Effects of Vegetation on Surface Winds

Vegetation is part of the friction surface which determines how the wind blows near the ground. Forests and other vegetated areas are characteristically rough surfaces and thus contribute to air turbulence, eddies, etc. They also have the distinction of being somewhat pervious, allowing some air movement through, as well as over and around, the vegetation.

Wind speeds over open, level ground, although zero at the very surface, increase quite rapidly in the first 20 feet above the ground. Where the surface is covered with low-growing, dense vegetation such as grass or brush, it is satisfactory, for most weather purposes, to consider the effective friction surface as the average height of the vegetation, disregarding the air flowing through it. In areas forested with trees, however, airflow within and below the tree canopies is important.

The leaf canopy in a forest is very effective in slowing down wind movements because of its large friction area. In forests of shade-tolerant species where the canopy extends to near ground level, or in stands with understory vegetation, wind speed is nearly constant from just above the surface to near the tops of the crowns. Above the crowns, wind speed increases much as it does over level ground. In forest stands that are open beneath the main tree canopy, air speed increases with height above the surface to the middle of the trunk space, and then decreases again in the canopy zone.

How much the wind speed is reduced inside the forest depends on the detailed structure of the forest stand and on wind speed above the forest canopy, or as measured out in the open away from the forest. The drag of any friction surface is relatively much greater at high wind speeds than it is with low speeds. At low wind speeds, the forest may have only a small effect on the speed of the wind. For example, a 4-m.p.h. wind measured in the open might be slowed to 2.5-m.p.h. at the same height inside the forest. But a fairly high wind speed in the open will be slowed in the forest in much greater proportion. Thus, a 20-m.p.h. wind might be reduced to 4- or 5-m.p.h. in an 80 foot-tall stand of second-growth pine with normal stocking. The reduction would vary considerably, however, among different species and types of forest. Deciduous forests have a further seasonal variation, because although trees bare of leaves have a significant effect in limiting surface wind speeds, it is far less than when the trees are in full leaf.

Local eddies are common in forest stands and are found in the lee of each tree stem. These small eddies affect the behavior of surface fires. Larger scale eddies often form in forest openings. The higher winds aloft cause the slower moving air in these openings to rotate about a vertical axis, or roll over in a horizontal manner. The surface wind direction is then frequently opposite to the direction above the treetops.

The edges of tree stands often cause roll eddies to form in the same manner as those associated with bluffs. Wind blowing against the stand often produces small transient eddies on the windward side, while those in the lee of a forest are mostly larger and more fixed in location, with subeddies breaking off and moving downwind.

Strong surface heating, as on warm, sunny days, adds to the complexity of these forest airflow patterns. Thermal turbulence is added to the generally turbulent flow through open timber stands as it is to the flow above a closed forest canopy. The flow beneath a dense canopy is affected only slightly by thermal turbulence, except where holes let the sun strike bare ground or litter on the forest floor. These become hotspots over which there is a general upwelling of warm air through the canopy. This rising air is replaced by gentle inflow from surrounding shaded areas. Thermal turbulence on the lee side of a forest stand may often be enough to disguise or break up any roll eddies that tend to form (see Effects of Vegetation on Surface Temperature).