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Penetration of Photosynthetically Active and Ultraviolet Radiation into Alfalfa and Tall Fescue Canopies

^a Geophysical Inst., Univ. of Alaska, Fairbanks, AK 99775
^b School of Natural Resources, Univ. of Nebraska, Lincoln, NE 68583-0728
^c Dep. of Plant Pathology, Univ. of Nebraska, Lincoln, NE 68583-0722
^d Dep. of Agronomy and Horticulture, Univ. of Nebraska, Lincoln, NE 685893-0724

View larger version (19K): [in a new window]   Fig. 1. Average transmitted irradiance for an alfalfa crop under clear skies on DOY 195 at an LAI of 4.2 (40° solar zenith angle) and at an LAI of 1.0 (20° solar zenith angle) for sample numbers 1, 5, 10, 20, and 40. (a) PAR; (b) UV-A; and (c) UV-B. Error bars are shown for the average transmitted irradiance.

View larger version (23K): [in a new window]   Fig. 2. Measured average UV-B transmittance (points) by plot with fitted line (solid line with standard error bar), as a function of LAI for (a) alfalfa under clear sky conditions; (b) alfalfa under overcast sky conditions; and (c) tall fescue under clear sky conditions.

View larger version (31K): [in a new window]   Fig. 3. Measured maximum (open symbol) and minimum (shaded symbol) UV-B transmittances by plot with fitted line (dashed line), as a function of LAI for (a) alfalfa under clear sky conditions; (b) alfalfa under overcast sky conditions; and (c) tall fescue under clear sky conditions. Fitted lines were determined using a weighted estimate of k (inverse of standard error), thus, unequal weighting of data points (i.e., best fit line is influenced by some points more than others).

View larger version (38K): [in a new window]   Fig. 4. Ratios of UV-B/PAR and UV-B/UV-A as a function of leaf area index for the three canopy/sky conditions of alfalfa under clear sky conditions (1997), alfalfa under overcast sky conditions (1997), and tall fescue under clear sky conditions (1998) for (a) above-canopy and (b) below-canopy.

View larger version (33K): [in a new window]   Fig. 5. Comparison of average transmitted irradiance as determined from 20 samples along the transect with a point quantum sensor with the average transmitted irradiance as determined from 20 measurements using a line quantum sensor in alfalfa under clear and overcast sky conditions (open and solid symbols) and in tall fescue under clear sky conditions (shaded symbols).