Quantitative genetics of spring and fall cold hardiness in seedlings from two Oregon populations of coastal Douglas-fir

Abstract

Abstract Genetics of fall and spring cold hardiness were investigated in two western Oregon breeding populations (Coast and Cascade mountains) of Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco). Seedlings from 40 open-pollinated families from each population were grown in raised nursery beds and subjected to two soil-moisture regimes (well-watered and mild drought) to evaluate the influence of summer drought on ranking of families for cold hardiness. Artificial freeze testing (AFT) of detached shoots, followed by visual scoring of injury, was used to evaluate needle, stem and bud cold hardiness on three dates in the fall (September, October and November) after the second growing season, and once in the following spring (March). The Cascade population suffered significantly less cold injury than the Coast population in fall AFT. However, in spring AFT the Cascade population was less cold hardy, although population differences were seldom significant. Families within both breeding zones varied significantly in cold hardiness, with mean estimates of individual heritabilities greater in spring ( h i 2 =0.57) than fall ( h i 2 =0.37) , greater in the Coast ( h i 2 =0.52) than in the Cascade ( h i 2 =0.42) population, and greater in the wet ( h i 2 =0.54) than in the dry moisture regime ( h i 2 =0.40) (fall means based on October tests). A single test date seems adequate to assess fall cold hardiness, because estimated genetic correlations for cold injury between fall test dates were strong ( r A =0.80) . Genetic correlations between spring and fall cold injury, however, were moderately negative ( r B =−0.66 and −0.21, Coast and Cascade, respectively), indicating that cold hardiness needs to be managed as two traits (i.e. fall and spring cold hardiness). Selection for cold hardiness based on a single shoot tissue is expected to increase cold hardiness in the other tissues as well, because genetic correlations between tissues in cold injury were moderately-to-strongly positive in both fall ( r B =0.67) and spring ( r B =0.84) . Seedlings grown under summer drought incurred significantly less cold injury in the fall than those that were well-watered; nevertheless, strong genetic correlations in fall cold injury between moisture regimes ( r B =0.91) indicate that summer moisture conditions had little influence on family rankings for fall cold hardiness. Correlations of injury resulting from a natural frost event in November of the first year with injury from AFT in the fall of the second year ( r A =0.72 and 0.78 for needle and bud injury, respectively) confirmed that AFT reliably predicts cold hardiness to natural frost events.