Refugia within refugia: the case study of a canopy tree (Eurycorymbus cavaleriei) in subtropical China

Aim Eurycorymbus cavaleriei (Levl.) Rehd. et Hand.-Mazz. (Sapindaceae) is a Tertiary relict tree endemic to subtropical China. This area is a centre for speciation and evolution within the East Asia biome and one of its most important refugial locations. In this study we aim to elucidate the phylogeographical patterning in E. cavaleriei, in order to identify the locations of the species' main refugia and the predominant patterns of migration that have led to the contemporary spatial genetic structure of chloroplast variation. Location Subtropical China. Methods We sampled 18 populations of E. cavaleriei throughout its geographical range. Chloroplast DNA (cpDNA) sequence data from two non-coding regions ((trnS/trnG and pl20/5'-rps12) were obtained from 170 individuals for phylogeographical analyses. Relationships among cpDNA haplotypes were determined using median-joining networks. Genetic structure was examined by spatial analysis of molecular variance (SAMOVA). Population differentiation was estimated by G(ST) and N-ST statistics. Results Ten distinct haplotypes were identified. The level of differentiation among populations was relatively high (G(ST) = 0.817), and N-ST was significantly higher than G(ST) (P < 0.05), indicating that strong phylogeographical structure is exhibited by this species. The SAMOVA revealed five diverging groups of related haplotypes, which coincide with major landscape features in this region. Main conclusions The high differentiation among populations of E. cavaleriei may be a combined effect of historical and contemporary processes, such as the low effective population size for the chloroplast genome of a dioecious species, long-term range fragmentation and limited seed dispersal for the species. Clear-cut geographical distributions of ancestral haplotypes of the species suggest multiple potential refugia across subtropical China. The identified refugial regions have long been recognized as centres of plant diversity and endemism for China and have also been suggested as glacial refugia for many other plant species. The combination of these factors means that these locations should be considered as the highest priority for inclusion in conservation policies and sustainable forest management strategies for subtropical China.