Soil thermal buffer and regeneration niche may favour calcareous fen resilience to climate change

Abstract

Calcareous fens are azonal habitats permanently saturated by groundwater. This is expected to have a buffer effect on soil temperature, alleviating climate changes and allowing plant communities to occupy diverse climatic regions. We analysed the extent of such buffering and its relation with a relevant plant trait, the seed germination niche breadth, along altitudinal gradients in fens of the Cantabrian Mountains (Spain) and the Western Carpathians (Slovakia). In each fen we recorded soil temperature for several years and compared it with WorldClim predictions for air temperature. We also collected seeds from five Cyperaceae fen specialists to evaluate the influence of soil temperature on germination. Although the soil temperatures and WorldClim predictions were strongly correlated, their absolute values differed substantially, showing a narrower thermal amplitude and warmer minimum winter temperature in the soil. The greatest differences in soil temperature and germination niche breadth were those between mountain regions. Narrower germination niches correlated with the colder Slovak winter. Our results suggest that the soil thermal buffer allows species to escape frost temperatures in winter, but also high summer temperatures in warm regions, explaining their wide distribution ranges. The warm regeneration niche does not match the cooler soils, but shows variability and potential for adaptation. While these findings support resilience to climate warming, changes in precipitation rather than temperature seem to be the main threat for fen persistence.