Flatwood Forests Within the Southern Till Plain Natural Division in Illinois

During the 1800s as Government Land Office (GLO) surveyors traveled north away from the Shawnee escarpment, they observed glaciated lands with flat uplands and broad valleys. One forest type which they discovered was flourishing on level upland ground. This forest land exhibited a wet nature. The surveyors described these forested lands as “level, second rate” or “flat, wet, second rate” and found those forests to be extensive, occasionally covering more than an entire section.

Now the area through which these surveyors traveled is referred to as the Southern Till Plain Natural Division. The forest type which they found within central Illinois is now called “flatwoods.” A flatwoods is characteristically an oak-dominated community established upon soils derived from glacial till, wind-blown silt known as loess, and from deposits of former glacial lake plains. The flatwood forest’s nature is attributed to its slowly permeable soils, the clayey subsoils and hardpan subsurface which disallow quick drainage after rain events, and which result in perched water tables. During a rainy season the flatwoods surface soil remains saturated or even inundated. Ephemeral ponds form in depressions. However, during a dry season the flatwood forest’s surface soil layer becomes extremely dry. Evaporation and transpiration remove the soil moisture accumulated during rainy times. Water from the subsurface is prevented from restoring moisture to the shallow root zone because the hardpan acts as a barrier between soil layers. Xeric conditions occur. The forest’s vegetative ground layer reveals this situation of seasonal very wet and very dry soils. Apparent is the presence of xeric upland and bottomland species as well as of those species most frequently associated with mesic and even hydric conditions.

An early study (1929) of the plant communities of flatwoods in Illinois indicated that post oak was the most prevalent tree species. Common also were blackjack oak, other oaks and hickories. Forest grasses, forbs, woody vines and shrubs dominated the ground layer vegetation. Additionally, prairie grasses and wildflowers grew in large openings and the forest edges. Plant surveys within the flatwoods have listed mesic forest plant species common in Illinois, such as woodland sunflower (Helianthus divaricatus) and Pennsylvania sedge (Carex pensylvanica). Woody vines of mesic upland forest edge, such as common dewberry (Rubus flagellaris), have been observed. Also having been found is poverty oat grass (Dianthonia spicata) although its habitat is normally glades and dry upland areas. Surveys have listed stout wood reed (Cinna arundinacea), which grows most often in floodplain woodlands, swamps, and damp savannas. Another plant with a preference for wet conditions and full sunlight, needle spikerush (Eleocharis verrucosa), has been observed within the flatwoods. This spikerush can even adapt to shallow water. Among the native prairie species of the flatwoods were Sampson’s snakeroot (Orbexilum pedunculatum) and Culver’s-root (Veronicastrum virginicum).

Within the flatwoods, open areas, associated with large tree canopies, were populated by highly diverse understory species. The large open areas of flatwoods with ground cover somewhat characteristic of prairies may have been dubbed “barrens” by early settlers and travelers. During the early study, discoveries of extensive fire scars marring the old, gnarled trees of an age up to 300 years led scientists to wonder if periodic fire, which is a known requirement for regeneration and growth of fire-adapted species of barrens, is also necessary for maintenance of the floristic character and open nature of flatwoods. Fire disturbances historically maintained the importance of oak species in flatwoods.

Between Interstate 70 and Route 14 lie five flatwoods remnants which have been the subject of research. Situated near the intersection of I-70 and I-57, Lake Sara Post Oak Flatwoods Natural Heritage Landmark (Effingham County), also studied extensively, has received careful stewardship practices. A 1992 study of this flatwoods found it to be well-maintained. In an effort to keep the woods open, land managers had conducted prescribed burns yearly for at least 12 years prior to 1991. A more recent study was conducted about the flatwoods’ response to a fire regime spanning across the years from 1989 to 2011. From that study came a supposition that frequent prescribed fires are necessary to maintain the open aspect of the flatwoods. However, changes found in the forest’s woody understory and herbaceous vegetation, especially the reduction of large saplings, has encouraged scientists to ponder what fire intervals are needed to permit occasional ingrowth and to maintain stability at century time scales.

Sources
Ebinger, John E., et.al., Changes in Woody Understory and Herbaceous Vegetation, Lake Sara Post Oak Flatwoods, Effingham County, Illinois 1989 to 2011. Erigenia, Number 27, Summer 2016.

Flatwood Forests by Illinois Department of Natural Resources.

Taft, John, et.al., Vegetation ecology of flatwoods on the Illinoian Till Plain, Journal of Vegetation Science, 6(5), Oct. 1995.

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For years, Patty Gillespie shared her enthusiasm for language and nature and got paid for it at a public school and at a nature center. Now she plays outdoors as often as she can and writes for the sheer joy of it.