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Abstract. The duration and meteorological history of winter and thaw periods in the boreal forest affect carbon exchange during the growing season. Characteristics of conifer canopies exert important control on the energy exchange at the forest floor, which in turn controls snow cover processes such as melting. This analysis investigated the role of the conifer tree characteristics, including height and canopy density. Canopy and snow models estimated radiation incoming to the snow surface, the net energy budget of the snow, and melting rates of snow cover under conifer forests with different canopy density and tree height. This analysis assumed that canopy effects dominated snow surface energy exchange under conifers in the boreal forest. We used data layers of forest characteristics from the Boreal Ecosystem-Atmosphere Study (BOREAS) modeling subareas in Saskatchewan and Manitoba to guide the choice of modeled tree height and canopy density. Modeled stand characteristics assumed random location of trees and used a uniform tree height within a stand and regular crown geometry scaled to tree height. Measurements during winter and thaw in 1994 of incoming solar and longwave radiation, humidity, and wind speed above the forest canopy provided input to the models, along with air temperature measured in the canopy. Results showed the importance of canopy density and tree height as the firstorder controls on cumulative incoming solar radiation at the forest floor for the range of these variables in the BOREAS test area. The combined canopy and snow models showed a large range of snow ablation within conifers, which showed the trade-offs between canopy density and tree height. Solar fluxes dominated the net transfer of energy to the snow in the north, while sensible heat exchange, net solar, and net longwave radiation played important roles in the south.

The complex architecture of conifer forests results in strong landscape heterogeneity with respect to radiation at many spatial scales since a light ray may be trapped in or transferred through any element of the canopy. Currently, a limited understanding of the radiation environment within snow-covered boreal landscapes leads to large uncertainties in the surface radiation inputs used in climate, hydrology, and vegetation interaction studies [Bonan et al., 1995].The purpose of this paper is to pursue an improved understanding of the effects of conifer canopy structure on the radiation regime within and below boreal conifer forests, with the primary motivation being to provide inputs to snowmelt models ( BackgroundRadiation transmission in a plant canopy is affected by many factors, including the source distribution (proportions of incident beam irradiance and diffuse skylight and its spectral properties), the canopy structure, as well as the spectral properties of the canopy elements and the canopy background. The proportions of beam irradiance and diffuse skylight and its spectral properties depend on atmospheric conditions. Radiation 29,555

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