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Spectral fluorescence signatures of tulip poplar, red maple and sweet gum under increasing nitrogen fertilization levels.
Butcher, LaDean*,1, Middleton, Elizabeth1, Entcheva-Campbell, Petya1, Corp, Lawrence1, McMurtrey, Jim2, Chappelle, Emmett1, 1 Laboratory for Terrestrial Physics, Biospheric Sciences Branch, Greenbelt, MD, USA2 Hydrology and Remote Sensing Laboratory, Beltsville, MD, USA
ABSTRACT- An experiment was designed to characterize the spectral fluorescence and reflectance properties of the foliage of several tree species over a range of nitrogen (N) conditions that they may encounter in agricultural riparian zones. Three year old tulip poplar (Liriodendron tulipifera L.), red maple (Acer rubrum L.) and sweet gum (Liquidambar styraciflua L.) saplings were exposed to a total seasonal N application of 74, 37, 18 and 0 kg N/ha from May through October 2002. The trees were grown in pots, exposed to full sun, and provided daily water at the USDA Agricultural Research Service, in Beltsville, MD. Measurements of adaxial and abaxial leaf surfaces were made throughout the growing season and included fluorescence imaging, fluorescence emission spectra (300 - 800 nm, 5 nm resolution), reflectance spectra (350 - 2500 nm, 3 nm resolution) and various biophysical parameters. Vegetation fluorescence occurs when light energy is absorbed by a leaf surface, is not utilized for photosynthesis and is emitted at a lower energy and longer wavelength. The actively induced fluorescence spectra of green plants typically exhibit five distinguishable maxima centered in the UV (335 nm), blue (445 nm), green (525 nm), red (685 nm) and far-red (740 nm) regions of the spectrum. Fluorescence emission spectra were obtained at three discrete excitations: 280 nm (280 Ex), 360 nm (360 Ex) and 530nm (530 Ex). Significant differences among tree species were obtained for: a) adaxial emissions centered at 360 nm and 375 nm resulting from 280 Ex (p < 0.001); b) adaxial emissions in the blue region from 360 Ex (p < 0.001); and c) adaxial emissions in the red peak, due to 530 Ex (p ≤ 0.001). The adaxial green/far-red fluorescence ratio (360 Ex) also exhibited species separation (p ≤ 0.01). In addition, there were significant N treatment differences in the red/far-red fluorescence ratio (360 Ex) in red maple, with the fluorescence ratio increasing with N treatment (r2 = 0.74, p ≤ 0.01). These findings demonstrate that individual tree species can be discriminated using spectral signatures and that N related differences could be determined using fluorescence measurements. These results are encouraging, particularly in light of the fact that they are observations from the first year of a multiple year N application study.
Key words: riparian zones, nitrogen fertilization, fluorescence, red maple, tulip poplar, sweet gum