Assessing bias from boles in calculating leaf area index in immature Douglas-fir with the LI-COR canopy analyzer

Abstract

Measurements of leaf area index (LAI) taken with photometers such as the LAI2000 are biased by boles and branches which stop part of the light from reaching the photometer. We estimated how much of the sky would be obscurred by boles in an immature stand of Douglas-fir ( Pseudotsuga menziesii (Mirb) Franco) that was stem-mapped and of known diameter, height and height-to-live-crown. These yielded gap fractions which were then converted to bole area index (BAI) via the algorithm used by the LAI2000. The mean BAI, as a fraction of the effective leaf area index (LAIe) calculated by the LAI2000, ranged from <0.01 for Douglas-fir at 24 years of age to about 0.1 at 48 years. Comparing readings on Douglas-fir trees defoliated by root rot with readings on fully foliated trees yielded a proportion of 0.41, which included branches. Measurements were also taken on clumps of maple ( Acer macrophyllum Pursh) and alder ( Alnus rubra Bong.) both before and after leaf-fall, yielding corresponding proportions, including branches, of 0.41 and 0.25, respectively. The values obtained this way are probably upper limits. The contributions of boles may be appreciable, particularly in mature unthinnned stands with closed canopies.