Deployment of improved black spruce (Picea mariana (Mill.) B.S.P.) seedlots in New Brunswick (NB) is a standard silvicultural practice. Most plantations have been established using elite-stand seedlots in the 1980s, first-generation seedling seed orchard seedlots in the 1990s and thereafter second-generation clonal seed orchard seedlots. A large-plot realized gain test was established at six sites in NB to provide estimates of actual gains from planting these improved seedlots. The test compared four improved seedlots, representing seedlots collecting from an elite stand, a first-generation seed orchard, two second-generation orchards and a mix of eight elite full-sib families identified in second-generation testing populations, with one unimproved checklot. This paper presents height measurements taken on trees at age 5. Results indicate that realized genetic gains from two-cycle selection and breeding are reasonably high and, in general, the more improved the seedlot, the higher the gain. Planting the elite-stand or the first-generation orchard seedlot would produce a gain of about 10% (relative to the checklot) in 5-yr height and an additional 5.8% gain could be obtained from planting the second-generation orchard seedlot. Even higher gain could be obtained via the deployment of the elite full-sib families, which was estimated to be about 8.6% more than the deployment of the second-generation orchard seedlot. While the realized gain varied with site, the ranking of gain achieved for the different seedlots was similar between sites. The above information was further used to optimize testing efforts for realized gain tests. To detect a typical gain of 5 to 10% at a significance level of 0.05 with a predetermined power of 0.80, each test should include 5 to 10 blocks per site (the number of sites is fixed at 4) or 4 to 6 sites (the number of blocks per site is fixed at 6) paired with planting 36 to 49 trees per plot. These approximate numbers of sites and blocks per site should be modified, depending on the interactions of seedlot with site and with block within site.
