Physiology and Growth of Containerized Coastal Douglas Fir Seedlings Given Different Durations of Short Days to Induce Dormancy

We compared the effects of different durations of short days (SDs) as a dormancy-induction regime on bud development, bud endodormancy, and morphology of first-year containerized coastal douglas fir [ Pseudotsuga menziesii (Mirb.) Franco var. menziesii ] seedlings in the nursery together with seedling survival and growth after one growing season in a common garden. In early July, four durations of 8-h SDs were applied: 3, 4, 5, and 6 weeks. During the first week of SDs, budscale initiation started and was completed; then initiation of needles for next year's leading shoot (leader) began. Needle initiation was completed 10 weeks after the start of the regime in seedlings given 5 or 6 weeks of SDs and 13 weeks for those given 3 or 4 weeks of SDs. In early October, duration of SDs had no effect on bud endodormancy; 50% to 88% of terminal buds were endodormant. On this date, seedling height and shoot dry weight were unaffected by duration of SDs, whereas root dry weight and shoot diameter were significantly reduced in seedlings given 6 weeks of SDs compared with other durations. After one growing season, duration of SDs had no effect on seedling survival, leader length, shoot dry weight, root dry weight, or shoot diameter. We recommend the 3-week duration of SDs for coastal douglas fir crops.