Biologically Active Radiation in the Vicinity of a Single Tree

The horizontal photon flux density of photosynthetically active radiation (PAR) and flux density of ultraviolet A (UVA) and ultraviolet B (UVB) radiation were measured in the vicinity of isolated single trees during the summer of 1996. Measurements were made under shade and sunlit conditions along a transect aligned with the solar disk and the tree trunk. Flux density measurements were normalized by the flux density at a reference location away from the tree. Results showed (1) a more rapid decline in the flux density of UVA and UVB radiation than PAR with decreasing distance to the tree trunk on both the sunlit and shaded side of a tree and (2) more rapid changes in the flux density of UVB radiation UVA radiation, and PAR with distance from the tree on the sunlit side of the tree than the shaded side of the tree. The UVB/PAR ratio was found to increase in the shadow of a tree with increasing distance from the tree to between 4 and 6 for the conditions of the study. The potential for detrimental effects by UVB flux density under conditions of the high ratio may be mitigated by sunflecks at a given location over the course of a day.