The amount of radiant energy exchanged between two surfaces depends on the temperature difference between them. Most of the sunlight is absorbed by the leaves in the forest canopy, resulting in a layer of air in the canopy region that is warmer than the air close to the ground or the air at the height of the tree trunks. Sun and Mahrt (1995) compared temperatures within a black spruce (
) forest in Canada during midday over a 3-day period of sunny weather in September. They found that the mean temperature in the forest canopy was 27
C; the forest floor was between 19 and 25
C beneath the shade of trees and 25 and 33
C in the sun (Figure 7.11). Hence, there is considerable mixing of air between the sunny and shaded parts of the canopy and between the upper canopy and the boundary layer above it. However, there is less mixing of air between the upper canopy and the subcanopy. In one study of airflow in an old-growth Douglas fir forest, Lee and Black (1993) also found that a temperature inversion occurred on sunny days between the warmer tree canopy and cooler forest floor. This inversion greatly suppressed the vertical movement of air near the forest floor and in the subcanopy.