Photo by Howard Dew
Brad St. Clair in seed orchard explaining
results of the height growth study.
The Willamette Valley Ponderosa Pine Conservation Association established a seed orchard at the Oregon Department of Forestry's Schroeder Seed Orchard as part of their objective to provide an assured supply of ponderosa pine seed for the Willamette Valley. The seed orchard was planted in 1996 with one-year-old planting stock using a replicated block design with twelve replications and six trees per family in a row within each replication. This design made it possible to evaluate differences among families. Six of the twelve replications were measured in December, 1998, for height and forking. The objectives were: (1) to evaluate the likelihood of achieving genetic gains from selection either within the orchard or as part of a tree improvement program, and (2) to look for evidence for geographic genetic variation that might indicate genetic differentiation for adaptation to different environments within the Valley.
The seed orchard was very good at distinguishing family differences in heights at four years of age. Families differed significantly, and estimates of heritability were particularly high (h2=0.58). (Heritability is a measure of how much of the total phenotypic variation is due to genetic variation, and, thus, is a measure of the degree to which a trait can be transmitted from parents to offspring.) The tallest family was 1.37 m on average and the shortest family was 0.99 m on average. Given these results, we might expect that selection in a tree improvement program could lead to considerable genetic gains in growth (dependent upon the proportion of parents selected). Gains may also be expected from selection of trees within the seed orchard. Plans for the seed orchard include removing five of the six trees in a family row plot, leaving the best individual of a family. Based on results from this analysis, we can expect that the progeny from the rogued seed orchard will be 11% taller at four years of age compared to an unrogued orchard. If we were to also remove some of the poorer families, we could expect some additional gain (3% additional gain from removing the poorest 25% of families). The difference between seedlings from the rogued seed orchard and seedlings from parents in natural stands may be expected to be even larger given the findings from Geoff Gooding's research that many of the stands are inbred. Mating in the orchard among unrelated parents originating from different stands will likely result in breaking up any inbreeding depression.
Families in the seed orchard did not differ in the amount of forking, indicating that forking was not heritable. Fifteen percent of the trees in the seed orchard were forked at four years of age. It is unclear what caused the forking or whether the forks will persist.
No evidence was found to indicate geographic differentiation within the Valley. Taller families came from both the south and the north. The lack of evidence for geographic differences in height and forking does not preclude other traits from showing differences; however, climatic differences within the range of ponderosa pine in the Valley are small, making it less likely that adaptive differences would be large.