Multiple glacial refugia across northern and southern China and unexpected patterns of spatial genetic diversity in Betula albosinensis: a widespread temperate deciduous tree species

The central-marginal hypothesis (CMH) predicts high genetic diversity at the species geographic centre and low genetic diversity at the species geographic margins. However, most studies examining the CMH have neglected potential effect of past climate. Here, we test six hypotheses, representing effects of past climate and contemporary range position, for their ability to explain the spatial patterns of genetic diversity in 37 populations of Betula albosinensis. Ecological niche modelling (ENM) revealed large and continuous suitable habitats in north, southwest and southeast China during the last glacial maximum (LGM) but a contraction of suitable habitats since the LGM. Pollen records further confirmed the existence of multiple refugia in north and south China. The spatial pattern of genetic diversity (i.e., expected heterozygosity, gene diversity and allele richness) were best explained by distance to the southern edge and distance to the range edge but also showed longitudinal and latitudinal gradients. Hypotheses accounting for the effects of climate (climatic suitability, climatic stability and climatic variability) had comparatively little support. Our findings show partial support for the CMH and illustrates that the existence of multiple LGM refugia, and suggests species abundance and past species movement play a role in shaping genetic diversity across species ranges.