Transmission of genetic variation from the adult generation to naturally established seedling cohorts in small forest stands of pedunculate oak (Quercus robur L.)

Natural regeneration is being increasingly encouraged as the preferred regeneration method for sustainable forestry. However, the benefits of natural regeneration may be jeopardized in small and low-density forest stands, as genetic drift and inbreeding may reduce genetic diversity of the standing population. Particularly in light-demanding tree species, which are characterized by a narrow recruitment window, conservation of genetic diversity during natural regeneration can be challenging in small forest stands. In this study, we investigated intergenerational transmission of genetic diversity in stands of the lightdemanding tree species pedunculate oak (Quercus robur L.) by comparing genetic diversity between adults and recently established seedling cohorts (1–3 years old) in four small stands of Q. robur (<4.5 ha) that varied in tree density. We also quantified mating patterns and gene flow to investigate their role in shaping the genetic diversity and spatial genetic structure of the studied populations. Because of the closed canopy cover of these stands, only recently established seedlings could be studied. When all stands were pooled, the adult cohort showed a slightly but significantly higher allelic richness (Ar) than the established seedling cohort (Ar = 11.5 and 10.7, respectively). However, at the stand level, no significant negative effects on the genetic diversity of the offspring generation were found in the small and low-density forest stands. As expected, acorn dispersal resulting in seedling recruitment was restricted to a few meters from mother trees, resulting in significant small-scale spatial genetic structure in the offspring. Pollen inflow from outside the study plots varied strongly among stands, but all plots showed a significant correlated paternity (rp), with higher estimates of rp found in forest stands with lower effective population sizes (Ne) and lower tree densities. Given that high rp-levels may increase the probability of biparental inbreeding and may incur fitness costs in subsequent offspring generations, we recommend that in small scale forestry of Q. robur, Ne and tree densities should be sufficiently large to maintain genetic diversity over the long term. Allowing gene flow between tree populations through reducing the spatial isolation among forest stands, can help to increase Ne in small-scale silvicultural systems of pedunculate oak, but also in other species characterized by narrow recruitment windows.