Influence of second flushing on genetic assessment of cold hardiness in coastal Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco)

The consequences of second flushing for fall cold hardiness in coastal Doug las-fir (Pseudotsuga menziesii var. menziesii (Mirb.) France) was investiga ted in 4-year-old trees from two genetic tests: 42 polycross families in Br itish Columbia and 8 full-sib families in the state of Washington. Cold inj ury to needle and stem tissues was assessed in October and November in samp les of both second-flushed and non-second-flushed shoots from the same tree s following artificial freezing. Freeze damage to needles and stems in the second-flushed portion of shoots was 50-60% greater than in non-second-flus hed shoots. Great care, therefore, should be taken to consistently sample t he same shoot type when comparing cold hardiness of genotypes (or families) using artificial freeze testing. However, because the estimated genetic co rrelations in freeze injury between second-flushed and non-second-flushed s hoots were moderately positive, scoring all trees for hardiness of one shoo t type should provide fairly accurate rankings of genotypes for cold hardin ess of both shoot types. We recommend scoring non-second-flushed shoots bec ause the frequency of second flushing decreases relatively rapidly with inc reasing age in coastal Douglas-fir. Hardening of both second-flushed and no n-second-flushed shoots was delayed in trees with higher proportions of sec ond-flushed shoots in their crown. Thus, foresters should avoid planting fa milies with high propensity to second flush on high fertility sites (i.e. s ites that promote second flushing) susceptible to fall frost events. (C) 19 98 Elsevier Science B.V.