Genetic gain and diversity caused by genetic thinning in a clonal seed orchard of Pinus densiflora

Estimates of genetic gain (in volume growth) and diversity (expressed as status number, N-s) were determined after the application of genetic thinning in a Pinus densiflora clonal seed orchard. The genetic thinning was based on: 1) clonal breeding values (represented by general combining ability, GCA) obtained from progeny tests, 2) clonal fertility estimated by strobilus production, and 3) clonal size variation determined by the ramet numbers per clone. Genetic gain and diversity estimates were determined under assumptions of 30% pollen contamination and inferior genetic value of contaminating pollen. Thinning 45% of seed orchard (from 67.5 to 41.9 ramets/clone) raised genetic gain to 6.3% in volume growth and reduced the status number to 28.1% of the census number. Further volume growth gain of 11.6% and diversity reduction (N-s = 29.2%) were attained after 70% thinning (from 41.9 to 24.8 ramets/clone). The orchard clones were grouped into 10 GCA groups to allow for the linear deployment of clones (i.e., clones were deployed in proportions that reflect their gain estimated), which was implemented for the six top groups. Some ramets from lower groups were intentionally left or removed to avoid the creation of wide gaps or clumps. The effect of pollen contamination on the genetic gain and the consequence of genetic thinning for seed production in the clonal seed orchard were also discussed.