Spring and fall cold hardiness in wild and selected seed sources of coastal Douglas-fir

Breeding for increased growth in coastal Douglas-fir (Pseudotsuga menziesii (MIRB.) FRANCO) could affect the level of cold hardiness of seedlings used for reforestation. If increased growth is achieved by initiating growth earlier in the spring or prolonging growth later into the fall, cold hardiness could be reduced during these seasons. Cold hardiness was measured in top-cross and first generation seed orchard trees selected for increased growth rates, and wild stand trees throughout one growing season by visual assessment of artificial freeze tests. Significant differences in freezing damage between genetically selected and wild stand trees were found during both the spring and fall. In April, LT,, of top-cross trees was 0.7 degrees C to 2.4 degrees C below that of wild stand trees, while in October, LT50 of wild stand trees was 1.9 degrees C to 3.4 degrees C below that of top-cross trees. Mitotic index was investigated as an indicator of dormancy, and a negative correlation between mitotic index and cold hardiness was found. A significant difference in mitotic index between the genetic groups was found once in March when mitotic index in wild stand and seed orchard trees was 1.4% and mitotic index in top-cross trees was 0.9%. There were no significant differences in mitotic index at any other times during the year. Date of bud burst and rates of shoot extension were related to levels of cold hardiness in the three groups of seedlings. The stage of bud burst in May was significantly correlated with levels of hardiness found earlier in March and April. Trees that completed shoot extension earlier in the season were significantly more hardy in the fall. Top-cross trees may extend their growing season later into the fall, thereby gaining a height advantage over wild stand seedlings. These top-cross families do not have an increased mid to late fall frost damage risk, and in fact may have reduced risk of critical spring frost damage due to delayed deacclimation.