The effect of tending and commercial thinning on the genetic diversity of Scots pine stands

The objective of this study was to estimate the effect of pre-commercial tending and commercial thinning on the genetic diversity parameters, especially rare allele loss and diameter-heterozygosity associations, in Scots pine stands by retrospectively modelling the removal of inferior/superior trees. Modelling was based on empirical data of DNA polymorphism in (a) a 60-year-old natural stand and (b) a 20-year-old young stand planted with seeds collected in a seed orchard. Within each of these stands, approximately 400 trees were systematically sampled within 1-ha plots (800 trees in total) and genotyped at 5 neutral and 7 EST-derived nuclear microsatellite markers. There was no significant association between heterozygosity, common allele number and tree diameter in either stand. Even at a high intensity, both simulated tending and commercial thinning had minor effects on the heterozygosity and allelic diversity but caused a significant loss of rare alleles. However, there was a nonlinear relationship between the loss of rare alleles and the thinning intensity in the young stand, such that below 30 % thinning intensity, the rare alleles were lost at a markedly lower rate. In conclusion, the association between commercial value and genetic diversity is weak in Scots pine. Thinning causes loss of rare alleles; however, for specific cases, it could be possible to identify the margins for a slow rate of rare allele loss.