Broad-scale genetic homogeneity in natural populations of common hazel (Corylus avellana) in Ireland

Hazel (Corylus avellana) has been a key species in European woodlands throughout the Holocene (10 KYA–present). Like many tree species, it is increasingly under threat from climate change, habitat loss and fragmentation, invasive species and emergent pathogens. As knowledge of the genetic structure of natural populations of trees is vital for managing these threats, as well as an essential basis for selection of material for replanting and restocking, we analysed levels and patterns of genetic diversity in the species at a range of spatial scales using high-resolution microsatellite markers. Our findings indicate that hazel populations exhibit high levels of genetic diversity along with low levels of population differentiation, suggesting extensive gene flow. Fine-scale genetic structuring was observed in some of the woodlands studied, probably resulting from restricted dispersal of the heavy nuts produced by the species. This, coupled with higher levels of pollen-mediated gene flow, resulted in a weak but significant pattern of isolation by distance. These results suggest that replanting following potential loss of hazel populations may not necessarily require the use of material from the same locality and mirror findings in other broadleaved tree species from the same area.