Beyond temperature: The environmental constraints of high‐mountain microrefugia

It has been hypothesized that fine‐scale climatic heterogeneity of above‐treeline landscapes may enable alpine plant species to persist in nearby cold microrefugia when climate warms.

However, temperature is not the only driver of species distributions in alpine environments. To what extent additional drivers constrain the suitability of potential microrefugia for high‐mountain plant species has not yet been evaluated.

We assessed if vascular plant assemblages occur in outposts below their optimal elevation due to specific temperature or substrate conditions. We therefore combined 824 species composition samples distributed across 5 km ² of above‐treeline terrain with fine‐scale environmental proxies by means of a generalized dissimilarity model (GDM) and used this GDM to predict the potential low‐elevation occurrence of species combinations that are characteristic of higher elevations in the study landscape. To quantify the relative impact of temperature and soil carbon content on the incidence of these low‐elevation vegetation outposts, we subsequently compared these projections to interpolated maps of low‐elevation occurrence of temperature and soil conditions that are characteristic of higher elevations.

In the GDM, temperature was the strongest driver of species composition, followed by soil carbon content. The GDMs projected that approximately 33% of the current alpine landscape potentially carry low‐elevation outposts of higher‐elevation vegetation. However, of this large area about 30% was characterized by outposts of low‐soil carbon content, mostly scree and rock faces, 2.9% by a combination of low‐carbon content and cold temperature, and only 0.1% exclusively by outposts of cold temperature. Vice versa, about 2.1% of the landscape exhibited unusually cold temperature, but were not projected to carry low‐elevation outposts of higher‐elevation vegetation.

Synthesis . In the current landscape, the potential occurrence of plant assemblages at unusually low elevations is concentrated in areas with particular soil conditions. Sites with unusually cold temperatures for their elevation are much rarer, and a significant share of them has soil properties that many species of the current alpine flora are unlikely to tolerate. Our findings therefore suggest that the rescue effect of cold microrefugia for alpine plants in a warming world will be limited and that edaphic factors deserve more attention alongside temperature when assessing microrefugia suitability.