GENETIC-STRUCTURE OF WESTERN RED CEDAR POPULATIONS IN THE INTERIOR WEST

Forty-one seedling populations of western red cedar (Thuja plicata Don n ex D. Don) were compared in five environmentally disparate common ga rdens. Separate tests of 25 wind-pollinated families from a single pop ulation 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 m odels with R2 values as high as 0:53 related genetic variation among p opulations to the elevation and geographic origin of the seed. The slo pe of the clines, however, was so gentle that populations must be sepa rated by about 600 m in elevation or by 2-degrees latitude before one can be reasonably certain of genetic differentiation. While genetic va riation among families suggests that gains in the growth of western re d cedar can be made through tree breeding, establishing evaluation tri als for estimating genetic variances, breeding values, and responses t o selection is problematic on sites in the interior.