Before parent trees are included in seed orchards, their offspring are tested in progeny tests over several sites in the zones where the seed orchard seed is to be deployed. Parent trees are selected based on the performance (height growth, survival, stem and/or tree form, and other factors such as pest or disease resistance) of their offspring across all test sites. Within these progeny tests, many parent trees with very different growth potentials are grown together; however, only the best few will be selected at a test age of 10-15 years. Due to this type of test design, it is possible that fast-growing seedlings are surrounded by slowergrowing seedlings, potentially leading to height differences that are exaggerated due to competitive effects. The seedlings that will grow fastest will do so partially at the cost of their neighbours. As a result, the genetic superiority of the test seedlings, and consequently their parents, may be overestimated and may not be realized when seedlings from orchard parents are grown together in an operational plantation.
At the time of selection, a measure of each parentís genetic quality is estimated based on the differences in overall performance of their offspring (progeny) across all test sites relative to the performance of a test control seedlot obtained from wild stands. This estimate of a parent treeís genetic quality is termed its breeding value and reflects the expected growth increment in volume at rotation age 60.
Realized genetic gain tests are established to verify the estimated genetic quality of parent trees (parental breeding values) based on progeny tests by growing seedlings from parent trees of similar genetic quality together in large blocks (as in an operational plantation planted with orchard seed). The select seedlots are then compared to the growth of blocks established with seedlings grown from seed collected in naturally regenerated stands (Woods et al. 1995; Dhakal et al. 1996; St. Clair et al. 2004; Ye et al. 2010).
In this extension note, we describe the results of a realized genetic gain test planted on six low-elevation sites in British Columbia using coastal Douglas-fir of three distinct genetic classes. To evaluate possible effects of spacing and early onset of competition, the tests were established at four spacings, two of which are generally not used in operational forestry.
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Updated March 08, 2011