It's the time of year when blackened soil covers acres of our natural areas.
Black soil created by prescribed or controlled burns. Stewards love the blackened areas of our prairies, savannas and woodlands. Controlled burns and black soil have many benefits to our native ecosystems. These ecosystems evolved with fire, initially caused by lightning, and also set by indigenous people, who burned to increase forage for game. So now our ecosystems are fire dependent, more healthy when they burn regularly.
Prescribed fires remove thatch, a layer of dead dried plant material, that can prevent seeds from contacting the ground and germinating properly. Fire also releases nutrients to the soil that were formerly tied up above ground in plant material. In the spring, black soil warms up quickly because its dark color absorbs sunlight, encouraging early plant growth. In our wooded areas, fires also set back or kill young woody plants, including thin barked natives like maples, ashes, gray dogwood and elms, as well as invasive species like buckthorn and honeysuckle. The presence of these shade tolerant woody plants makes it difficult for acorns from oaks to sprout, and also makes our woods less flammable. Oak leaves contain tannins that burn easily, encouraging fire, while maple and ashes leaves lack these chemicals, so those leaves are harder to burn.
When the public land surveyors surveyed this area in the 1830's, the most common trees were white oaks and bur oaks; after hundreds of years of fire suppression, the Field Museum reports that our most common tree is non-native buckthorn. Extra sunlight reaching the ground after fires also benefits our native woodland grasses like wood reed grass, silky wild rye, and ear-leaved brome, and forbs like elm-leaved goldenrod and Short's aster that need the dappled light found under oak trees, to flourish.
With the right fuel, prescribed fire is a powerful tool that can even jump wet areas. See above for blackened soil in a sedge meadow, an old oxbow of the middle fork of the Chicago river, where the fire crossed swales and watery low places. Look for blackened soil when you visit or volunteer in the forest preserves, and celebrate the life-supporting burn that gave rise to the black.
Guest column and photo by Eileen Sutter, North Branch Restoration Project.