The Science of Prairie Burning

Fire is one of the most destructive forces that can wreak havoc on our communities. Forest fires and house fires threaten tenants, destroy sentimental belongings, and ruin everything in their wake. Although we typically view large fires as unruly and harmful beasts, there is one case in which fire actually rejuvenates the environment: prairie burning.

I experienced my first prairie burning as a confused first grader who had just finished lunch (trading the healthy stuff away in exchange for potato chips) and wasn’t allowed to go into the field during recess. As we were all herded outside for recess by chaperones with bullhorns, I ran to keep up with my friends, but then noticed that the field behind the playground was on fire. A big fire. Alarmed, I noticed that none of the adults seemed to be doing anything about it, and that seemed odd to my seven-year-old self. I approached one of the playground chaperones and told her the field was on fire – as if she somehow didn’t already see it. She told me they were burning the prairie, that they do it every year, and not to go near it. She put extra emphasis on that third point, but I was more interested in the first two. Sitting on the swings a few minutes later, I watched the smoke rise from the tall grasses, still very confused. Over the years, I witnessed seasonal prairie burns and gradually learned that the burns were counter-intuitively conducted in order to maintain the prairie behind the school.

UW Arboretum staff manage a prescribed prairie fire burn at Greene Prairie during autumn. © UW-Madison University Communications, Photo by: Jeff Miller

The Midwest is filled with prairies, and each year these prairies must be carefully maintained to keep them safe and healthy. Oddly, burning them down is an effective way to keep prairies healthy in the long term. Each spring and each fall, a certified team of Burn Managers checks the weather for precise conditions and ignites a contained and highly-controlled fire in Midwestern prairies.  The goals of lighting these fires are to restore nutrients to the soil by removing them from dead organic matter, to prevent invasive species from out-competing native prairie plants, to remove dry lingering “tinderbox” plant matter, and to maintain a grassy environment where woody trees would otherwise takeover.

Prairie Grass Root Systems

Here’s why prairie burning is important:

1. If dry plant matter builds up on the prairie floor, a spontaneous and uncontrolled fire is more likely to break out. Removing dead organic matter by burning reduces this likelihood and returns the nutrients to the soil at the same time.
2. Burning removes invasive plant species, which would otherwise takeover and out-compete native plants for nutrients.
3. A grassy environment would naturally transition into a wooded forest over time. Burning helps maintain the grasslands so that animals living there will still have a home.
   

   Prairie Dog Burrow

Speaking of animals having a home – What lives in a prairie, and where do the animals go during the burn?

Many of the animals living in prairies, such as prairie dogs, are burrowers.  They live in self-dug tunnel systems under the ground, and they will pop down into their subterranean homes for safety during the burn.

Fun fact: My elementary school’s mascot was the prairie dog.

Prairie Dogs

For safety, a burn is only prescribed and conducted under precise weather conditions; this also helps the animals survive.  To begin a burn, the wind must be under 20 mph, the humidity must be 30-55%, and the temperature should be between 20-60 ºF.  Too much wind would cause the fire to burn out of control. Too much humidity would make it hard for the fire to spread where it needs to, while too little humidity would make it spread too fast.  The temperature affects how well the fire can be controlled as well.  Fires cannot be set during droughts.  In fact, it’s important for soil to be wet during a prairie burn in order to protect plant root systems and animals that have burrowed into the ground. If the soil is not wet enough, the plants will not grow back, and the animals’ habitat would be destroyed.

Prescribed Prairie Burn. Flanking head fire at Dahms Suck Restoration. Photo by Chris Helzer/The Nature Conservancy.

In addition to the animals, the main plant in a prairie is tall grass. Some of the grasses reach taller than 6 feet high, so it’s pretty easy to get lost if you’re not on a trail – especially as a tiny first grader. Some varieties of grasses most commonly found in prairies in the Midwest include switch grasses, blue grasses, and Indian grasses.

Do you live near a prairie? Have you seen a controlled burn? There are many prairies around Illinois, so go check out the plants that live there and see how quickly they regenerate after a burn.

Prairie Grassland

Disclaimer: Never, ever start a burn yourself. An uncontrolled burn is extremely dangerous and can spread quickly – causing damage to natural habitats, peoples’ homes, and most importantly, you.