Genetic selection for cold hardiness in coastal Douglas-fir seedlings and saplings

Genetic control of cold hardiness in two-year-old seedlings was compared wi th that in 7-year-old saplings of 40 open-pollinated families in each of tw o breeding populations (Coast and Cascade) of coastal Douglas-fir (Pseudots uga menziesii var. menziesii (Mirb.) Franco) from western Oregon. In additi on, the efficacy of bud phenology traits as predictors of cold hardiness at the two stages was explored. Fall and spring cold hardiness were assessed using artificial freeze testing. Similar genetic control of cold hardiness in seedlings and saplings is suggested by strong type-B genetic correlation s (r(B)) between the two ages for fall and spring cold injury traits (r(B) greater than or equal to 0.78) and by similar trends in individual tree her itability estimates (h(i)(2)), e.g., h(i)(2) was greater in spring ((h) ove r bar (2)(i) = 0.73) than in fall ((h) over bar (2)(i) 0.36) and greater in the Coast population ((h) over bar (2)(i) = 0.69) than in the Cascade popu lation ((h) over bar (2)(i) = 0.40) at both ages. Strong responses to direc t selection are expected for spring cold hardiness at both ages and for fal l cold hardiness in seedlings, even under mild selection intensities. Simil ar heritabilities in seedlings and saplings, and strong genetic correlation s between ages for cold-hardiness traits, ensure that selection at one age will produce similar gains at the other age. Type-A genetic correlations (r (A)) between fall and spring cold hardiness were near zero in the Cascade p opulation (r(A) = 0.08 and -0.14 at ages 2 and 7, respectively) but were mo derate and negative in the Coast population (r(A) = -0.54 and -0.36, respec tively). Bud-burst timing appears to be a suitable surrogate to artificial freeze testing for assessing spring cold hardiness in both seedlings and sa plings, as is bud set timing for assessing fall cold hardiness in seedlings , but bud set timing is a poor predictor of fall cold hardiness in saplings .