Optical Properties of Coniferous Forests

Researchers: Sampo Smolander, Pauline Stenberg, Elja Arjas

Historically, the theory of radiative transfer has been developed within several diverse fields. Astrophysicists have studied the scattering and penetration of light in interstellar clouds, geophysicists and meteorologists have studied the optical properties of the atmosphere, and nuclear reactor theory considers the scattering of neutrons in the reactor medium.

The fundamental idea is to describe the radiation field within the medium with a partial differential equation. The information needed to pose the problem consist of the scattering phase function of a single scatterer, the density distribution of the scatterers in the medium, and the boundary conditions for the radiative forcing on the medium boundary. Radiative transfer theory has been quite successfully applied to model the reflective properties of broadleaved forest canopies, and via model inversion this has resulted in methods that can be used for mapping forests based on satellite images. However, there is a well known discrepancy between the current models and the measured reflective properties of coniferous forests.

In our study, we have noted that the description of the scattering medium (forest) by a density distribution based approach -- which essentially assumes infinitesimally small scattering objects -- may have some inherent problems when applied to conifers. The geometric structure of conifers is hierarchically clumped in many scales: needles are clumped into shoots, shoots into branches and branches into tree crowns. Such a structure is not easily described by any simple non-hierarchical density distribution. As a first step, we have described the effect of shoot-level clumping to the optical properties of simulated forest canopies by using a homogenous higher-level structure. We also propose a simple modification to the traditional leaf-based models, so that they can be used for conifers, taking into account the shoot-level clumping. We continue the study to explore whether similar ideas will also help us to deal with the higher-level clumping in conifers.

Publications

S. Smolander and P. Stenberg (2001): A method for estimating light interception by a conifer shoot. Tree Physiology 21:797-803

S. Smolander and P. Stenberg (2002): Small scale clumping in conifers changes the relationship between LAI and canopy spectral albedo. Journal of Geophysical Research D - Atmospheres (Under review)

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