The vegetation assemblage that I call "Sand Hill Savanna" is about an acre and a half of tallgrass prairie vegetation (a variety of forbs, otherwise known as wildflowers, and grasses) and scattered trees, mostly oaks and hickories, located near my home on the west half of the property, near Havana, Illinois. I planted the first small patch of prairie about fifteen years ago, enlarging it to its present size about eight years ago, while occasionally adding species as opportunities arose. Fire is the main tool used to maintain the prairie and keep the always encroaching woody vegetation in check. It is well known that given Illinois' overall climate, without fire, prairie can rapidly be overtaken by trees and shrubs (White 2001; Taft, Anderson, and Iverson 2009; Betz 2011).
Even with periodic fires burning off all of the above-ground grasses and forbs and top-killing small trees and shrubs, everything resprouts again from root systems the following year. This process of trees and shrubs resprouting from the intact root systems that are undamaged by fires is normal and, in fact, has long been noted, as ecologist John White illustrated when quoting from an 1836 guidebook for immigrants to Illinois (White 2001):
"When the fires are stopped, these barrens produce timber...Dwarfish shrubs and small trees of oak and hickory are scattered over the surface, where for years they have contended with the fires for a precarious existence, while a mass of roots, sufficient for the support of large trees, have accumulated in the earth. As soon as they are protected from the ravages of the annual fires, the more thrifty sprouts shoot forth...."
This process has drawn my special attention because it brings the human influence into the equation, not just on my prairie-savanna restoration, but within the context of the grassland ecosystem that once covered millions of acres in North American for thousands of years. That is because this ecosystem was maintained by fire caused by either lightning strikes or purposely set by the Native Americans.
Just how important were these fires in keeping trees and shrubs from overtaking the prairie is illustrated by how rapidly and easily the top-killed woody plants resprout after a fire. To see this effect, I photographed selected woody plants on my prairie-savanna as the resprouts first appeared in spring, following a fire the previous fall, and then later in the growing season when regrowth had mostly reached the final state for the season.
The following photographs show the same plants on April 21, 2015 and August 8, 2015 (click on each photograph to enlarge).
Figure 1. Black oak, April 21, 2015.
Figure 2. Black oak, August 21, 2015.
Figure 3. Black walnut, April 21, 2015.
Figure 4. Black walnut, August 8, 2015.
Figure 5. Hazelnut, April 21, 2015.
Figure 6. Hazelnut, August 8, 2015.
Figure 7. Pignut hickory, April 21, 2015.
Figure 8. Pignut hickory, August 8, 2015.
Figure 9. Smooth sumac, April 21, 2015.
Figure 10. Smooth sumac, August 8, 2015.
Figure 11. American elm, April 21, 2015.
Figure 12. American elm, August 8, 2015. Of all the species illustrated above, elm is the most sensitive to fire. While elm trees can be found in a variety of habitats, historically, elms, tolerant of flooding, were most common on floodplains or other moist environments (Adams and Anderson 1980), where landscape-scale fires were probably rare events.
Sand Hill Savanna is small and surrounded by woods that support several aggressive species (non-native black locust, Siberian elm, white mulberry, and bush honeysuckle; native sassafras and black walnut) that would be problematic in any case even if it were not for the high perimeter-to-area ratio that ensures the savanna is easily invaded from the outside. It is clear that with any relaxation on my part, as far as controlled fires, the prairie vegetation would soon lose out to woody invasion. This illustrates one of the challenges involved in maintaining or restoring prairie and savanna habitats in Illinois.
Selected References
Adams, D.E. and Anderson, R.C. 1980. Species responses to a moisture gradient in central Illinois forests. American Journal of Botany 67(3): 381-392.
Betz, Robert F. 2011. The prairie of the Illinois Country. DPM Ink, Westmont, Illinois.
Taft, J.B., R.C. Anderson, and L.R. Iverson. 2009. Vegetation ecology and change in terrestrial ecosystems. Pages 35-72 in C.A. Taylor, J.B. Taft, and C. E. Warwick, editors. Canaries in the catbird seat: the past, present, and future of biological resources in a changing environment. Illinois Natural History Survey Special Publication 30. Champaign, Illinois.
White, J. 2001. A survey of native vegetation in the Big Rivers Blufflands of Calhoun, Greene, and Jersey counties with recommendations for protection, restoration, and management. Report to the Illinois Department of Natural Resources. Ecological Services, Urbana, Illinois.
Note: Additional background on Sand Hill Savanna can be found by following this link and this link.