A Survey of Ultraviolet-B Radiation in Forests

Abstract

1 Ultraviolet-B radiation (UV-B, 280-320 nm) was surveyed in a mixed deciduous forest in Maryland, USA, using a Robertson-Berger meter. A smaller number of comparable measurements were made in closed-canopy forests in Chile, Panama, and Washington State, USA, and under two canopies recently disturbed by hurricanes, in Virginia, USA, and Mexico. Simultaneous measurements of photosynthetically active radiation (PAR, 400-700 nm) were also made. 2 UV-B near the forest floor was generally low compared to UV-B incident on the outer canopy, and had a positively skewed frequency distribution. Under closed canopies, geometric mean UV-B transmittance was only 1-2% of incident radiation; under disturbed canopies, 8-17%. In Maryland, geometric mean UV-B transmittance increased to 30% during the leafless season. Gaps received larger UV-B exposures over time than shaded understorey locations. Thus UV-B transmittance depends strongly on canopy structure. UV-B transmittance did not, however, have a detectable dependence on solar elevation. The vertical extinction of UV-B through a closed canopy was rapid: in Maryland, about 40-70% of incident UV-B was absorbed by the top 25% of the canopy. 3 The spatial and temporal variation in UV-B and PAR within the canopy were qualitatively similar. However, UV-B varied less dramatically than PAR, and the two wavebands also had different patterns of variation in canopy space. These differences were probably due to the greater diffuse component of incident UV-B, and led to wide fluctuations of UV-B:PAR ratios. The UV-B:PAR ratio was low in sunflecks and high in some partially shaded areas such as gap edges. In Panama and the two disturbed sites, UV-B transmittance was significantly greater than PAR transmittance; in the other sites no significant differences were found. 4 These results were combined with a published model of the atmospheric transmission of UV-B to estimate present and future UV-B exposures in the Maryland forest, assuming a decline in stratospheric ozone. The greatest increases in UV-B exposure should come in the summertime in the upper canopy, and in the spring in the lower canopy.