RADIATIVE AND AERODYNAMIC EFFECTS OF AN OPEN PINE SHELTERWOOD ON CALM, CLEAR NIGHTS

Net radiation represents a disequilibrium between incoming and outgoing radiative fluxes at a surface. This disequilibrium is caused by the non-radiative, i.e. the convective, conductive and chemical (mainly photosynthetic and respiratory) heat fluxes. Contrary to the prevailing common sense regarding net radiation in shelterwoods, a more negative (negative in this context refers to any energy flux directed away from, i.e. cooling, the surface) net radiation was observed beneath the canopy of an open pine shelterwood than in a nearby clearcut area. These observations were made during comparative net radiation surveys at Vindeln, Sweden, using 22 net radiometers at each site. In the evening the net radiation was initially more negative in the clearcut, but later became more negative in the shelterwood by about 10 W m-2. This was a consistent pattern on all calm, clear nights during the survey. On windy nights the net radiation was slightly more negative in the clearcut than in the shelterwood. Since non-zero net radiation can only be caused by the non-radiative fluxes, and the nocturnal chemical heat fluxes are small, this difference is caused either by the ground heat flux or the convective fluxes or both. The temporal variations in measured ground temperatures and net radiation suggest that convective rather than conductive heat fluxes are responsible for the net radiation difference we observed on calm nights. On such nights, light winds interacting with the upper parts of the tree crowns induce slight movements of the air in the near-surface layers of the shelterwood. In the clearcut there are no such aerodynamic effects. Thus, in calm conditions there is, in addition to the radiative shelterwood effect, also an aerodynamic effect of the open shelterwood.