Measurements of branch area and adjusting leaf area index indirect measurements

Abstract

Estimates of leaf area index obtained with indirect measurement techniques, which are replacing more arduous destructive sampling methods, are frequently questioned due to light interception by woody elements and a non-random distribution of foliage elements. Usually, branches are assumed to be positioned randomly with respect to leaves or shoots in the canopy. However, in this study of boreal forest architecture, branches are shown to be preferentially shaded by other non-woody elements (e.g. shoots or leaves) in both coniferous and deciduous species of the boreal region. A new instrument called a Multiband Vegetation Imager (MVI) is used to capture two-band (Visible, 400–620 nm and Near-Infrared, 720–950 nm) image pairs of contrasting Canadian boreal forest canopies during the BOReal Ecosystem-Atmosphere Study (BOREAS). The spatial relationship of branches and photosynthetically active foliage is studied to estimate the fraction of the effective branch hemi-surface area index ( B e) that is masked by leaves and shoots. We suggest an approach that corrects indirect LAI measurements using the LAI-2000 or a similar instrument by correcting for the following biases: (1) the effective canopy branch hemi-surface area that is not masked by leaves or shoots in the canopy, (2) the amount of stem hemi-surface area beneath crowns, (3) leaf (or shoot) (Ω e( θ)) and branch (Ω b( θ)) non-random spatial distributions in the canopy, and (4) the fraction of maximum LAI resulting from defoliation in the canopy. In boreal aspen, MVI image analysis shows that 95% of the effective branch hemi-surface area is masked by other foliage in the canopy. In jack pine and black spruce forests, 80–90% of the effective branch hemi-surface area is masked by other foliage in the canopy. These estimates suggest the fraction of indirect LAI that consists of branches intercepting light is less than 10%. Therefore, branches generally do not intercept a significant amount of beam radiation in boreal forests, and do not significantly bias indirect LAI measurements. However, stems, which comprise 30–50% of the total woody area in this study, may not be preferentially shaded by leafy foliage. Therefore, stem contribution to indirect LAI estimates measured with the LAI-2000 or a similar instrument cannot be overlooked. MVI estimates of the total branch hemi-surface area index agree to within 10–40% of direct measurements made in similar species; however, the error between indirect and direct measurements may be due largely to difficulties associated with obtaining adequate sampling so that the error may fall within the noise level of measurements.