Trees: The Invasion Continues!

by Douglas J. Spencer, Rangeland Management Specialist
Natural Resources Conservation Service (NRCS)
Marion, Kansas

Many travelers drive by pastures in the Flint Hills of Kansas seeking the opportunity to view a vast, open prairie. Unfortunately, the vast, open prairie is continually diminishing due to tree invasion. Lowland range sites are prime examples of this fight the prairie is losing. Why is there such a high victory rate for the trees? Let’s think about it.

Put yourself in the mindset of an invasive tree seed. If the seed could pick the place it wanted to invade, where should it make the attack to have the best chance for survival? The seed would search for a place that is free of competition from other plants, bare soil readily available for optimum seed-to-soil contact, and adequate sunlight to germinate and grow. It would also seek a place with abundant water and nutrients and areas where natural enemies (such as fire) do not frequent.

Where do these conditions exist?

Optimum conditions can exist in several areas of the pasture, but especially in lowland range sites. These sites have deep, nutrient-rich soils and abundant water due to soil capabilities and runoff from the uplands. Lowland sites are often heavily infested with cool-season grasses that do not allow intense fires to frequent and tend to be heavily grazed even though the upland sites in the same pasture are adequately grazed.

Why do these conditions exist?

Animals prefer grazing lowland sites because of the abundant vegetation they produce, especially if cool-season grasses such as brome or fescue have invaded. The lowland sites continue to provide vegetative growth even during dry periods because of the available moisture from the deep soils. If brome or fescue is abundant in the lowland sites and your grazing period extends into September and October, the animals will seek these areas for the green, nutritious fall growth. Another way to view this occurrence is while the uplands are serving broccoli, the lowlands are serving ice cream. Continued heavy grazing in these lowland areas allows for more and more cool-season invaders to dominate. Once cool-season grasses dominate and are heavily grazed, fire from prescribed burns is unable to damage new brush sprouts. Ground disturbance from animal concentration, vehicles, or other means also prevents fire from entering these areas and provides a very hospitable place for brush seeds to germinate. The invasion begins!

Why be concerned?

The lowland sites that these trees begin to develop on are our most productive range sites. Keeping just one acre of this lowland range site productive can provide the equivalent of 2 to 3 acres of upland. Lowland range sites can produce upwards of 6,000 to 8,000 lbs. of air dry forage per acre. An upland range site with moderately deep soils might produce 3,500 to 5,000 lbs. while a shallow upland site only provides 2,000 to 2,500 lbs. of air dry forage. In other words, if you wanted to let 10-acre blocks of trees develop for some other purposes in a pasture, you would have to rent or buy 20 to 30 acres of upland per block of trees to run the same amount of grazing livestock. The grass production lost to tree invasion alone is a big concern, but there are also water quality issues that arise. Once trees develop a shading canopy, livestock will congregate under them seeking relief during the hot summer months. I’m sure we have all seen that tree or group of trees with little to no vegetation around it with manure piles concentrated so heavily that everybody would have to split the pot on cow chip bingo. Having these trees in lowland areas is concerning because rain events can easily wash this denuded area free of manure and topsoil increasing sediment loads and fecal coliform bacteria counts in our surface waters.

How do we stop the invasion?

The best option is to prevent the brush trees from getting started. Proper use of the key forage species in these lowland areas is crucial in order to prevent an invasion. Grazing systems with key rest periods can help accomplish this goal. Proper use of key species also allows prescribed burning to be used to control any young sprouts. If invasive trees are already present, they must be controlled through mechanical or chemical means. When considering controlling these brush species, start as soon as you identify them! As tree size increases, so does cost to control. Contact your local Natural Resources Conservation Service (NRCS) office for technical advice and potential cost share to help stop the invasion! Even if the removal or control of the brush trees is on a selective basis, the benefits can be readily seen as native grasses regain dominance on these lowland sites.

Take a little time this winter or next spring to look for some of these areas in your pastures. Can you make a positive environmental as well as an economic impact on your pasture by stopping the invasion? Make a difference now!

For more information about NRCS programs, visit the Kansas NRCS Web site at www.ks.nrcs.usda.gov.

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Last Modified: 08/20/2008