The solar radiation environment of greenhouse-grown douglas-fir seedlings

Abstract

An increasing number of conifer seedlings destined for reforestation programs are propagated in styroblock containers located in greenhouses. This study investigates one important component of the microclimate of these seedlings: the spectral distribution of solar irradiance. The spectral composition of solar irradiance on a horizontal surface was measured outdoors, and above and below the seedling canopy in two greenhouses: one covered with four year old fiberglass, the other covered with new ultraviolet-stabilized polyethylene. The polyethylene greenhouse transmitted more solar radiation than did the fiberglass greenhouse throughout all wavelength intervals sampled here. The difference was greatest in the far ultraviolet and least in the infrared and was attributed mainly to the nature of the materials rather than to differences in age or cleanliness. Greater irradiance in the polyethylene greenhouse would favor sturdier, more robust seedlings. The pattern of spectral transmittance through the seedling canopy was similar to that most commonly reported for mature stands. The dense seedling canopy transmitted less than 0.2% in the photosynthetically active radiation (PAR) wavelength interval in late summer. Irradiance at the base of the canopy was insufficient to directly affect sporulation of Botrytis cinerea, a fungus that is becoming a major problem on container-grown Douglas-fir and western hemlock seedlings. The low irradiance, however, may contribute to needle senescence which provides a favorable host environment for the fungus.