RADIATIVE PROPERTIES OF HARDWOOD LEAVES TO ULTRAVIOLET-IRRADIATION

Spectral reflectance and transmittance of leaves to ultraviolet irradi ation were determined under laboratory conditions for seven species of hardwood trees, namely red oak (Quercus rubra, L), black oak (Q. velu tina, Lamarch), white oak (Q. alba, L.), sugar maple (Acer saccharum), Norway maple (A. plantanoides), hickory (Carya tomemtosa), sweetgum ( Liquidambar styraciflua), and black oak litter. The experimental syste m consisted of a solar simulator, an integrating sphere, and a spectro radiometer. Experiments were repeated three to five times for both ada xial and abaxial surfaces of fresh leaves chosen at randomly. The spec tral distributions and simple averages of the radiative properties in the wavelength ranges of ultraviolet-B (UV-B, 280-320 nm) and ultravio let-A (UV-A, 320-400 nm) were determined. The spectral distributions o f reflectance were similar between adaxial and abaxial surfaces, altho ugh the magnitude varied among tree species. Leaf reflectance was very low for the ultraviolet spectrum in general and varied among species and between adaxial and abaxial surfaces. It was generally higher over the UV-A waveband compared to UV-B, and higher on the abaxial than ad axial surface. The broadband reflectance in the UV-A range (over all s pecies) was 5.0 and 3.9% for abaxial and adaxial surface, respectively , compared to 3.5 and 2.8% in UV-B. The transmittance through leaves w as extremely small in the UV-B (<0.1%) and nearly zero in the UV-A spe ctral range. Consequently, the absorptance of ultraviolet radiation by leaves, as determined from the measured reflectance and transmittance , was quite high, being more than 90% for all the combinations of spec ies and wavebands examined. The reported results are useful for studie s requiring spectral radiative properties of the examined leaves with respect to ultraviolet irradiation.