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Published in Agron. J. 96:1562-1571 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

Agroclimatology

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

Martha D. Shulskia, Elizabeth A. Walter-Sheab,*, Kenneth G. Hubbardb, Gary Y. Yuenc and Garald Horstd

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

* Corresponding author (ewalter-shea1{at}unl.edu)

Received for publication August 13, 2003.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Ultraviolet-B radiation (UV-B, 280–320 nm) reaching the earth's surface has deleterious effects on plants. The degree of susceptibility to UV-B is dependent on the amount of energy present in longer wavelengths of ultraviolet-A radiation (UV-A, 320–400 nm) and photosynthetically active radiation (PAR, 400–700 nm). This study was conducted to quantify the UV and PAR light environment and describe the UV-B/UV-A and UV-B/PAR ratios above and below developing vegetative canopies. Transmitted irradiant flux densities of UV-B, UV-A, and PAR in a developing alfalfa (Medicago sativa L.) canopy and a tall fescue (Festuca arundinacea Schreb.) canopy were measured at varying solar zenith angles under clear and overcast sky conditions. Extinction coefficients for average transmittance differed for alfalfa and tall fescue; a single equation for each waveband and canopy/sky condition sufficed to describe the average transmittance. Canopy structure, LAI, and, to a lesser degree, the extent of direct and diffuse radiant energy were found to influence penetration more than sun angle. An envelope of transmittances defined by equations representing the maximum and minimum light transmittance illustrates the variability in transmittances and was broadest under clear skies and narrowed with decreasing wavelength. Leaf area altered the average ratios of above-canopy UV-B/UV-A and UV-B/PAR ratios. The average ratios of above-canopy UV-B/UV-A and UV-B/PAR ratios varied slightly with year and sky condition. Differences between the two canopies indicate the need to consider canopy architecture in determining the amount of light penetrating a canopy in the UV-B, UV-A, and PAR.

Abbreviations: ARDC, University of Nebraska Agricultural Research and Development Center • CFCs, chlorofluorocarbons • DOY, day of year • LAI, leaf area index • PAR, photosynthetically active radiation, wavelength range 400–700 nm • RMSE, root mean square error • UV-A, ultraviolet radiation-A, wavelength range 320–400 nm • UV-B, ultraviolet radiation-B, wavelength range 280–320 nm • UV-B/UV-A, ratio of UV-B irradiance to UV-A irradiance (above- and below-canopy ratios) • UV-B/PAR, ratio of UV-B irradiance to PAR irradiance (above- and below-canopy ratios) • UV-MFRSR, ultraviolet multi-filter rotating shadowband radiometer • VIS-MFRSR, visible multi-filter rotating shadowband radiometer • WMO, World Meteorological Organization


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ALTHOUGH ENERGY in the ultraviolet spectrum represents a small fraction of the total solar radiant energy, it is of great biological importance. Ultraviolet radiation (UV) is defined by three consecutive wavebands: UV-C (200–280 nm), UV-B (280–320 nm) and UV-A (320–400 nm). Extraterrestrial irradiances (solar constant values) for these wavebands are 6.4 W m–2 for UV-C, 21.1 W m–2 for UV-B, and 85.7 W m–2 for UV-A, representing 0.5, 1.5, and 6.3%, respectively, of the total solar radiant energy (Frederick et al., 1989). At the earth's surface, UV irradiances are considerably lower than corresponding extraterrestrial values due to scattering and absorption by atmospheric constituents; UV-C is totally attenuated and is therefore not a direct biological concern at the earth's surface (Correll et al., 1992). However, UV-B radiation has negative overall effects on vegetation such as reductions in photosynthesis, transpiration, plant growth, flowering, and yield (Tevini and Teramura, 1989; Caldwell, 1998).

Ozone is a major absorber of UV-B radiation; however, ozone concentrations in the stratosphere have declined in recent decades due to the influx of chlorofluorocarbons (CFCs) that destroy ozone molecules (Rowland, 1990; Lumsden, 1997). Because of declining ozone concentrations, surface UV-B levels are expected to increase, and this has raised concern about possible detrimental effects on aquatic and terrestrial ecosystems (Tevini, 1993, p. 125–153, 229–240; Caldwell et al., 1994, 1995; Caldwell, 1998). Species, cultivar, and age are determining factors in the degree of susceptibility of plants to increased UV-B radiation (Tevini and Teramura, 1989; Caldwell et al., 1995). Organisms inhabiting plant surfaces also are susceptible to increases in ultraviolet radiation. Microorganisms (bacteria and fungi) lacking effective UV tolerance mechanisms would be particularly affected in survival and development by a slight increase in UV-B (Hader, 1986; Sundin, 2002). Because UV-A and photosynthetically active radiation (PAR, 400–700 nm) can ameliorate the damaging effects caused by UV-B radiation by inducing photoreactivation processes in living cells (Cullen et al., 1992; Smith et al., 1992; Caldwell et al., 1995), the ratios of UV-B/UV-A and UV-B/PAR determine the susceptibility of organisms, as well as plant tissue, to UV exposure (Tevini, 1993, p. 125–153, 229–240; Deckmyn et al., 1994; Barnes et al., 1996).

Solar angle, leaf angle distribution, leaf and soil optical properties, and the fraction of diffuse light in the incident beam can all influence the ratio of UV-B/PAR (Grant, 1991, 1997b, 1999). Ultraviolet and PAR leaf optical properties differ with UV reflectance being two-thirds that in the PAR band and UV leaf transmittance is essentially negligible while PAR leaf transmittance is less than 10% (Gates et al., 1965; Grant 1993). Ultraviolet-B penetration is less variable with leaf inclination angle than is PAR, because of the higher diffuse component in the UV region compared with PAR (Caldwell, 1981; Deckmyn et al., 2001). This implies that the UV-B/PAR ratio should change with canopy leaf area and leaf architecture.

The goal of this research is to investigate the natural UV and PAR radiation environment within vegetative canopies under field conditions. There were two objectives: (i) determine the penetration of UV-B, UV-A, and PAR in canopies of alfalfa (Medicago sativa L.) and tall fescue (Festuca arundinacea Schreb.) under clear (no clouds) and overcast (complete cloud coverage) sky conditions, and (ii) determine the ratios of UV-B/UV-A and UV-B/PAR in both canopies under clear and overcast sky conditions.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Site
Field measurements were taken in 1997 and 1998 at the University of Nebraska Agricultural Research and Development Center (ARDC) near Mead, NE (41.13°N, 96.48°W; 353 m above sea level). An investigation was conducted in 1997 from 14 July (day of year [DOY] 195) to 23 August (DOY 235) in three adjoining plots (12.2 by 12.2 m alfalfa) of alfalfa selected in a larger alfalfa field (planted August 1995 in east–west rows at 15-cm row spacing). During the study, the field was irrigated when soil moisture content (by weight) fell below 20% of field capacity. Every 2 wk a different plot was cut to a height of 5 cm, with a period of 6 wk between cuttings in a given plot. This rotation supported height increments of approximately 20 cm and corresponding differences in leaf area from one plot to the next most recently cut plot. Among the three plots, leaf area index (LAI) varied from 0.3 to 4.9 with a corresponding variation of mean tip angle of 41 to 90° (the variation being a result of the periodic cutting).

A second investigation was conducted in 1998 at the ARDC from 20 July (DOY 201) to 25 Aug. 1998 (DOY 237) in three adjoining 4.5 by 4.5 m plots selected in a tall fescue (c.v. Kentucky-31) field. The plots were managed under forage conditions and irrigation was equal to 80% of the evapotranspiration to prevent water stress. Each week a different plot was cut to a height of 10 cm. Just before mowing, the tall fescue reached a height of approximately 40 cm. Leaf area index varied from 1.7 to 5.2 among the four plots with a corresponding variation of mean tip angle of 43 to 65°.

Irradiance Measurements
Incident and transmitted irradiances of PAR, UV-A, and UV-B were measured in the alfalfa and tall fescue plots using a set of three portable instruments. The PAR photon flux density (µmol m–2 s–1) was measured with a point quantum sensor (Model LI190-SA, Li-Cor, Lincoln, NE; 24 mm diam. by 25 mm high) and subsequently converted to irradiance (W m–2) using a conversion factor of 4.6 µmol quanta J–1 (Li-Cor, 1991). The UV-A and UV-B irradiances (W m–2) were measured with two photodiode-based sensors (BW20 Analog UV Light Monitors, Vital Technologies, Bolton, ON, Canada; 30 mm diam. by 33 mm high). The UV-A sensor responds to wavelengths in the 310 to 385 nm range (50% relative response at 315 and 375) and the UV-B sensor to 275 to 320 nm (UV-B) wavelengths (50% relative response at 290 and 315 nm). The UV sensors were selected based on size, performance, and facility for interfacing with a datalogger. World Meteorological Organization (WMO) tests indicated the device response is within 10% of actual values for solar zenith angles 0 to 50° (Leszczynski et al., 1995). In the tall fescue canopy study in 1998, however, the UV-A instrument failed and no replacement was available.

A subsurface transect track system was installed in each alfalfa and tall fescue plot approximately 1 mo before the study period. The track system was installed after an initial cutting of the vegetation so that the vegetation was short, minimizing canopy disturbance. The system consisted of a narrow trench (7 cm wide, 12.5 cm deep, and 1.6 m long) and instrument support mechanisms to position sensors at soil level. Aluminum sheeting, painted matte-black, covered the bottom and sides of the trench. Two posts driven into the ground through holes cut in the aluminum at either end served to anchor the sheeting in the trench. The posts extended approximately 16 cm above the soil surface to support a track. A trolley sliding in the track was used to carry the sensors along the length of the trench. The track was constructed of metal tubing with a slat milled to the shape of the slider at the base of the trolley. The track was removable and the height of the track could be adjusted for leveling purposes. The three sensors were mounted 1.3 cm apart on the same trolley so that the detecting face of each sensor was positioned at the same height. Together, the sensors and trolley measured 11 cm high and 14 cm long. Cords attached to the trolley and pulled from the opposite end of the track were used to position the trolley at various positions where transmitted irradiance measurements were obtained with sensors facing up at soil level. Sensors and trolley were then moved to the next plot for measurement.

To obtain a spatial average of the transmitted flux, measurements were taken at specific stops as the sensors were moved along an east–west oriented 1-m transect. The three sensors did not measure at the exact same location but all sensors were sampled at each stop. Chartier et al. (1993) noted the inherit difficulty in sampling transmitted fluxes at an LAI < 1, as attested by discrepancies from study to study. Grant (1997a) reported large spatial variability in irradiances under sparse canopies and suggested a method using spatially and temporally averaged measurements of radiation penetration.

A test was performed to determine how many measurements (stops) are required to characterize the below canopy transmittance. In the test, tracks were installed in two alfalfa canopies, one with low and the other with high leaf area. A population of 40 samples was collected at approximately 40° and 20° solar zenith angles for clear skies on DOY 195 (1997). The population was subsampled to yield averages for 1, 5, 10, and 20 measurements to determine (i) the effect that sample number has on estimates of average transmitted radiation and (ii) the minimum number of samples required to reliably estimate the 1-m population average. A PAR line quantum sensor (model LI191-SA, Li-Cor, Lincoln, NE) was slid into place below the canopy next to the track during the alfalfa transmitted irradiance point measurements, and into the track for the tall fescue immediately following the point measurements (the plant spacing in the tall fescue canopy did not allow parallel measurements). Twenty measurements along the track provided an average comparable to the population average (n = 40) (Fig. 1); thus, all below canopy transmittances were sampled at n = 20.



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  		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.

 
Total (direct and diffuse) incident irradiances were measured with the same instruments and track except that the track was placed 4 m north of the transects and positioned so that sensor heads were 1 m above the soil surface. Total irradiance was measured for the purpose of calculating transmittance through the canopy. At each solar zenith angle on each day of measurement, a set of measurements were made with all three sensors in each plot (of PAR, UV-A [alfalfa only] and UV-B), as follows: incident irradiance (above-canopy), transmitted irradiance (20 independent, evenly spaced point measurements and line measurements along each transect), and a second incident irradiance measurement. The completion of one data set required 6 to 7 min; therefore, three independent plots (each with a different LAI due to the cutting schedule) were measured in roughly 20 min. This 20-min time interval was centered on solar zenith angles of 50°, 40°, and 30° and those measured about solar noon for each day of measurement (solar noon zenith angles varied from 19° on DOY 195 to 28° on DOY 232 in 1997 and 20° on DOY 201 to 28° on DOY 237 in 1998). Estimates of LAI and mean tip angle were obtained nondestructively by placing a Model LAI 2000 Plant Canopy Analyzer (Li-Cor, Lincoln, NE) in each transect at soil level.

Incident irradiance data for PAR, UV-A, and UV-B were also available from a UV-B monitoring station (located 0.8 km northeast of the alfalfa site and 2.8 km southwest of the tall fescue site). The station is part of the USDA UV-B Monitoring Program coordinated by Colorado State University, located at Ft. Collins, CO (Bigelow et al., 1998). The ultraviolet multi-filter rotating shadowband radiometer (UV-MFRSR) and visible multi-filter rotating shadowband radiometer (VIS-MFRSR) sensors (developed by Harrison et al., 1994) were used for checks and comparisons with the Li-Cor PAR and Vital UV-A and UV-B sensor output. The MFRSR sensor output includes corrections made to the direct beam component to adjust for the cosine response of the instrument as outlined by Michalsky et al. (1995). Sensor stability is estimated to be on the order of 1% decline per year as a result of filter instability and precision is found to be within 8% of the estimated extraterrestrial solar irradiance (Bigelow and Slusser, 2000) (further information is also available through the Program website at http://uvb.nrel.colostate.edu; verified 12 July 2004). Diffuse, direct, and total irradiances (W m–2) in discrete 2 nm bandwidths centered on 300, 305, 311, 317, 325, 332, 368, 415, 500, 610, and 665 nm were used to verify clear and overcast days. Overcast days had nearly 100% diffuse irradiance in each waveband while on clear days direct irradiance was dominant at the 610 nm band window. These data were also used to check the stability of the sensors between years. An independent estimate of incident radiant flux density was derived for PAR, UV-A, and UV-B wavebands against which the Vital and Li-Cor sensors were compared at the end of the 1997 and 1998 study periods (DOY 197, 212, 227, and 232 of 1997 and on DOY 201 and 209 of 1998 centered at the various solar zenith angles). The observations compared well with these independent estimates for clear sky data with root mean square error (RMSE) values of 0.15, 2.6, and 33.0 W m–2 throughout the day and during the course of the measurement period under clear sky conditions, respectively, for the UV-B, UV-A, and PAR sensors. Because the USDA UV-B sensors were not calibrated during the period, any source of drift in an intercomparison could not be assigned and would therefore be inconclusive. However, an intercomparison (under the assumption that a stable relationship between the two sets of sensors would not likely be caused by both the Vital and the USDA UV-B sensors drifting at the same rate) was made for clear sky days (as verified by examining the diffuse and direct irradiances at the USDA site). The intercomparison involved a multiple linear regression where the Vital and Li-Cor readings were the dependent variables and the irradiance values in the different wavebands on the USDA UV-B sensors were the independent variables. Drift was suspected if the regression coefficients changed significantly from day to day while stability was assumed if the regression coefficients did not change from day to day.

Data Reduction
Transmitted irradiance, Ti j({theta}, {lambda}, LAI), was measured at 20 locations (i = 1,20) along the 1-m transect for each waveband ({lambda}, leaf area index, LAI; solar zenith angle, {theta}; and canopy/sky condition, j) in each plot. Incident irradiance, Ii,j({theta}, {lambda}, LAI), was estimated based on a time-based linear interpolation of the incident irradiance measured before and immediately after each below canopy transect measurement. For overcast skies, the average of the two incident irradiances bracketing the transmitted irradiance measurement set was used as an estimate of incident irradiance Ii,j. Average transmittance was calculated from the 20 samples in each data set per plot as:

[1]
where j = canopy/sky condition (1 = alfalfa, clear; 2 = alfalfa, overcast; 3 = tall fescue, clear).

Average transmittance values obtained from a plot over the course of a study was related to LAI using a general form of Beer's law (Campbell and Norman, 1998) on a plot basis:

[2]
where kj({theta}, {lambda}) is the extinction coefficient. Strictly speaking, this equation is applicable to only direct beam radiation. Equation [2] provides an opportunity to test for trends in extinction coefficients; any failure in performance points to a need for integrating more sophisticated models with field measurements. The value of kj({theta}, {lambda}) for each plot was found using SAS Proc NLIN for each combination of solar zenith angle, waveband, and canopy/sky condition. The kj({theta}, {lambda}) values were tested for three-way and two-way interactions among canopy/sky condition, solar zenith angle, and waveband using SAS Proc GLM. Average k values then were derived depending on results from testing for interactions. For example, if no significant interaction was found between {theta} and {lambda}, a weighted average k value was determined across all {theta} or {lambda} using the inverse of the standard error as the weight for corresponding kj({theta}, {lambda}) values. Average transmittance values were plotted along with exponential relations (Eq. [2]) as a function of LAI. The coefficient of determination (r2) and the root mean square error (RMSE) were determined for these exponential curves (refer to Willmott, 1982 for further discussion of RMSE).

Minimum (shade) and maximum (sunfleck) transmitted irradiances for canopy/sky condition, solar zenith angle, waveband, and LAI were taken as the lowest and highest readings, respectively, from the 20 samples per plot. Under clear sky conditions, incident irradiance occurring at the time of the maximum or minimum transmitted irradiance, Imax,j or Imin,j, was estimated using a time-based linear interpolation. For overcast skies, the average of the two incident irradiances bracketing the transmitted irradiance measurement set was used. Minimum and maximum transmittances were related to LAI for each plot and sky condition separately:

[3a]

[3b]

Extinction coefficients for minimum transmittances (kmin,j) were calculated using the same procedure as with the average transmittances. In the case of maximum transmittances, extinction coefficients (kmax,j) were determined from data sets in which LAI values were 1.0 or higher; maximum transmittances were assumed to be 1.0 for LAI < 1.0. Maximum and minimum transmittances and their exponential relation as defined by the appropriate extinction coefficient were compared to the average as a means of showing the variation about the mean. The coefficient of determination (r2) and the root mean square error (RMSE) were determined for the resulting exponential relation.

Above- and below-canopy ratios of UV-B/UV-A (alfalfa only) and UV-B/PAR were determined for each solar zenith angle of each measurement day (averaged over plots) and canopy/sky condition. Above-canopy ratios were calculated as the ratio of the respective incident irradiances. Below-canopy ratios were calculated as the ratio of the average transmitted irradiances. The means of the ratios were tested for two-way interactions and simple effects using SAS Proc GLM where the interacting variables are solar zenith angle and canopy/sky condition. Ratios were further tested for three-way and two-way interactions and simple effects where the interacting variables are canopy/sky condition, above- and below-canopy position, and LAI (where LAI is categorized as low [LAI ≤ 2] and high [LAI > 2]) to identify changes in ratio due to vegetative cover.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Average Transmittance
No significant difference was detected among extinction coefficients, kj({theta}, {lambda}), due to solar zenith angle ({theta}) when considered by waveband ({lambda}: PAR, UV-A, UV-B) and canopy type (i.e., no three-way interactions of extinction coefficients between waveband, canopy/sky condition, and solar zenith angle). Since there was no significant interaction of canopy/sky condition and waveband with solar zenith angle, transmittance values were averaged over all solar zenith angles for a particular canopy/sky condition and waveband. A difference in extinction coefficients was detected when averaged across solar zenith angles (i.e., a significant canopy/sky condition x waveband interaction [P < 0.10]). Average plot transmittance as a function of LAI for the three canopy/sky conditions of each of the three wavebands followed Beer's law of attenuation (Fig. 2). Extinction coefficients describing average transmittance for each canopy/sky condition and waveband, kj({lambda}), ranged from 0.52 to 0.54 for the alfalfa canopy under clear skies (r2 ≥ 0.85, RMSE ≤ 0.11), 0.50 to 0.54 for the alfalfa canopy under overcast skies (r2 ≥ 0.78, RMSE ≤ 0.12), and 0.41 to 0.49 for the tall fescue canopy under clear skies (r2 ≥ 0.78, RMSE ≤ 0.08), indicating higher transmittance in tall fescue than in alfalfa at comparable LAI values. The k value for PAR waveband under the tall fescue canopy was significantly lower than in the other combinations of {lambda} and canopy/sky condition indicating highest transmittance in fescue in the PAR waveband. This could be due to the greater penetration of direct beam as compared with the diffuse radiation in a canopy of erectophile orientation, in combination with the particular sampling strategy applied, since PAR transmittance has been found to correspond with direct beam penetration while that of UV transmittance is more closely associated with the view factor of sky diffuse radiation. Grant (1999) noted that normalized PAR values were higher than normalized UV-B values at particular sun/surface orientations (sunlit but not perpendicular and away from the sun), which are likely to occur in an erectophile canopy. Higher PAR leaf transmittance and reflectance would result in higher scatter in the canopy. No trend in transmittance as related to waveband was found in alfalfa. These results differ from those found for canopies of different architecture; a slight decrease in transmittance with increasing wavelength was observed in a grass (Lolium perenne L.) canopy (Deckmyn and Impens, 1998) as well as in an oak (Quercus sp.) forest canopy (Yang et al., 1993).



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  		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.

 
There was a difference in extinction coefficients due solely to the canopy/sky condition [i.e., a highly significant simple effect for canopy/sky condition (P < 0.0001)]. Extinction coefficients across all wavebands and solar zenith angles were 0.53, 0.52, and 0.47 for alfalfa/clear skies, alfalfa/overcast skies, and tall fescue under clear skies, respectively. The extinction coefficient for tall fescue was significantly different from that of alfalfa (clear and overcast sky conditions), which could be attributed to the different canopy structures of the tall fescue (more erectophile orientation) and of the alfalfa (more planophile orientation) and corresponding differences in penetration.

There was no significant effect for solar zenith angle. Values for k, however, tended to increase (from 0.49 to 0.53) as solar zenith angle increased from solar noon to 50°. This trend supports that of Deckmyn and Impens (1998) that solar angle and irradiance had little effect on transmittance in a grass canopy, as compared with LAI and percent diffuse component. A UV canopy transmittance modeling study by Allen et al. (1975) found little change in the radiation penetration under solar zenith angles less than 60°. Similarly, Deckmyn and Impens (1998) found no significance of solar angle on the penetration of UV-B and PAR in a maize canopy (Zea mays L.); rather, LAI and percent diffuse light were highly important. Perhaps the inclusion of larger solar zenith angles in the present study may have shown a greater solar zenith angle effect on transmittance.

Maximum and Minimum Transmittance
As with average transmittances, there were no significant three-way interactions of canopy/sky condition, solar zenith angle and waveband for maximum and minimum transmittances while two-way interactions of canopy/sky condition with waveband were highly significant (P < 0.001). Beer's law (Eq. [2]), with appropriate extinction coefficients, provided a fit to maximum and minimum transmittance values averaged over all solar zenith angles as a function of LAI (Fig. 3). Extinction coefficients describing maximum transmittance for each canopy/sky condition and waveband, kmax,j({lambda}) ranged from 0.11 to 0.19 for the alfalfa canopy under clear skies (r2 ≥ 0.42, RMSE ≤ 0.26), 0.42 to 0.47 for the alfalfa canopy under overcast skies (r2 ≥ 0.69, RMSE ≤ 0.24), and 0.07 to 0.22 for the tall fescue canopy under clear skies (r2 ≥ 0.27, RMSE ≤ 0.26), indicating higher maximum transmittance values under clear skies than under overcast sky conditions. Extinction coefficients describing minimum transmittance for each canopy/sky condition and waveband, kmin,j({lambda}), ranged from 1.01 to 1.81 for the alfalfa canopy under clear skies (r2 ≥ 0.68, RMSE ≤ 0.09), 0.69 to 0.77 for the alfalfa canopy under overcast skies (r2 ≥ 0.66, RMSE ≤ 0.11), and 0.93 to 1.11 for the tall fescue canopy under clear skies (r2 ≥ 0.36, RMSE ≤ 0.05). Thus, under overcast skies the minimum transmittance values were higher than under clear sky conditions, meaning a greater penetration under overcast skies of diffuse radiant energy, which dominates above the canopy as well. The findings reported here are consistent with the fact that radiation is diffuse under overcast skies. A point among the 20 points below the canopy that represents the maximum (sunfleck) under clear skies, is expected to have a view factor that includes a portion of the sky in or near the solar disk (essentially direct beam) and therefore relatively more radiant energy transmitted. However, this point would not be favored under overcast skies because the diffuse irradiance arriving from the direction of the solar disk under cloudy conditions is not much higher than that arriving from other portions of the sky. Similar results are found in forest (Brown et al., 1994) and maize canopies (Gao et al., 2002). Grant and Heisler (1996) found that penetration of UV in a suburban street-tree canopy was more closely associated with the view factor of sky diffuse radiation while PAR penetration corresponded with direct beam penetration. This effect can be used to explain differences in transmittance among the three wavebands in alfalfa canopies in this study. However, in the tall fescue canopy under clear skies, the UV-B waveband with a higher diffuse component had a higher k value than the PAR waveband. This could be attributed to the tall fescue canopy structure being more amenable to direct beam penetration in comparison with that in alfalfa.



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  		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).

 
By providing the fitted lines for the maximum and minimum values, an envelope of transmittance values was created for each waveband and sky condition (Fig. 3). For clear skies, there was a wide range of transmittances along the track between the maximum (sunfleck) and minimum (shade) points, creating a broad envelope of transmittance for both the alfalfa and tall fescue canopies. The variation is likely due to the interaction of the canopy and the incoming irradiance; however, wind variations might cause an oscillation of the canopy and movement of the sunflecks to further complicate the sampling. In this study no effort was made to address wind effects. The spread between {tau}j,max({lambda}, LAI) and {tau}j,min({lambda}, LAI) for UV-A and UV-B is less than that for PAR, meaning there was greater variation in the individual PAR transmittances along the transect than there was for UV-A or UV-B (large for maximum transmittance, small for minimum transmittance). Differences in transmittance "envelopes" between wavebands can be attributed to different proportions of sky direct and diffuse radiation (i.e., under clear sky conditions, incident UV radiation is composed of up to 50% diffuse while incident PAR is roughly 10% diffuse, making the UV-B light environment more spatially uniform than the PAR environment), their associated penetration probabilities, and leaf and soil scattering characteristics. Heisler et al. (2003) show large fluctuations in PAR relative to UV-B under a tree with clear sky conditions, noting the importance of direct beam radiation for PAR. Brown et al. (1994) found that positions below a mixed forest canopy with only diffuse light available had proportionately more UV-B, whereas positions in sunflecks were comparatively rich in PAR. The envelope of exponential curves for each waveband for overcast skies was much tighter than the corresponding set of curves for clear skies. Grant (1999) found that minimum normalized UV-B values were greater than minimum normalized PAR values due to the greater importance of diffuse sky radiation to UV-B penetration in vegetative canopies. The selective penetration for clear skies and more uniform penetration for overcast skies has been noted elsewhere and has been attributed to the position of gaps in the canopy and the resulting differences in penetration of direct and diffuse radiation (Holmes and Smith, 1977). Deckmyn and Impens (1998) found no significant difference for transmittance between waveband under overcast skies. Moreover, Deckmyn and Impens (1998) found that the differences in penetration decreased as percentage diffuse increased.

Standard errors for minimum transmittance were smallest under clear skies and equal to that of average transmittance under overcast conditions. Standard errors were largest for maximum transmittance under all sky conditions. The precision at which maximum and minimum transmittances in the understory of a canopy can be determined is linked to the sampling strategy. In the case reported here, there is a 5% chance the observed maximum from the 20 measurements taken along a transect can be exceeded by the true maximum. The sampling variability for the maximum and minimum could be reduced by taking more samples. For example, the probability that the true maximum exceeds the observed maximum could be reduced to about 1% if 100 samples were measured. This would reduce also some of the scatter associated with the maximum and minimum transmittance curves. The sampling strategy, however, was selected specifically for finding the average transmittance and the desirability of reducing the maximum and minimum sampling error must be weighed against the time it takes, relative to solar elevation changes, to collect 100 samples.

Differences in maximum transmittance extinction coefficients for the different canopies (averaged over wavebands) were detected due to solar zenith angle (i.e., two-way interaction of canopy/sky condition with solar zenith angle was significant [P = 0.015]) but not for average and minimum transmittance (although the trend was for average k values to increase as solar zenith angle increased). Values of kmax,j({theta}) generally increased with increasing solar zenith angle (Table 1) with kmax,j at extreme solar angles under clear sky conditions, that is, kmax,j(50) was consistently larger than kmax,j(solar noon) and kmax,j(30). Given the maximum transmittance values are dominated by radiant energy in the direction of the solar disk, it is reasonable that solar zenith angle would affect maximum transmittance. Irradiance associated with maximum transmittance is likely dominated by the direct beam component, which dominates the above canopy irradiance while irradiance associated with the minimum transmittance is primarily diffuse (and therefore multi-directional). The irradiance associated with the average transmittance is the combination of directional and diffuse components (thus, the directional effect is likely muted in the averaging process). This may explain why the extinction coefficients for the maximum transmittance are so different from the coefficients for either minimum or average transmittance since the maximum transmittance is dominated by sunflecks, which in turn are dominated by direct beam radiation. Other research has found that solar angle has not been a major factor influencing UV canopy transmittance; rather, LAI and percentage diffuse light are more important contributing factors (Allen et al., 1975; Deckmyn and Impens, 1998; Gao et al., 2003).


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  		Table 1. Extinction coefficients describing maximum transmittance as a function of solar zenith angle and canopy/sky condition (averaged over all three wavebands).

 
UV-B/UV-A and UV-B/PAR Ratios
Statistically, most UV-B/UV-A ratios were significantly different from each other across solar zenith angles between clear and overcast skies and across sky conditions for a particular solar zenith angle (Table 2); the exceptions were with ratios between clear and overcast skies at low sun positions (large solar zenith angles of 40° and 50°) and between ratios at solar noon and 30° solar zenith angles within the same sky condition. Average UV-B/UV-A ratios were slightly larger under overcast skies than under clear skies; the highest ratios occurred at the small solar zenith angles. Likewise, most UV-B/PAR ratios were significantly different from each other across solar zenith angles in a particular canopy/sky condition and across canopy/sky conditions for a particular solar zenith angle (Table 2). The exceptions include the above-canopy ratios at solar zenith angles of near solar noon over the alfalfa (1997) and fescue (1998) under clear sky conditions and, as with UV-B/UV-A ratios, at low sun positions (solar zenith angles of 40° and 50°) when comparing overcast conditions with clear conditions. The results indicate a difference in clear sky conditions between 1997 and 1998; however, the differences are not as great as between clear and overcast conditions in 1997 (Fig. 4a). On average, UV-B/PAR ratios under overcast skies in 1997 (alfalfa) were larger than those for clear sky conditions in 1998 (fescue), which were larger than those for clear sky conditions in 1997 (alfalfa). Deckmyn and Impens (1998) found cloudiness affected PAR more than UV-B, resulting in higher ratios under overcast skies, which is compatible with the results from the current study. The mean ratios of UV-B/PAR across solar zenith angles found in this study are higher than 0.0036 found by Yang et al. (1993) above a forest canopy. However, their ratio was determined under partly cloudy sky conditions and falls on the low end of the data range reported here.


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  		Table 2. Above-canopy ratios of UV-B/UV-A and UV-B/PAR for the three experimental conditions: for 1997 (clear and overcast skies over the alfalfa canopy) and 1998 (clear skies over the tall fescue canopy).

 


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  		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.

 
No differences were detected among below-canopy ratios averaged by canopy/sky condition and by solar zenith angle (i.e., no two-way interaction of below-canopy ratios between canopy/sky condition and solar zenith angle); however, UV-B/PAR ratios averaged over canopy/sky condition or by solar zenith angle differed (i.e., single effects of canopy/sky condition and solar zenith angle were significant) and UV-B/UV-A ratios averaged over canopy types differed (Table 3). These contrast to the results with above-canopy ratios, indicating an effect of the canopy on trends in the ratios (Fig. 4b). Solar zenith angle affected the ratios significantly, with smaller solar zenith angles having the higher ratios for UV-B/UV-A and UV-B/PAR (Table 3). The below-canopy UV-B/PAR ratios for the alfalfa canopy under overcast skies exhibited greater variation with solar zenith angle than in the other canopy/sky conditions. This can be attributed to differences in radiation penetration of the two wavebands under clear skies (each having different proportions of direct and diffuse radiation) and to cloud movements and differences in cloud thickness, water content and layering, and the consequential fluctuations in incident irradiance.


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  		Table 3. Below-canopy ratios of UV-B/UV-A and UV-B/PAR for the three experimental conditions: for 1997 (clear and overcast skies over the alfalfa canopy) and 1998 (clear skies over the tall fescue canopy).

 
Ratios of UV-B/UV-A and UV-B/PAR were further analyzed by defining the ratios into categories of low LAI (LAI ≤ 2) and high LAI (LAI > 2) and comparing above- and below-canopy ratios to elucidate changes due to vegetative cover. Three-way interactions among LAI category, canopy/sky condition, and above- and below-canopy mean UV-B/PAR ratios were detected. No differences in above- and below-canopy ratios were detected under low LAI conditions, but highly significant differences in ratios were detected for high LAI conditions for alfalfa clear and alfalfa overcast with a trend (but not statistically significant) for fescue. No difference in the effect of sky condition on UV-B/UV-A ratios was detected; the lack of difference in response is attributed to the diffuse nature of UV radiation regardless of sky condition. The UV-B/PAR and UV-B/UV-A ratios measured in alfalfa canopies under clear and overcast sky conditions were higher than those measured above the canopy, while ratios within the tall fescue canopy under clear sky conditions were lower than those measured above the canopy (Fig. 4b). This contrast may be due to the influence of solar angle, leaf angle distribution, leaf and soil optical properties, and the fraction of diffuse light in the incident beam on the ratio of UV-B/PAR (Grant, 1991). Ultraviolet-B penetration is less variable with leaf inclination angle than is PAR, because of the higher diffuse component in the UV region compared with PAR (Caldwell, 1981; Grant, 1999). Given the different architectures and leaf shape of the two canopies studied here, differences are expected.

Sensor Stability
In the intercomparison between the Vital and Li-Cor readings and the USDA UV-B sensors, the regression coefficients did not change between days or between years 1997 and 1998; thus, the sensors were considered stable during the study period.

In addition, all line quantum sensor transmittances from the 2-yr study were compared with average PAR transmittances for each solar zenith angle and all LAI values for both clear and overcast sky conditions, as a post-study check of sensor stability. The method used to sample transmitted irradiance below the canopy proved to be effective particularly for overcast skies in the alfalfa and clear skies in the tall fescue; RMSE values were 51.1 W m–2 for alfalfa under clear skies, 19.2 W m–2 for alfalfa under overcast skies, and RMSE of 25.3 W m–2 for tall fescue under clear skies with an overall RMSE of 33.1 W m–2 (Fig. 5). Discrepancies between the results from the line quantum sensor and those from the point sensors, particularly for alfalfa under clear skies, may be attributed to (i) the path of the point quantum sensor was parallel but slightly displaced from the line segment measured by the line quantum sensor, (ii) the highly anisotropic nature of clear skies result in greater variability under the canopy than under the near isotropic conditions of an overcast sky, and (iii) given the heterogeneous clumping nature of the alfalfa, 20 samples may not adequately represent the variation along a 1-m transect below this canopy as evidenced by the larger variability in the transmitted irradiances for clear skies than under overcast skies and at low LAI alfalfa canopies. The difference between point and line quantum sensor readings is much smaller in the tall fescue, which is attributed to the homogeneous nature of the tall fescue canopy and to the practice of operating both the line and point sensors in the same line segment.



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  		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).

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The simplistic representation of transmittance as a function of LAI represented by Beer's Law performed reasonably well for both average and minimum transmittance (RMSE ≤ 0.12). The resulting estimator performed poorly for maximum transmittance (RMSE > 0.24). To attain better performance (e.g., RMSE < 0.05) a more complete theory and supporting measurements are needed. Transmitted irradiance in canopies under clear skies is difficult to characterize because clumps of vegetation and associated sunflecks lead to a highly variable light environment. Line sensors give a good measurement of the average transmitted irradiance, provided length is not a limitation. However, because UV and PAR have different penetration probabilities in shade and sunfleck regions, the results reported here indicate it is best to use point measurements when information regarding the variation in transmittance is needed. It is possible that the variation in transmittance as indicated by upper and lower limits will be more critical to understanding UV effects in the canopy than the average transmittance, particularly in regard to the development of specific plant organs (e.g., buds, leaves, fruit) within the canopy and in regard to the survival of plant-associated microorganisms in various positions in the canopy.

Although the method used to sample transmitted irradiances below the canopy proved to be effective, future improvements are recommended. More intensive tests are needed at the beginning of each experiment under a variety of LAI and solar zenith angle conditions, specifically for clear skies, to identify a sampling procedure that produces reliable estimates of both the central tendency (average) and dispersion (probability distribution) of the irradiance. The number of below-canopy samples will be dependent on the "clumpiness" of the vegetation with more samples required for a more heterogeneous canopy (e.g., alfalfa plants clump together more than tall fescue so that more samples are needed). For a given canopy architecture, the sampling strategy should be guided by the agreement between the sample mean (for a given number of point measurements) and the population mean (derived from a large number of point measurements).

On average, transmittance was similar between the UV and PAR regions for a particular canopy, but differences were evident in individual point measurements of transmittances, which could have consequences for plant function and the survival of UV-susceptible microorganisms within the canopy environment (Yuen et al., 2002). Transmittance in shaded regions (minimum transmittances) below the canopy was low in the PAR region but relatively rich in UV light so that microorganisms and lower leaves in the canopy would be exposed to a greater fraction of incident light from the UV spectral region than from the PAR. Sunfleck regions (maximum transmittance) provided comparable richness in PAR so that plants and organisms could potentially benefit from a position in a sunfleck due to the ameliorating effects from the longer wavelength radiation.

Equations similar to those presented for the maximum and minimum transmittance potentially could be used to determine the range in irradiance due to shade and sunfleck regions below a particular canopy. The transmittance response varies for sky condition, where radiation under clear skies will have much greater variation across LAI than radiation under overcast skies.

In this study, the mean UV-B/PAR and UV-B/UV-A ratios were dependent on the overlying vegetative canopy. The ratios at individual positions below the canopy were subject to variation due to penetration differences in diffuse and direct beam radiation. Since this ratio is important in determining plant and organism sensitivity to UV-B radiation, the distribution of shade and sunfleck areas is useful in determining UV susceptibility. Although lacking the additional sensors to completely replicate this experiment, a range of UV and PAR transmittances is reported, as well as their ratios, within two typical crop canopies with varying physical properties. With further knowledge of the UV and PAR light environment, development of specific plant organs and the survival of microorganisms within vegetative canopies may be better understood.


   ACKNOWLEDGMENTS
 
This research was supported by the University of Nebraska Agricultural Research Division Interdisciplinary Research Program. The authors thank Naomi LaRue for her assistance with fieldwork, Mark Mesarch and Karl Blauvelt for their assistance in instrumentation and data logging, Dave Earl and Sheldon Sharp for their technical assistance and preparation of the measurement tracks, and David Marx and Yuli Xie for their assistance in the statistical tests. Support for the permanent UV-B irradiance monitoring site was provided by the USDA's UV Radiation Monitoring Program.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This research was supported by the Univ. of Nebraska Agric. Res. Div. Interdisciplinary Research Program. Support for the permanent UV-B irradiance monitoring site was provided by the USDA's UV Radiation Monitoring Program. Journal Series no. 12734, Nebraska Agric. Res. Div.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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