Regulation of Douglas-fir Seedling Growth and Hardiness by Controlling Photoperiod

Abstract

Abstract Three experiments were conducted to determine how treatments with various photoperiods affect the growth, dormancy, and frost hardiness of Douglas-fir seedlings. In the first experiment, seedlings were exposed to an 8-h main photoperiod plus an 8-h period of supplemental light of varying intensities. Seedling growth was significantly greater than the control when the intensity of the supplemental light was 37 or more microwatts per square centimeter. In the second experiment, aimed at determining the effects of a wide variety of nighttime lighting regimes on seedling growth, a broad range of lighting treatments effectively extended both the magnitude and duration of seedling growth in the spring. The third experiment determined how various daylight periods and various intensities of light leakage affect the development of frost hardiness of seedlings in the late summer and early fall. Artificially shortening the daylight periods substantially increased the level of hardiness, but relatively low levels of light leakage greatly reduced this hardiness. These results indicate that a controlled photoperiod can be an effective tool for raising Douglas-fir seedlings in greenhouses in the Pacific Northwest when off-season planting or multiple cropping is desired. Forest Sci. 24:142-152.