Estimation of light interception properties of conifer shoots by an improved photographic method and a 3D model of shoot structure.
ABSTRACT The spherical mean of the shoot silhouette-to-total leaf area ratio (STAR) and the shoot transmission coefficient (c) are two key structural parameters in radiative transfer models for calculating canopy photosynthesis and leaf area index. The standard optical method for estimating these parameters might introduce errors in the estimates for species with flexible shoots and needles by changing shoot inclination relative to its inclination in situ. We devised and tested two methods to address this problem. First, we modified the standard optical method by designing an apparatus that allows shoots to be photographed in their original orientation. Second, we developed a faster, model-based approach to replace photography and tested the results against the established approach. We used shoots of three pine species, Pinus echinata Mill. (needle length ~50 mm), P. taeda L. (~150 mm) and P. palustris Mill. (~300 mm). Values of the parameters simulated by the model were similar to those measured from the photographs. In our data, STAR varied about twofold among the pine species and was ~40% higher in shade shoots than in sun shoots of P. taeda. The transmission coefficient for P. taeda shade shoots was also ~40% higher than that of sun shoots of all three species. We tested the versatility of the model by employing it on shoots of two other pine species (P. strobus L. and P. thumbergiana Parl.) as well as on shoots of Tsuga canadensis L. Carr. and Picea pungens Engelm. Regardless of shoot characteristics, the model generated values of shoot structural parameters similar to those estimated with the optical method. Although species-specific and vertical gradients in parameter values are best for modeling radiative transfer in conifer canopies, our results suggest that, in the absence of adequate data, STAR can be approximated as 0.16 for a wide range of shoot structures. For applications requiring angle-dependent parameterization, our new model facilitates rapid generation of these radiative transfer parameters.
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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 Ecosystem-Atmosphere 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.Aspen Bibliography. 01/1997;
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ABSTRACT: The LAI-2000 plant canopy analyzer (Li-Cor, Inc., Lincoln, NE) was tested at six experimental plots of Scots pine (Pinus sylvestris L.) in central Sweden at peak leaf area in August and after litterfall in October 1990. An independent estimate of leaf area index for August 1990 was obtained based on an empirically derived regression of needle area on stem sapwood area, and the decrease in leaf area between the two measurements was estimated from measurements of litterfall. A strong linear relationship was found between estimates by the LAI-2000 (L(Li-Cor)) and the indirect estimates of leaf area index (taken as half of total surface area) (L). The finding that L(Li-Cor) was considerably smaller than L was explained theoretically. It was shown that if shoots, instead of individual needles, are randomly distributed in the canopy, L(Li-Cor) corresponds to L multiplied by a factor (beta) characterizing the mutual shading of needles on the shoot. The shading factor, beta, was equal to the ratio of spherically projected shoot area to spherically projected needle area, where the spherically projected area is defined as the average projection (silhouette) area taken over all directions in space. The quantity betaL was defined as the shoot silhouette area index (SSAI), and an equation for the relationship between SSAI and the mean silhouette to total area ratio (mean STAR) of shoots was derived. Measured values of mean STAR for Scots pine indicated that L(Li-Cor) corresponds to SSAI rather than L. However, the decrease in leaf area index due to litterfall occurring between August and October was only partly detected by the LAI-2000, possibly because SSAI did not change to the same degree as L, i.e., there was an increase in the factor beta. This hypothesis is supported by data showing a large increase in mean STAR with shoot age.Tree Physiology 02/1994; 14(7_9):981-995. · 2.85 Impact Factor
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ABSTRACT: A stochastic radiative transfer equation for the mean field and its solution for the case of discontinuous vegetation canopies is discussed in this article. The equation set satisfies the law of energy conservation and is amenable to numerical solution by the method of successive orders of scattering approximations. Special attention is given to analytical analysis of the effect of spatial discontinuity on the radiation field. Research indicates that a complete description of the radiation field in discontinuous media is possible, using not only average values of radiation over total space, but averages over space occupied by absorbing elements is also required. A new formula for absorbtance, which extends the formula for a homogeneous case, was obtained for the general case of discontinuous media. The internal as well as the emergent radiation fields were validated using available radiative transfer models (one- and three-dimensional) and Monte Carlo model of computer-generated maize canopy. Additionally, the canopy reflectance simulation is assessed by comparisons with field data from shrublands. In all cases, the simulations compare well with both the Monte Carlo results and the field data.Remote Sensing of Environment - REMOTE SENS ENVIRON. 01/2000; 74(1):125-144.