Woody-to-total area ratio determination with a multispectral canopy imager

Leaf area index (LAI) -- defined as one half of the total green leaf area per unit ground surface area -- can be determined by direct or indirect methods. Three major sources of errors exist in indirect LAI measurements: within-shoot clumping, beyond-shoot clumping and non-photosynthetic components. The effect of non-photosynthetic components on LAI measurements can be described by the woody-to-total area ratio, α; however, no convenient and efficient indirect methods have been developed to estimate α, especially the variations in α with zenith angle theta, α(theta). We describe the development and use of a multispectral canopy imager (MCI) to estimate α and α(theta) by considering the effects of non-random distributions of canopy elements and woody components and the overestimation of needle-to-shoot area ratio on woody components. The MCI, which mainly comprises a near-infrared band camera (Fujifilm IS-1), two visible band cameras (Canon 40D), filters and a pan tilt, was developed to measure clumping index, woody-to-total area ratio and geometric parameters of isolated trees. Two typical sampling plots (Plots 1 and 5) chosen from among 16 permanent forest experiment plots were selected for the estimation of α and α(theta). The non-random distributions of canopy elements and woody components were estimated separately at eight zenith angles (from 0° to 70° in increments of 10°) using MCI images based on the gap size distribution theory. The visible/near-infrared image pairs captured by the MCI were able to discriminate among sky, leaves, cloud and woody components. Based on three methods of estimation, we obtained woody-to-total area ratios of 0.24, 0.19, 0.19 for Plot 1 and 0.23, 0.18, 0.17 for Plot 5. If clumping effects were ignored, α values were overestimated by as much as 21% and 24% at Plots 1 and 5, respectively. We demonstrated that α(theta) varied with the zenith angle, with variations in the range of 3-33% at Plot 1 and 2-65% at Plot 5. A new formula for the precise determination of LAI is proposed.