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.

Abstract Distribution of genetic diversity among and within plant populations may depend on the mating system and the mechanisms underlying the efficiency of pollen and seed dispersal. In self-incompatible species, negative frequency-dependent selection acting on the self-incompatibility locus is expected to decrease intensity of spatial genetic structure (SGS) and to reduce population differentiation. We investigated two populations (peripheral and more central) of wild service tree (Sorbus torminalis (L.) Crantz), a self-incompatible, scattered tree species to test the differences in population differentiation and spatial genetic structure assessed at the self-incompatibility locus and neutral nuclear microsatellites. Although, both populations exhibited similar levels of genetic diversity regardless of the marker type, significant differentiation was noticed. Differences between FST and RST suggested that in the case of microsatellites both mutations and drift were responsible for the observed differentiation level, but in the case of the S-RNase locus drift played a major role. Microsatellites indicated a similar and significant level of spatial genetic structure in both populations; however, at the S-RNase locus significant spatial genetic structure was found only in the fragmented population located at the north-eastern species range limits. Differences in SGS between the populations detected at the self-incompatibility locus were attributed mainly to the differences in fragmentation and population history.