ENERGY-BALANCE ANALYSIS OF NIGHTTIME LEAF TEMPERATURES AND FROST FORMATION IN A SUB-ALPINE ENVIRONMENT

Nocturnal leaf temperatures depend primarily on convective heat exchange with the air and longwave radiation exchange with the surroundings. Consequently, the leaf boundary layer and upper hemisphere infrared radiation dominate the energy balance of a leaf. Longwave radiation from the night sky may be particularly low in the subalpine environment because of the decreased atmospheric density, as well as the low moisture content of the atmosphere. The sky infrared radiation input during the growing season was often as low as 230 W m-2 in the subalpine, equating to a blackbody equivalent sky temperature of less than -20-degrees-C. Leaf temperatures predicted from energy balance equations were compared with field measurements of leaf temperatures for Erigeron peregrinus, a perennial herb common to subalpine meadows with a characteristic leaf dimension of approximately 2 cm. Differences between predicted and measured temperatures generated a root-mean-square error of 0.9-degrees-C, while the root-mean-square deviation between the air and leaf temperature observations was 3.9-degrees-C (n = 461). Leaf temperatures were modelled in simulations addressing the effects of ambient air temperature, wind speed, sky infrared radiation, and sky exposure characteristic of this high elevation environment. Leaf temperature responses to air temperature and sky infrared radiation were linear, while the sensitivity of the leaf temperature response to wind speed increased substantially under calm conditions (less than 0.4 m s-1). Low wind in combination with clear, cold nights generated simulated leaf temperature depressions of over 6-degrees-C below air temperature, frequently resulting in frost episodes during the entire growing season.