Current issue: 54(2)
Populations at species’ range margins are expected to show lower genetic diversity than populations at the core of the range. Yet, long-lived, widespread tree species are expected to be resistant to genetic impoverishment, thus showing comparatively high genetic diversity within populations and low differentiation among populations. Here, we study the distribution of genetic variation in the pedunculate oak (Quercus robur L.) at its range margin in Finland at two hierarchical scales using 15 microsatellite loci. At a regional scale, we compared variation within versus among three oak populations. At a landscape scale, we examined genetic structuring within one of these populations, growing on an island of ca 5 km2. As expected, we found the majority of genetic variation in Q. robur to occur within populations. Nonetheless, differentiation among populations was markedly high (FST = 0.12) compared with values reported for populations of Q. robur closer to the core of its range. At the landscape level, some spatial and temporal sub-structuring was observed, likely explained by the history of land-use on the island. Overall, Q. robur fulfils the expectation of the central-marginal hypothesis of high differentiation among marginal populations, but the notable population differentiation has most likely been influenced also by the long, ongoing fragmentation of populations. Finnish oak populations may still be adjusting to the drastic habitat changes of the past centuries. Preservation of genetic variation within the remaining stands is thus an important factor in the conservation of Q. robur at its range margin.
Genetic variation in 5 natural stands of Quercus robur L. in Finland was analysed electrophoretically for 13 isozyme loci. Stands were on average polymorphic at 49.2% of the loci, with 2.1 alleles per locus. Observed heterozygosities, ranging from 13.6% to 16.9%, were slightly lower than estimates reported for German stands. The majority of the species’ genetic variation was found within each studied stand, and only 5.5% was between stands. Mean genetic differentiation (∂) was the same as that found in the primary range of the species, but the differentiation estimates (D) for single Finnish population were more variable.