When does habitat fragmentation lead to changes in populations gene pool of pedunculate oak (Quercus robur L.)?

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Development of agriculture in the forest and forest-steppe zones of the vast East European plain over the past millennium has resulted to a significant decrease of broad-leaved forests habitat. It is expected that the disintegration of the forest tree species distribution ranges will lead to the erosion of the population genetic diversity of due to limited gene flow, impacts on gene pools of genetic drift and inbreeding. To investigate this threat, we analyzed the genetic diversity and differentiation among 20 populations of pedunculate oak (Quercus robur L.) from the Ural Mountains (Russia) and neighboring lowlands and foothills. A set of 412 nuclear SNPs loci was used for our study. A positive correlation (r = 0.168, p < 0.05) was observed between genetic and geographical distances. The most genetic variation was distributed within populations (fixation index FST = 0.054). It was found that the values of genetic diversity are similar in large continuous mountain stands (effective number of alleles ne = 1.340, observed heterozygosity HO = 0.213, expected heterozygosity HE = 0.200), in spatially highly fragmented forests of the Cis-Urals (ne = 1.340, HO = 0.210, HE = 0.200) and geographically isolated marginal habitats with populations of hundreds individuals (ne = 1.338, HO = 0.206, HE = 0.198). Only in populations with few dozen individuals, we revealed a statistically significant decrease in the genetic diversity (ne = 1.312, HO = 0.211, HE = 0.184) and increase of numbers of full sibs and level of kinship (the kinship-coefficient Kin = 0.078–0.128 vs Kin = -0.002–0.035 in other populations). Overall, our study demonstrates that the current gene pool of Q. robur populations in the study area remains stable under conditions of significant forest decline and habitat fragmentation. The historically short period of these processes in the region and the genetically successful gene flow between populations over long distances might be reasons for this phenomenon.