DESIGN OF GREENHOUSES FOR THE MANIPULATION OF TEMPERATURE IN TUNDRA PLANT-COMMUNITIES

Passive greenhouses can be used to elevate the temperature of natural communities, but they also introduce other effects. We tested the effects of potential greenhouse materials-clear polyethylene plastic film, polyester fabric, and rigid fiberglass panels-on light transmission, photosynthesis of Salix planifolia, elevation of air and soil temperature, and thaw depth. Plastic had the greatest light transmittance and caused the least depression of photosynthesis (- 5%). Greenhouses covered with plastic elevated daily maximum and daily mean air temperatures by an average of 7.8 and 2.0-degrees-C and depressed daily minimum temperature by 1.1-degrees-C compared with the control. Plastic is impervious to gases and may alter CO2 concentration and humidity within greenhouses. Fiberglass had lower transmittance, especially of short wavelength radiation. Fabric had the lowest light transmission and reduced photosynthesis by 10%, but it has the advantage of permeability to CO2 and water vapor. Greenhouses covered with fabric, alone, produced only a small effect (daily mean temperature elevated 0.4-degrees-C above controls). A mixed greenhouse design (plastic and fabric) raised daily mean temperatures by 0.9-degrees-C and may minimize adverse effects on gas diffusion. Because of the effect of the materials on amount and spectral distribution of radiation and on photosynthesis, the appropriate treatment control for any greenhouse design is an open plot shaded with the same material. Soil temperature at 10 cm depth was elevated in all greenhouses, but no effect on depth of thaw was detected.