Impacts of silviculture on genetic diversity in the native forest species Eucalyptus sieberi

Potential impacts of regeneration practices on genetic diversity in the Australian native forest species Eucalyptus sieberi L.A.S. Johnson. (silvertop ash) were assessed using DNA markers. Three different silvicultural treatments were examined: clear-felling with aerial re-sowing, and the seed tree system with site preparation by either burning or mechanical disturbance. In addition, two unharvested stands were chosen as controls. A total sample of 825 trees were genotyped at 35 Mendelian markers: 26 single-copy nuclear RFLPs and 9 microsatellites. No significant differences were found among the treatments in any of four population genetic statistics: allelic richness, effective number of alleles, expected heterozygosity and the panmictic index (f). Rare alleles were prevalent, and a Monte Carlo simulation showed that the apparent loss of four rare alleles from the sapling regenerants was highly statistically significant. There was no evidence for recent bottlenecks from analyses of either the levels of expected heterozygosity relative to that expected under mutation drift equilibrium, or the allele frequency profiles. A dendrogram of the relationships between the sampled populations suggested that the seed tree system may result in the promotion of genetic drift (slight expansion of the dendrogram) while aerial re-sowing of clear falls with the same seedlot will lead to genetic homogenisation (contraction of the dendrogram). The apparent genetic robustness of E. sieberi to native forest regeneration practices is attributed to its local abundance combined with the favourable properties of its reproductive biology, as well as to the limitation that only a single rotation was examined.