Effects of harvesting on genetic diversity in old growth eastern white pine in Ontario, Canada

Genetic diversity measures at 54 isozyme loci coding for 16 enzymes in megagametophytes were compared between preharvest and postharvest gene pools of two adjacent virgin, old-growth (similar to 250 years) stands of eastern white pine (Pinus strobus L.) in the Galloway Lake Old Pine Area of central Ontario. The concurrence of genetic diversity changes between the stands suggests that real and repeatable genetic erosion occurred in these gene pools as a result of harvesting. The total and mean number of alleles detected in each stand were reduced by approximately 25% after tree density reduction of 75%. The percentage of polymorphic loci dropped by about 33% from preharvest levels. About 40% of the low frequency (0.25 > p greater than or equal to 0.01) alleles and 80% of the rare (p < 0.01) alleles were lost from each stand because of harvesting. Hypothetical multilocus gametic diversity was reduced by about 40% in each stand after harvesting. Latent genetic potential of each stand was reduced by about 50%, suggesting that the ability of these gene pools to adapt to changing environmental conditions may have been compromised. Heterozygosity estimates in the postharvest stands did not reflect reductions in allelic richness due to harvesting. Observed heterozygosity increased by 12% in one stand after harvesting, even though other genetic diversity measures decreased. Gene frequency changes due to harvesting imply that gene pools of naturally regenerated progeny stands may be quite different from the original parental stands. Silvicultural practices should ensure that the gene pools of remaining pristine old-growth stands have been reconstituted in the regenerating stands.