Genetic and environmental components of variation of site index in inland Douglas-fir

Journal Article

Development

Results from two disparate and independent studies of inland Douglas-fir (Pseudotsuga menziesii [Mirb] Franco var. glauca ) were combined to answer the following questions: First, how much variation in site index is associated with genetic variation? Second, what is the relative importance of the genetic and environmental components of phenotypic variation in explaining differences in mean height among stands of mature trees? Third, what is the magnitude of the interaction? An index to genetic variation, based on 3-year seedling height in provenance tests, accounted for approximately 40% of the variation in both 50-year and 100 year dominant height (i.e., site index) among 135 natural stands in northern Idaho and western Montana. Combining the genetic index and simple environmental variables (elevation, habitat series, latitude, longitude) accounted for nearly half the variation in site index. Path analyses estimated a strong correlation (0.76) between genotype and environment, reflecting the steep adaptive clines that are well known for this species. The analysis also indicated that the genotype is about a third more important than the contemporary environment in determining phenotypic variation in dominant height of natural stands. The results suggest that a combined genetic/mensuration model has potential to improve both tree breeding efforts and stand management.