Modelling the location of interglacial microrefugia for cold-adapted species: insights from the terrain-mediated distribution of Rhododendron tomentosum in a temperate region in central Europe

Rhododendron tomentosum is a vascular plant common in northern regions of Europe, Asia and North America. In central Europe, this species is considered a glacial relict. In recent years, systematic floristic mapping of the distribution of this species in the Bohemian Switzerland National Park has revealed that this rare and highly sensitive species is strongly associated with a specific type of habitat. Rhododendron tomentosum adapted to environmental changes throughout the Holocene: soil acidification, repeated fires and negative effect of forestry on species composition. To identify the survival mechanisms of R. tomentosum, suitable sites in well-preserved parts of a sandstone landscape with assumed minimal human influence were selected. Based on data from intensive field sampling and a set of environmental variables for the whole area of the National Park, habitat suitability maps for R. tomentosum were developed. These maps were then combined with the palaeoecological data and used to identify sources of pollen with greater precision and extend the knowledge of the potential distribution of R. tomentosum outside the intensively sampled areas. Palaeoecological data was particularly useful for reconstructing the past population dynamics of R. tomentosum. The results indicate the long-term stability of sites suitable for this species, supported by locally diversified vegetation development in sandstone areas. This research could therefore provide the first direct evidence of the persistence of R. tomentosum micropopulations throughout a large part of the Holocene.