Microclimate factors influencing the frequency and duration of growth season frost for subalpine plants

The subalpine environment of the Medicine Bow Mountains, Wyoming (USA) is characterized by frost events throughout the summer. As a result, growing season length is difficult to define in terms of frost-free days. For example, such a definition would have limited the 1993 growing season for many plants to only 5 days. The frequency of summertime frost events appeared particularly dependent on net losses of infrared energy to a cold night sky (radiation frost). These radiative losses to the night sky were strongly dependent on microsite sky exposure and leaf characteristics as well as meteorological conditions. During the summer of 1993, air temperatures at a height of 8 cm at the center of a subalpine meadow were below 0°C on 26 of 67 nights, while leaves of a broadleaf species ( Erigeron peregrinus) frosted on 41 nights compared to 25 nights for a coniferous sapling ( Abies lasiocarpa). In E. peregrinus, a strong association occurred between the frequency and duration of freezing episodes and the amount of sky exposure, with plants sheltered by forest canopy experiencing frost only 10% to 40% as often as those in the center of the meadow. At exposed locations, longwave radiation minima from the night sky were strongly correlated with the occurrence of leaf temperature minima. Thus, microsite sky exposure as well as the variation in longwave radiation from the atmosphere may have important implications for estimating growing season length and plant distribution patterns at higher elevations. Growth habit also influenced the occurrence of frost because plant height spanned a substantial vertical gradient in air temperature above the soil surface. This was particularly true on nights with low sky infrared radiation and low wind. On calm, clear nights, the coldest leaf temperatures ( T leaf ≈ −11°C with T air ≈ −6°C) occurred because of the combined effects of low sky infrared radiation and low air temperature at plant heights. However, even on relatively windy nights with air temperature above freezing, net radiative losses to clear skies resulted in leaf frost. Flower temperatures of both E. peregrinus and Polygonum bistortoides tended to be significantly warmer (≈ 2°C) than leaf temperatures due to their greater height above the ground. Thus, leaf size, plant height, microsite exposure, and daily variation in sky infrared radiation all have important influences on the determination of frost frequency, duration, and intensity, as well as the length of the g