the growing season length and the soil temperature. Eighty-seven mountain summits across ten European mountain ranges, covering three biomes (boreal, temperate, Mediterranean), were inventoried as part of the GLORIA project. Vascular plant cover was estimated visually in 326 plots of 1 × 1 m. Soil temperatures were measured in situ for 2–7 years, from which the length of the growing season and mean temperature were derived. The climatic conditions corresponding to 40% plant cover were defined as the thresholds for alpine grassland. Closed vegetation was present in locations with a mean growing season soil temperature warmer than 4.9 °C, or a minimal growing season length of 85 days, with the growing season defined as encompassing days with daily mean ≥ 1 °C. Hence, the upper limit of closed grasslands was associated with a mean soil temperature close to that previously observed at the treeline, and in accordance with physiological thresholds to growth in vascular plants. In contrast to trees, whose canopy temperature is coupled with air temperature, small-stature alpine plants benefit from the soil warmed by solar radiation and consequently, they can grow at higher elevations. Since substrate stability is necessary for grasslands to occur at their climatic limit, the grassline rarely appears as a distinct linear feature.

Climate Change Affects Vegetation Differently on Siliceous and Calcareous Summits of the European Alps

“…Therefore, the hypothesis (1a) that vascular plant cover and litter should prevail on siliceous bedrock, whereas bare ground, scree, and rock should dominate on calcareous bedrock could only be partially confirmed. Independently of bedrock, mean vascular plant cover decreased strongly along the elevation gradient and thus, is strongly related to the adiabatic temperature gradient (Bürli et al, 2021). Also, the proportion of scree and rock was independent of bedrock types.…”

High-elevation limits and the ecology of high-elevation vascular plants: legacies from Alexander von Humboldt

“…Grabherr et al (1995) present not only elevational limits of plants in tropical, subtropical, and temperate mountains in East Africa, South America, and Europe but also the upper limits of continuous vegetation. In Table 4, I summarise information of the highest elevation (HE) of the mountains considered in Table 3, along with the highest known elevational limits of vascular plants (PL) on mountains in the major mountain regions within the tropical, subtropical, and temperate zones, as well as the potential climatic treeline (TL; based on Körner 2007, 2012, Paulsen and Körner 2014, the highest elevation when closed vegetation (VL) ('grassline' of Bürli et al 2021) occurs, and the ratios of PL/TL, PL/HE, and PL/VL. The data sources used in Table 4 include Grabherr et al (1995), Körner (2003Körner ( , 2012, publications cited earlier for the basic upper-elevation limits for vascular plants in particular mountain regions, and my own field observations, particularly for the closed vegetation limit (≥40% vascular-plant cover).…”

Why Is the Alpine Flora Comparatively Robust against Climatic Warming?

“…As already mentioned, this estimation does not account for effects of soil development and altered moisture availability. It had been proposed that there is an alpine grassline in analogy to alpine treeline, meaning that connecting the position of patches of closed, very high elevation grassland communities also follows a common isotherm [42]. Since such grassland fragments are tied to stable substrate and certain topographic niches, they are unlikely to shift upslope, but are likely to retain their position, given they are currently representing the cold range limit of this type of alpine vegetation with a substantial leeway towards warmer conditions.…”