Radiation frost susceptibility and the association between sky exposure and leaf size

Plants growing in exposed and sheltered habitats have characteristic leaf structure and physiology that are traditionally associated with the total amount of incident sunlight. However, greater sky exposure also increases the susceptibility of leaves to radiation frost. Plants with large horizontal broadleaves are particularly susceptible to both overheating during the day and freezing at night. Moreover, the combined effects of high daytime sun-exposure and nighttime frost susceptibility could be particularly stressful to plant tissues. The purpose of this study was to evaluate the influence of elevation and microsite exposure (i.e. net loss of longwave radiation) on frost susceptibility, as well as the corresponding intraspecific variation in leaf size in the subalpine daisy (Erigeron peregrinus). Measured decreases in upper hemisphere infrared radiation (sky IR) of 0.014 W m^-2 m^-1 occurred with increasing elevation, beyond decreases predicted due to changes in air temperature and water content, resulting in an average decrease of 0.029 W m^-2 m^-1. Previous equations of sky IR based on air temperature and humidity were improved by adding this elevational term (r ² improved from 0.52 to 0.71). In contrast, a mean decrease of 6.5 W m^-2 m^-1 occurred with increasing sky exposure across a subalpine meadow. Leaf size in Taraxacum officinale decreased linearly with increasing elevation and a corresponding decline in sky IR. No difference in daily solar irradiance was measured across the same elevational gradient. Also, E. peregrinus had smaller leaves at high elevation microsites with greater sky exposure and decreased sky IR, while there was a much weaker association between leaf size and the amount of total daily solar irradiance. Differences in plant leaf structure and physiology traditionally associated with daytime sun-exposure may also be influenced by nighttime sky exposure and the susceptibility to radiation frosts.