Pedunculate and sessile oaks (Quercus robur L.; Q. petraea [Matt] Liebl.) often coexist in mixed forest stands.

However, species-specific investigations and forest management actions in such populations require reliable

methods of identification of the species status of individuals. We investigated genetic diversity and species differentiation

of adult and naturally established seedling cohorts in a mixed forest stand composed of Q. robur and

Q. petraea, located in the Jamy Nature Reserve in north-central Poland. Using nineteen nuclear microsatellite

loci and a model-based clustering approach as a tool for species delineation, we efficiently identified 105 and

60 adults, as well as 191 and 456 seedlings of pedunculate and sessile oaks, respectively. While the adult trees

of both species were randomly distributed throughout the sample plot, the seedlings demonstrated significant

spatial clustering, which was particularly evident for Q. petraea. The two oak species exhibited similar levels of

genetic diversity in adult and offspring cohorts. Inbreeding was found to be low and significant only at the stage of

seedlings. The estimates of effective population size were higher for Q. robur than Q. petraea, despite the overall

greater reproductive success of the later one. There was a significant level of differentiation between the studied

oak species, as measured by Fst coefficient (0.084 – adults; 0.099 – seedlings). The results on genetic diversity and

species differentiation obtained in the studied indigenous near-natural stand of Q. robur and Q. petraea could be

considered as a reference for other population genetic studies of oaks.

Genetic diversity is often considered a major determinant of long term population persistence and its potential to adapt to variable environmental conditions. The ability of populations to maintain their genetic diversity across generations seems to be a major prerequisite for their sustainability, which is particularly important for keystone forest tree species. However, little is known about genetic consequences of demographic alterations occurring during natural processes of ecological succession involving changes in the species composition. Using microsatellites, we investigated genetic diversity of adult and offspring generations in beech (Fagus sylvatica L.) and oak (Quercus robur L.) populations coexisting in a naturally established old-growth forest stand, showing some symptoms of ongoing ecological succession from oak- to beech- dominated forest. In general, adult generations of both species exhibited high levels of genetic diversity (0.657 for beech; 0.821 for oak), which, however, depended on the sets of selected genetic markers. Nevertheless, several symptoms such as differences in genetic diversity indices between generations, significant levels of inbreeding (up to 0.029) and low estimates of effective population size (48-80) confirmed the declining status of the oak population. On the other hand, the uniform distribution of genetic diversity indices across generations, low levels of inbreeding (0.004), low genetic differentiation among adults and offspring and, most importantly, large estimates of effective population size (119-716), all supported beech as a successive and successful tree species in the studied forest stand.