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Agroforestry: Windbreaks

Agroforestry, discussed briefly in a previous Nature Notes, can be described as a combination of agriculture and forestry. It is designed to maximize yield from the land, especially in areas with limited space. Windbreaks are one of the six accepted agroforestry practices. It is the precise planting of trees so that wind is significantly blocked or reduced by trees. This practice is commonly used in areas that are flat and receive a lot of continuous wind.  
These will often be seen in areas where there is an abundance of farm land, particularly in the western part of the United States. 

A windbreak can be a very effective tool when implemented correctly; as it can break up damaging winds, prevent soil erosion, develop a more favorable microclimate, and divert snow. This tends to be true for areas that are vast and open. A windbreak is designed to divert and filter harsh winds through a hedgerow-type of planting arrangement. To be effective, they need to be made up of a few rows of trees that are variable in height. An example of this would be to plant small shrubs on the exterior, followed by medium height trees, and then tall trees. This keeps wind from penetrating all layers of the windbreak. With this in mind, having a windbreak that is too dense, not allowing wind to filter through, can create excessive turbulence on the leeward side. This can create conditions that increase wind speeds, making them even more damaging.


Windbreaks have uses other than just being utilized for wind. For instance, on the lower eastern shore of Maryland, it is common to find many chicken farms. Many of these farms are very large, and as many know, can be unsightly as well as smelly. It has become more common in recent years to plant medium height evergreens around the chicken houses. This has significantly blocked the smell of these farms, and has also naturalized (to some degree) these large industrial operations.

To fully understand how a windbreak works, one must comprehend windbreak aerodynamics. This can get fairly complex. To keep it simple, as wind moves toward the windward side, air is forced up and over, as well as through the windbreak. This allows the diversion of the high speed winds up and over what is being protected, while allowing the windbreak to filter through some residual wind at a decreased speed. If a windbreak is too dense, not allowing wind to filter through, an excessive buildup of wind at
the top of the windbreak can cause turbulence, pushing wind into what was supposed to be the protected area. If designed correctly, the high speed winds can be pushed 3 to 5 times the height of the windbreak before returning to the original wind speed.

Article and diagram by the Frederick County Forest Conservancy District Board
Nature Note for 3/10/2019