Measurements of light transmission were made in spring, summer and autumn in three overstory types: young aspen (Populus tremuloides Michx.); old aspen; and mixed old aspen-white spruce (Picea glauca (Moench) Voss). The spring, with relatively high solar elevation angles and the least shading, showed the greatest light intensitites penetrating all understory levels in all overstory types. At ground level, low light intensities prevailed in the summer, suggesting that white spruce seedlings may rely on high light conditions during the spring, and also in the autumn in pure aspen stands, for a large portion of their annual carbon gain. In a second study, percent light transmission was measured through individual crowns of white spruce and aspen over a range of solar elevation angles. Overall, white spruce intercepted approximately six times as much light as aspen. For both tree species, light transmission increased with decreased solar elevation angle.