Windbreaks for Livestock and Wildlife
Much of Utah and the West consists of wide-open spaces unprotected by native trees and shrubs. High winds, cold winter temperatures, hot summer temperatures, blowing snow, and blazing sun are common. All of these conditions can be problems for wildlife and livestock. Tree and shrub windbreaks can provide protection in these wide-open spaces for both livestock and wildlife. Windbreaks ease high winds, reduce wind chill in winter, provide shade and evaporative cooling in summer, and control snow drifting. Beef and dairy cattle, sheep, and pigs have all been shown to benefit from wind protection, especially in winter. For wildlife species that are at least partially dependent on trees or that prefer tree cover, windbreaks may be the only food, nesting, and escape areas available in cropland and rangeland.
How Do Windbreaks Work?
Wind that encounters a windbreak is deflected up and over the barrier, and eventually comes back down to ground level some distance downwind. This distance is typically 10 to 20 times the windbreak's height (H). The size and shape of the protected zone are controlled by windbreak design and placement. Factors that should be considered when designing a windbreak include:
Windbreaks that are dense let very little wind through. This results in a low pressure zone on the downwind side of the windbreak. This low pressure causes the wind going over the top of the windbreak to come down to the ground sooner. The result is a protected zone that is fairly calm but that does not extend very far downwind. Snow deposited behind such a windbreak occurs in a deep drift in the calm area. Windbreaks that are more porous let some air through, reducing the low pressure zone. The result is a larger protected zone that is not quite as calm as with a dense windbreak. Any snow drift tends to be shallower and extends farther downwind. Very porous windbreaks let too much wind through to be effective. Some porosity can be achieved by planting fewer rows, spacing trees farther apart within rows, and using deciduous trees and evergreens with less dense crowns.
Windbreaks protect the widest area when placed at right angles to the prevailing wind direction. For winter wind protection in much of the western U.S. this means having a windbreak to the north or west of the area to be protected, or possibly a combination of both. Wind tends to curl in around the ends of a windbreak, so windbreaks should extend beyond the area or object to be protected.
The cross-sectional shape of a windbreak affects how wind is directed up and over and thus affects the protected area. Windbreaks that start short and become tall (going in the downwind direction) act like dense windbreaks. Most wind is deflected up and over and very little gets through to reduce the low-pressure zone on the downwind side. Wind comes back to ground level fairly soon and the protected area is short. Windbreaks that rise abruptly on the upwind side tend to force some air through, resulting in a larger protected zone.
Windbreaks with more rows tend to be denser. This leads to a very calm zone directly downwind but a short zone of protection.
Since windbreaks protect for 10 to 20 times their height (H) downwind, taller windbreaks will protect a larger area. The zone of protection will extend farther as the trees grow. An area can be completely protected by short or tall windbreaks, but more of the short windbreaks will be needed since they protect for a smaller distance.
Gaps in windbreaks due to dead trees or for road access funnel wind and cause reduced protection. Replace dead trees and angle any planned openings through the windbreak at right angles to the prevailing wind direction.
Designing Windbreaks for Livestock and Wildlife
Though any windbreak will provide some protection, the ideal windbreak for livestock and wildlife will have at least four to five rows. In areas where heavy snow is common a windbreak with fewer rows may fill with snow and not provide enough winter protection. A mixture of large deciduous and evergeen trees in the interior and shrubs in the outer rows provides food, cover, and perch and nest sites. Good protection of livestock is also provided by such a dense windbreak. Snow will accumulate in a drift at about 2H to 3H, though, so any calving, feeding, or other livestock handling area should be located beyond this drift zone.
Large windbreaks benefit wildlife more than small ones, and some species may not even use a windbreak that is smaller than a certain size. One to three acres is a minimum size for some birds, and larger areas may be needed for other species. Several windbreaks that are close together or that are placed to connect other habitat areas are more effective than solitary windbreaks.
Snags are large dead trees that are still standing. Snags and trees with dead tops are useful as nesting, foraging, and perching sites for many wildlife species such as owls and woodpeckers. Snags may occur on their own in older windbreaks or can be created by girdling a few live trees. Nest boxes can also be added to young windbreaks to provide shelter for cavity nesters.
All windbreaks in areas where livestock are present should be fenced. Though livestock prefer the shade of trees in summer, browsing and trampling will ruin windbreak structure and health.
More information on windbreaks can be obtained from your local Cooperative Extension office. Technical assistance and financial help for windbreak establishment is available through the Soil Conservation Service and your District Forester.
Images taken from University of Nebraska Extension publication How Windbreaks Work (EC 91-1763-B).