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Windbreaks: trees as technology for protecting Wisconsin’s topsoil (Research Brief #22)

Posted October 1996

We need a lot more protection of erosion-prone land than we have. Windbreaks are an important component of this effort to control wind erosion. — Clive David, forestry researcher, University of Wisconsin-Stevens Point

Strong winds can stir up trouble for farmers in the Central Sands region of Wisconsin. Every year, according to area estimates, high speed gusts sweep several million tons of soil from cropland, scattering seeds and sandblasting fields. To protect against these windy forces, farmers can use nature to their advantage by planting or maintaining windbreaks. These long stretches of trees surrounding fields create natural barriers to wind that can reduce soil erosion, protect crops, and increase crop yields.

Despite this low-tech solution to wind erosion, windbreak numbers have fallen in the Central Sands area in recent decades because of tree damage, disease, or removal to accommodate center-pivot irrigation, says University of Wisconsin-Stevens Point forestry researcher Clive David. The result, says David, is that the region has lost track of one of its best natural defenses against wind. “We need a lot more protection of erosion-prone land than we have,” he says. “Windbreaks are an important component of this effort to control wind erosion.”

With support from the Center for Integrated Agricultural Systems (CIAS), David and his research team studied windbreak populations in Portage County, the Central Sands county where wind erosion is the most severe. After taking inventory of this area’s windbreak population, David’s team concluded that windbreaks in the region are in fair to good condition, but that more and better-maintained trees are needed to prevent wind erosion.

Taking stock of area windbreaks

In 1990, the Wisconsin Legislature created the Central Sands Wind Erosion Control Project to devise wind erosion strategies for Adams, Juneau, Portage, Wood, and Waushara counties. This five-county region loses an estimated 2.2 million tons of soil from cropland every year due to wind erosion, which is more intense during the spring when winds are stronger and cropland soils are exposed. Although windbreaks are considered an important part of soil conservation, since the 1930s no studies have assessed the windbreak population in the region.

David’s research team–which included Robert Brush, of the UW-Stevens Point forestry faculty; George Greil, a wind erosion control planner; and graduate student Vicki Rhyner–evaluated windbreak populations in Portage County using aerial photography and field measurements. The team also got advice in project design from area growers.

Researchers identified stretches of windbreaks that line agricultural fields. They mapped the size, number, orientation, and type of windbreaks throughout the 800-square-mile county. They also appraised a subset of windbreaks in the field by measuring tree height and width; gaps and spacing; porosity; and tree health and condition.

Profile suggests protection by default

The researchers identified 2,632 windbreaks covering 521 miles in Portage County and evaluated the condition, type, and maintenance of the windbreaks as well as their potential effectiveness against wind. The research team concluded the following:

  • The eastern half of the county was more densely populated with windbreaks compared to the western half. This may be due to windbreak removal in the western portion to accommodate center-pivot irrigation machinery.
  • Approximately two-thirds of the windbreaks evaluated were of natural origin, implying the majority of trees grew on their own rather than through managed plantings.
  • Northern pin oak, burr oak, boxelder, and cherry made up about two-thirds of windbreaks in the county. Conifers such as red pine, jack pine, and white pine made up a smaller amount.
  • Windbreaks averaged about 30 feet high and 30 to 32 feet wide.
    Less than half of the windbreaks (45 percent) were oriented north-south, a direction which cuts strong westerly winds in spring.
  • The researchers estimated that the current population could potentially protect between 11 to 68% of a field, based on a conventional field size of 10 to 160 acres.
  • The researchers rated windbreak conditions in the county as good to fair, based on tree health and growth. Windbreaks in the eastern half of the county were rated in better condition than the western half.

These data suggest that although windbreaks in the region offer protection, more and better maintained windbreaks are needed. David says county specialists recommend planting 800 additional miles of windbreaks for good wind erosion protection. “Even if that assessment is high, it is still indicating we need a lot more windbreaks than we have,” he says.

Portage County may also need to maintain existing windbreaks better, says David, pointing out that trees can deteriorate in several years unless well-managed. His study found that the majority of the region’s windbreaks grew naturally, rather than through managed plantings. “The implication is that two-thirds of the windbreak populations in Portage County are unmanaged, perhaps ignored,” says David. Although natural windbreaks can be just as effective as planted ones, a good maintenance strategy can mean the difference between using nature effectively and haphazard protection.

Eric Hurley, a Portage County wind erosion specialist, says the study reveals the scope of the problem and helps specialists target their efforts. “This study helps us understand what is happening to windbreaks over time in the county and provides us with descriptive tools to use in windbreak inventories in other counties.” He works with the Central Wisconsin Windshed Partnership to devise long-term solutions to wind erosion.

Windbreaks part of larger effort

What does this study mean for central Wisconsin growers? David hopes the research helps farmers become more aware of soil erosion and helps them devise strategies for conserving soil. Wind erosion may seem like a small problem, he says, but it may be damaging over the long-term. For instance, in one year, a grower may lose a tiny fraction of topsoil due to wind. Spread over an acre, that small loss could amount to 18 to 19 tons per year. “Taken over the long term–say, 8 to 10 years–such losses could translate into a real problem for the landowner, since it is the more important and productive fraction of the soil that is being lost during wind erosion,” David says.

What a farmer does about soil erosion depends on his or her philosophy, farm size, and costs, he says. Based on existing data, the expense of planting windbreaks are offset in 15 to 18 years through improved soil and increased land values. That may seem like a long time, he says, but it is a strategy that may pay off in the end.

Used in conjunction with farm practices such as contour planting, minimal tillage systems, cover crops, and crop rotation, windbreaks offer long-term strategies for preventing soil loss and protecting crops. These strategies, which are part of a systems approach that takes into account farming practices, economics, and land ethics, suggest using nature as a technology can protect soil and land resources in low-tech ways.

Table 1. What are some features of an effective windbreak?

  • Height. In general, the taller the windbreak, the more area it will protect downwind.
  • Length. Longer stretches of trees can protect larger areas of cropland.
  • Orientation. Effective windbreaks should lie perpendicular to the approaching wind.
  • Gaps. Denser windbreaks are more effective than sparse ones. Large gaps between trees reduce windbreak effectiveness and, in some cases, increase wind speeds by channeling air through.
  • Porosity. This is a measure of thickness and pore openings of the wind barrier. An effective windbreak cuts wind speed but still allows some air to pass through.
  • Species. Conifer, hardwood, or mixed species can affect windbreak growth and stability.
  • Condition. This includes tree age, health, vigor, crown condition, species diversity, and maintenance.
  • Origin. Whether the stand grew naturally or was planted influences tree condition and maintenance.

For more information about the study, contact:

Clive David
University of Wisconsin-Stevens Point
College of Natural Resources
Stevens Point, WI 54481
Phone: (715) 346-4552

Published as Research Brief #22
October, 1996