Seasonal patterns of light transmission through boreal mixedwood canopies

Measurements of light transmission were made at three heights (ground, 0.5 m, and 1.3 m) in six stands in each of three overstory types: pure young aspen (Populustremuloides Michx.); pure old aspen; and mixed old aspen–white spruce (Piceaglauca (Moench) Voss). Light transmission was measured in the spring, summer, and autumn. In the summer, the amount of light that reached the forest floor was very low in all overstory types (5.9% of incident light). In the mixed stands, the overstory intercepted 87% of above-canopy light, and the understory intercepted 52% of its above-canopy light. In the old aspen stands, there was more light transmission through the overstory, which led to increased understory cover and light interception at that level. In the autumn, 26% of light reached the forest floor in the pure aspen stands because of the loss of deciduous over- and understory foliage. In the mixed stands only 8% of above-overstory light reached the forest floor in the autumn. The deciduous leaf-off period in the spring with relatively high solar elevation angles showed the greatest light transmission through the overstory, ranging from 22% in the old mixedwood to 59% in the young aspen stand. Findings suggest that white spruce seedlings less than 0.5 m in height may be at or below the photosynthetic light compensation point for much of the summer. High light during the deciduous leaf-off periods in the spring and autumn may account for a large portion of their annual carbon gain.