Measurements of leaf orientation, light distribution and sunlit leaf area in a boreal aspen forest

A new instrument called a Multiband Vegetation Imager (MVI) (Kucharik et al., 1997), which uses a 16-bit charge-coupled device (CCD) camera and filter exchange mechanism to capture 2-band (visible and near-infrared) image pairs of plant canopies, has been used to measure the light distribution over sunlit leaves and indirectly infer leaf area index (LAI), sunlit LAI and leaf angle distribution (LAD) in a boreal aspen (Populus tremuloides) forest during the Boreal Ecosystem-Atmosphere Study (BOREAS). One purpose of this study is to demonstrate that by combining MVI measurements with numerical Monte Carlo simulations of forest canopy architecture, the LAD and sunlit LAI of aspen can be obtained indirectly. Our results show that this boreal aspen stand exemplifies an erectophile LAD, with a mean leaf inclination angle near 70 degrees. We also find that the values of the measured and modeled sunlit leaf area in aspen do not change dramatically for typical northern boreal latitude solar zenith angles (i.e. 30-70 degrees). A major problem with determining the sunlit LAI is deciding what range of light intensities constitute a sunlit leaf because penumbra create a smooth continuum of light intensities over sunlit and shaded leaves in the canopy. Therefore, we show that the upper and lower limits can be placed on sunlit LAT values in aspen by using different leaf illumination threshold levels to determine sunlit and shaded LAI in the canopy. Typically, sunlit LAI values in aspen (LAI=3.3) range between 0.8-1.0 at a 70 degrees sun zenith angle and 1.1-1.6 at a 30 degrees sun zenith angle. Monte Carlo simulations and MVI measurements suggest that canopy sunlit leaf area estimates are possible from below the canopy at modest LAI values (<similar to 4.0) but become unreliable in higher LAI (>5.0). Since a substantial fraction of the total sunlit leaf area can be viewed from below the canopy in aspen (40-60% of the total sunlit LAI), a representative light distribution can be measured and used to quantify the canopy leaf angle distribution. (C) 1998 Elsevier Science B.V. All rights reserved.