How geomorphology shapes unique alpine microrefugia: Plants and soils in cold scree slopes

Cold scree slopes are fascinating geomorphological features governed by a complex reversible internal ventilation system known as the chimney effect, which leads to a pedoclimate that is wetter and around 10 to 15°C colder than surrounding environments. They typically occur in montane and subalpine belts in the Alps (900-2,000 m a.s.l.) and are found across most temperate mountains worldwide. The constantly overcooled scree area harbours small azonal environments (a few hectares at most) with sparse dwarf trees, surrounded by the usual fir, beech or spruce forests. These conditions engender an open understorey, thereby fostering the establishment of boreo-arctic communities dominated by ericaceous shrubs, terricolous lichens, and bryophytes. The combination of the high primary productivity of bryophytes, which is due to the availability of light and wetness, and the cold pedoclimate, which inhibits the activity of soil fauna, results in atypical soils with remarkable accumulations of raw organic matter. Overall, the isolation provided by the thick layer of soil organic matter and bryophytes strengthens the ventilation system.

Thanks to their stable cold micro-pedoclimate, these unique ecosystems are considered as microrefugia: their boreo-arctic islands harbour isolated cold-adapted species surrounded by mountain forests since the end of the Pleistocene. Yet the cold scree slopes that we know today are under threat from climate change, as the chimney effect requires sufficiently cold winters in order to function. However, new ones are likely to occur at higher altitudes in the future, where similar ventilation systems are appearing on today's unvegetated scree slopes in the alpine belt. Identifying where such geomorphological processes could sustain future microrefugia is therefore critical for the conservation of cold-adapted species in warming temperate mountains.