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Solar Ultraviolet Radiation in Terrestrial Plant Communities¹

ABSTRACT

There has been a growing concern that NO_x effluents from supersonic or other highflying craft, or chlorofluoromethane refrigerants or aerosol can propellents that diffuse to the stratosphere, could cause a reduction of atmospheric ozone, which would result in a concomitant increase of penetration of solar ultraviolet radiation to the earth's surface with possible biological consequences. Spectral distributions of direct-beam and diffuse solar ultraviolet irradiance at the earth's surface, as a function of stratospheric ozone content and solar elevation angle, have been accurately measured or predicted by other researchers. Our objectives were to couple incident spectra to a plant canopy radiation penetration model to give the redistribution of middle ultraviolet radiation, or UV-B (280–315 nm), within plant canopies. Detailed comparisons were made between two ozone content conditions (0.32 cm, typical for 30° N latitude and 0.24 cm, representing a 25% ozone reduction) with a 60° solar elevation angle.

Predictions of UV-B radiation regimes in plant communities over ranges of architectural or structural characteristics, including erect-leaf, normal-leaf, and horizontal-leaf canopies, were computed. Clumped, random, and regular leaf distributions were modeled, as well as leaf area indices of 2.6, 3.3, and 4.0. Phyloelement optical properties included zero transmissivity and 5% reflectivity. Soil reflectivities of both 5% and 20% were used. Epidermal transmission spectra were used to predict UV-B radiation loads inside leaves.

Predicted penetration of UV-B radiation was much greater in erect-leaf than horizontal-leaf canopies. Upward-directed UV-B irradiance was greater near the ground level than near the top of the canopy.

In conclusion, the model prediction described the range of UV-B radiation regimes to be expected in plant communities under present stratospheric ozone content and under a 25% ozone reduction. Data from these idealized plant communities can be interpolated for other plant canopy types and soil types to predict upward or downward UV-B radiation loads and dosages.

Key Words: ozone • plant canopy • ultraviolet transmittance • ultraviolet reflectance • epidermis • radiation penetration model

¹ Contribution from Agricultural Research Service, USDA, Ithaca, New York and Weslaco, Texas, and the Cornell University Agric. Exp. Sta. Department of Agronomy Series Paper No. 1033. Based on a contribution (without financial support) to Monograph V, Climatic Impact Assessment Program (CIAP), U. S. Department of Transportation.

² Soil Scientist, USDA, ARS, Gainesville, Fla., and Assistant Professor, Agronomy Dep., Univ. of Florida (formerly Soil Scientist, USDA, Ithaca, N. Y., and Assistant Professor of Micrometeorology, Agronomy Dep., Cornell University); Plant Physiologist, and Physicist, USDA, ARS, Weslaco, Tex.

Received for publication September 3, 1974.

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