Leaf area index of boreal forests: Theory, techniques, and measurements

Abstract

Abstract. Leaf area index (LAI) is a key structural characteristic of forest ecosystems because of the role of green leaves in controlling many biological and physical processes in plant canopies. Accurate LAI estimates are required in studies of ecophysiology, atmosphere-ecosystem interactions, and global change. The objective of this paper is to evaluate LAI values obtained by several research teams using different methods for a broad spectrum of boreal forest types in support of the international Boreal EcosystemAtmosphere Study (BOREAS). These methods include destructive sampling and optical instruments: the tracing radiation and architecture of canopies (TRAC), the LAI-2000 plant canopy analyzer, hemispherical photography, and the Sunfleck Ceptometer. The latter three calculate LAI from measured radiation transmittance (gap fraction) using inversion models that assume a random spatial distribution of leaves. It is shown that these instruments underestimate LAI of boreal forest stands where the foliage is clumped. The TRAC quantifies the clumping effect by measuring the canopy gap size distribution. For deciduous stands the clumping index measured from TRAC includes the clumping effect at all scales, but for conifer stands it only resolves the clumping effect at scales larger than the shoot (the basic collection of needles). To determine foliage clumping within conifer shoots, a video camera and rotational light table system was used. The major difficulties in determining the surface area of small conifer needles have been largely overcome by the use of an accurate volume displacement method. Hemispherical photography has the advantage that it also provides a permanent image record of the canopies. Typically, LAI falls in the range from 1 to 4 for jack pine and aspen forests and from 1 to 6 for black spruce. Our comparative studies provide the most comprehensive set of LAI estimates available for boreal forests and demonstrate that optical techniques, combined with limited direct foliage sampling, can be used to obtain quick and accurate LAI measurements. IntroductionThe boreal forest is the second largest biome, and there is increasing interest in this biome for its role in the terrestial carbon cycle. One key to characterizing forest stands is obtaining accurate and meaningful measures of the forest canopy.

Retrieval of vegetation clumping index using hot spot signatures measured by POLDER instrument

“…However, we can note an opposite trend between YJP, OJP, and OBS sites. Indeed, the jack pines display a higher aggregation of needles to shoot, especially the young trees (Chen, Rich, Gower, Norman, & Plummer, 1997). Consequently, for the same amount of foliage in the crown, the probability of light penetration increases and the HDS ratio decreases.…”

Scaling Gross Primary Production (GPP) over boreal and deciduous forest landscapes in support of MODIS GPP product validation

“…The simple formulation of Beer's Law was used to derive FPAR in this study because that is the FPAR algorithm in the Biome-BGC model, and here the modeled APAR was used in combination with the modeled GPP to estimate e g . Particularly in the boreal forest, an equation relating LAI to FPAR that accounted for solar zenith angle and clumping factors would have produced more accurate estimates (Chen, Rich, Gower, Norman, & Plummer, 1997). A 30-day ramp for leaf-on in the case of the boreal hardwood forest cover type is also overly simplistic.…”

A method to account for shoot scale clumping in coniferous canopy reflectance models

“…The canopy structure is then described in terms of the spatial and angular distribution of shoots, and the geometrical and spectral properties of leaves are replaced with those of shoots. This approach, using the annual shoot as the basic structural unit, has long been applied in light interception models (Cescatti, 1998;Nilson, Anniste, Lang, & Praks, 1999;Oker-Blom & Kellomäki, 1983;Stenberg, Smolander, & Kellomäki, 1993) and LAI measurement techniques (Chen, Rich, Gower, Norman, & Plummer, 1997;Stenberg, 1996). A key parameter entering these models is the shoot silhouette to total area ratio (STAR) (Oker-Blom & Smolander, 1988), which is conceptually analogous to the G-function, or the mean projection of unit foliage area, defined for flat leaves (Nilson, 1971).…”