Genetics of cold hardiness in a cloned full-sib family of coastal Douglas-fir

Variation in cold-hardiness traits, and their extent of genetic control and interrelationships, were investigated among individuals (clones) within a single large full-sib family of coastal Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) from Oregon. Cold injury to needle, stem, an d bud tissues was evaluated in fall 1996 and spring 1997 following artifici al freeze testing of detached shoots collected from 4-year-old ramets (root ed cuttings). Variation among clones in cold-injury scores was significant (p < 0.01) for all shoot tissues in both fall and spring and averaged about three times the magnitude previously observed among open-pollinated famili es of this species. Thus, improving cold hardiness by within-family selecti on appears to hold much promise. Striking similarities in relative magnitud es of heritability estimates and genetic correlations in the full-sib famil y, compared with breeding populations, support the following hypotheses abo ut the quantitative genetics of cold hardiness in this species: (i) heritab ility of cold hardiness (both broad-and-narrow-sense) is stronger in the sp ring than in the fall; (ii) cold hardiness of different shoot tissues in th e same season is controlled by many of the same genes; and (iii) genetic co ntrol of fall cold hardiness is largely independent of cold hardiness in th e spring.