Cryptic no more: soil macrofossils uncover Pleistocene forest microrefugia within a periglacial desert

Despite their critical importance for understanding the local effects of global climate change on biodiversity, glacial microrefugia are not well studied because they are difficult to detect by using classical palaeoecological or population genetics approaches. We used soil macrofossil charcoal analysis to uncover the presence of cryptic glacial refugia for European beech (Fagus sylvatica) and other tree species in the Landes de Gascogne (southwestern France). Using botanical identification and direct radiocarbon dating (140 (14) C-dates) of macrofossil charcoal extracted from mineral soils, we reconstructed the glacial and postglacial history of all extant beech stands in the region (n = 11). Soil charcoal macrofossils were found in all sites, allowing the identification of up to at least 14 distinct fire events per site. There was direct evidence of the presence of beech during the last glacial period at three sites. Beech was detected during Heinrich stadial-1, one of the coldest and driest intervals of the last glacial period in Western Europe. Together with previous results on the genetic structure of the species in the region, these findings suggest that beech persisted in situ in several microrefugia through full glacial and interglacial periods up to the present day.