Effects of fifty years of shelterwood harvesting on genetic diversity and population structure of Oriental beech (Fagus orientalis L.) in the relict Hyrcanian forest

Genetic diversity is essential for sustainability, adaptation and evolution of forest tree populations, especially under changed climate conditions. Maintenance of standing genetic diversity and effective population size (Ne) in relict forest species is of special interest to preserve their genetic heritage. Forest harvesting and management practices can affect genetic diversity and population structure by affecting demography and several evolutionary processes. We examined the effect of shelterwood harvesting on genetic diversity, population structure and Ne of Oriental beech (Fagus orientalis L.) populations in the relict Hyrcanian forest region by comparing pairwise unmanaged and manged populations at five locations using nuclear microsatellites. High levels of genetic diversity and significant levels of fixation index were observed, and the populations showed geographical genetic structure. There was a trend of reduced genetic diversity and Ne in the managed compared to unmanaged populations, but the differences were not significant. The unmanaged populations had almost twice the number of private alleles that of the managed populations. The unmanaged populations were genetically differentiated from their managed counterparts to varying degrees. The two unmanaged populations from Asalem and Sangedeh which had other human disturbance of livestock grazing or illegal harvesting were genetically the most differentiated from their managed counterparts as well as all other populations. FST estimates indicated similar levels of genetic differentiation among unmanaged and among managed populations. Overall, our study shows that shelterwood harvesting although does not significantly reduce genetic diversity and Ne and increase inbreeding levels in the extant managed populations, which are only one generation apart from the unmanaged populations, it does cause some genetic diversity and Ne erosion. Our study has implications and applications for sustainable management and conservation of genetic resources of Oriental beech inside and outside of the Hyrcanian forest as well as for other beech species.