Genetic structure of western red cedar populations in the Interior West

Forty-one seedling populations of western red cedar (Thujaplicata Donn ex D. Don) were compared in five environmentally disparate common gardens. Separate tests of 25 wind-pollinated families from a single population were conducted on two of the sites. Genetic differences among populations and families were detected for variables related to growth and winter hardiness. Statistically significant multiple regression models with R2 values as high as 0.53 related genetic variation among populations to the elevation and geographic origin of the seed. The slope of the clines, however, was so gentle that populations must be separated by about 600 m in elevation or by 2° latitude before one can be reasonably certain of genetic differentiation. While genetic variation among families suggests that gains in the growth of western red cedar can be made through tree breeding, establishing evaluation trials for estimating genetic variances, breeding values, and responses to selection is problematic on sites in the interior.