MEASUREMENTS OF BRANCH AREA AND ADJUSTING LEAF-AREA INDEX INDIRECT MEASUREMENTS

Estimates of leaf area index obtained with indirect measurement techni ques, which are replacing more arduous destructive sampling methods, a re frequently questioned due to light interception by woody elements a nd anon-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, bra nches are shown to be preferentially shaded by other non woody element s (e.g. shoots or leaves) in both coniferous and deciduous species of the boreal region. A new instrument called a Multiband Vegetation Imag er (MVI) is used to capture two-band (Visible, 400-620 nm and Near-Inf rared, 720-950 nm) image pairs of contrasting Canadian boreal forest c anopies during the BOReal Ecosystem-Atmosphere Study (BOREAS). The spa tial 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 a pproach that corrects indirect LAI measurements using the LAI-2000 or a similar instrument by correcting for the following biases: (1) the e ffective canopy branch hemi-surface area that is not masked by leaves or shoots in the canopy, (2) the amount of stem hemi-surface area bene ath crowns, (3) leaf (or shoot) (Omega(e)(theta)) and branch (Omega(b) (theta)) non-random spatial distributions in the canopy, and (4) the f raction of maximum LAI resulting from defoliation in the canopy. In bo real aspen, MVI image analysis shows that 95% of the effective branch hemi-surface area is masked by other foliage in the canopy. In jack pi ne and black spruce forests, 80-90% of the effective branch hemi-surfa ce area is masked by other foliage in the canopy,These estimates sugge st the fraction of indirect LAI that consists of branches intercepting light is less than 10%. Therefore, branches generally do not intercep t a significant amount of beam radiation in boreal forests, and do not significantly bias indirect LAI measurements. However, stems, which c omprise 30-50% of the total woody area in this study, may not be prefe rentially shaded by leafy foliage. Therefore, stem contribution to ind irect 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 simil ar species; however, the error between indirect and direct measurement s may be due largely to difficulties associated with obtaining adequat e sampling so that the error may fall within the noise level of measur ements. (C) 1998 Elsevier Science B.V. All rights reserved.