Seasonal differences in clear-sky nighttime forage temperature in proximity to deciduous trees
Considerable research has been done on daytime forage shading by silvopasture trees since solar radiation is required for photosynthesis. However, trees also impact nighttime temperature on clear nights when trees also effectively shade forages from cold skies. Appalachia has a temperate climate and deciduous-tree silvopasture nighttime temperature patterns will differ between spring before tree leaves emerge and summer when trees are in full canopy. Longwave radiation sensors, which simulate forage (surface) temperature, were installed in triplicate at four locations differing in obstruction to open sky by trees. An open pasture site (O) had 90 [percent] exposure to open sky during summer. A site within an adjacent closed-canopy second growth mixed hardwood forest (F), with only small canopy gaps, had 10 [percent] exposure to open sky. Two intermediate sites within a 15 X 50m gap cleared into the forest had 20 and 40 [percent] exposure to open sky respectively (G2 and G4). Air temperature was recorded at 2m at all sites. Temperatures were measured every 10s and hourly averages recorded from 1-Feb.-2008 through 10-July-2008. Summer differences between surface and air temperature in response to radiation cooling during clear night hours were -6.7, -2.9, -1.5, and -0.7oC for sites O, G4, G2, and F respectively. Respective late winter and early spring temperature differences were -6.3, -6.4,-5.7 and -4.3oC. There was a threefold difference between longwave radiation forage cooling at O compared to F in summer compared to early spring. Also, summer surface-air temperature differentials increased linearly as sky exposure increased. During tree leafless periods, temperature differentials were constant down to 40 [percent] sky exposure but decreased exponential approaching 10 [percent] sky exposure. During periods with full cloud cover there were no site temperature differences between air and surface during either season. These results suggest different forage management strategies in response to different forage nighttime temperatures may be warranted.