Woody encroachment on prairie and savanna remnants (and the importance of annual burns)

Woody encroachment is a major problem throughout prairie and savanna sites in Central North America, and the principal reason for this is insufficient fire. The purpose of this paper is to make a strong case for annual burns on prairie and savanna remnants.

The area being considered is that part of the U.S. where the original vegetation was primarily grassland and savanna. Historically, the vegetation was maintained by frequent fire, either from natural or anthropogenic causes. Native Americans burned, and pioneer settlers burned. When the fire-spewing railroads came, burns continued. When the land was cultivated, woodlots and savannas that remained uncultivated were burned to induce “green-up” for pasture.

Hill country is especially favorable for fire, because fire readily carries to the top. One match at the bottom may be all it takes. Elderly farmers confirm that they burned every year. A fire often got started as an escape from a burn pile. Burns were generally in spring because grass was well cured and burns well. A fire might even be started in January when there is still snow at the top of the hill.

According to a recent analysis by the U.S. Forest Service (Nowacki and Abrams, 2008), in the Eastern U.S. frequent fire continued through the end of the 1930s but gradually diminished in the post-World War II period (see graph). Thus our area had frequent fires until about the mid 1950s. Air photos from that period still show extensive open land, especially in hill country. It takes a while for woody vegetation to get established and take over. However, once enough shade is created, prairie grasses can no longer grow. A “tipping point” is reached after which there is explosive growth of woody plants. Virtually all at once the forest is continuous and grasses are gone. Air photos show that this occurred in our area in the 1960s and 1970s. By the early 1990s most of what had once been open land was now closed. Only scattered remnants were left to provide a poor clue to what the land must have looked like.

Areas burned in the Eastern United States (from Nowacki and Abrams, 2008)

Current burn programs are failing

Why? We are still burning, so what is the problem?

We have too many sites and some of them are not being burned.

Those that are being burned are often on a three-year burn cycle. If woody encroachment is occurring, this is not frequent enough. How often should we burn?

The most extensive research has been carried out by Kansas State University at Konza Prairie in eastern Kansas. Here a long-term (over 30 years) study has been comparing annual burning with other burn cycles. These fire frequency studies have shown that shrub expansion is prevented with annual or biennial fire intervals. A three-year fire interval sometimes maintains a grassland, but not always. And with fire intervals >3 years shrubs consistently become well established and grasslands disappears. The worst offenders are clonal shrubs such as sumac and dogwood. 

Figure from Ratajczak et al. 2014

But many prairies are routinely burned on a 3-year cycle without problems of woody encroachment. How can that be?

Most of these are planted prairies which have been established in former agricultural fields. There is no woody legacy. These fields formerly had corn or soy beans, were often held in crops for many years, the crops were probably Round-up Ready, and at the time the prairies were planted the fields were completely free of woody vegetation. They may have never seen a sumac or dogwood.

Let me emphasize that burning prairies planted on ag fields is quite different from burning prairie remnants.

In conclusion: prairie remnants should be burned every year, or (at the least) every other year. If not, the danger of woody encroachment is real, even on a 3-year burn cycle. There is always the possibility that due to bad weather or other circumstances, a year might be missed in a 3-year cycle. And maybe another year? The Kansas work showed that after 5 unburned years, it is virtually impossible to restore prairie to its former state by burns alone. You will be facing a long siege of cutting and herbicide work to get that site back to its previous state.

How about savannas? The legacy of woody plants is much more significant in savannas, meaning one must be even more careful. Annual burns must be used, and they must be carried out (probably) for the duration of the site.

References (should be found by an Internet Search and can probably be downloaded as PDFs):

Briggs, John M., Alan K. Knapp, John M. Blair, Jana L. Heisler, Greg A. Hoch, Michelle S. Lett, and James K. McCarron. 2005. An ecosystem in transition: Causes and consequences of the conversion of mesic grassland to shrubland. BioScience 55: 243-254.

Briggs, John M., Alan K. Knapp, and Brent J. Brock. 2002. Expansion of woody plants in tallgrass prairie: a fifteen year study of fire and fire-grazing interactions. American Midland Naturalist. 147: 287-294.

Ratajczak, Zak, Jesse B. Nippert, John M. Briggs, and John M. Blair. 2014. Fire dynamics distinguish grasslands, shrublands and woodlands as alternative attractors in the Central Great Plains of North America. Journal of Ecology 102: 1374-1385. [Figure]

Gregory J. Nowacki and Marc D. Abrams. 2008: The demise of fire and “mesophication” of forests in the Eastern United States. BioScience 58: 123-138. [Figure]