Genetics of fall and winter cold hardiness of coastal Douglas-fir in Oregon

Genetic variation in fall cold hardiness was studied in two western Oregon breeding populations df coastal Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) France), one on the west slope of the Cascade Mountains and the other in the Coast Range. On six sampling dates (September, October, and November of 1992 and January, September, and October of 1993), shoot cuttings from 40 open-pollinated families in each of two progeny test sites for each breeding zone were subjected to artificial freezing at two test temperatures. Damage on each shoot was recorded as visible injury to needle, stem, and bud tissues separately. Considerable family variation was found for cold injury scores in all tissues in early fall to midfall, but differences were often smaller or nonsignificant in late fall and midwinter. Individual heritability estimates for needle cold injury were low (<0.40) and generally decreased in late fall and midwinter. Family rankings for fall cold hardiness, however, are expected to be relatively consistent over sites and years, although needles appear to display more family-by-site interaction than stems or buds. Genetic correlations between tissues in cold injury varied considerably and were sometimes weak, indicating that the evaluation of a single tissue is probably not adequate for assessing overall cold hardiness of genotypes. Fall and winter cold hardiness seem to be largely under separate genetic control since genetic correlations between hardiness at these two stages were weak. This study confirms earlier results in Washington breeding populations and shows that coastal Douglas-fir families can be effectively ranked for fall cold hardiness by conducting artificial freeze tests on cut shoots in midfall (October) and scoring damage to stems and at least one other tissue.