Effects of Vegetation on Surface Temperatures

In all situations, vegetation moderates air temperatures within the vegetative layer for several reasons. First, it intercepts both incoming and outgoing radiation and therefore has a marked effect on ground temperature; second, green foliage does not warm up as much as ground or dry litter; and third, leaf surfaces exchange heat with air through a deeper, less restricted boundary layer. These effects result in less pronounced temperature changes with height above the ground.

In all vegetative cover, the temperature distribution depends upon the nature and density of the vegetation. With plants, such as low brush, the leaves form a nearly continuous upper surface, and this surface acts as the effective ground surface. The maximum daytime temperatures and minimum nighttime temperatures are near the top of the brush or dense plant cover, although temperatures near the ground are not greatly different.

The crowns of trees in a heavy forest form a nearly continuous cover and the canopy thus becomes, in effect, the air contact surface. The highest daytime temperatures are found near the crown top, and the temperature will decrease gradually between this level and the ground. Maximum air temperatures near the crowns may be 18° to 20° warmer than air temperature near the ground. Above the tree crowns the temperature decreases fairly rapidly with height, although never as rapidly as over bare ground. This is because the temperatures of the tree crown surfaces in contact with the air are lower than bare ground, and because the air circulation around these surfaces is better.

Less dense vegetation will permit more solar radiation to penetrate to the ground than will a dense cover. The degree of partial ground shading provided by less dense vegetation determines the air temperature distribution between the ground surface and the canopy top. It will range between that found over bare ground and that under a closed canopy.

Air temperatures at the standard 4 1/2-foot height within the forest in the afternoon are likely to be 5° to 8° cooler than the temperatures in nearby cleared areas. Openings in a moderate to dense timber stand may become warm air pockets during the day. These openings often act as natural chimneys and may accelerate the rate of burning of surface fires that are close enough to be influenced by these "chimneys."

Night temperatures in dense timber stands tend to be lowest near the top of the crown where the principal radiation takes place. Some cool air from the crowns sinks down to the ground surface, and there is some additional cooling at the surface by radiation to the cooling crowns. Sparse timber or other vegetation will merely decrease the strength of the inversion just above the ground surface.