What is a Savanna?
The definition of savanna is an often debated topic among land managers. Simply stated, an oak savanna is an ecosystem consisting of two key layers: an overstory of primarily open-grown oak trees with the canopy cover ranging from 10-60 percent, and a groundcover composed of grasses, sedges, and wildflowers. Prairie fires, which frequently entered the savannas, were critical in maintaining this two-tiered structure. With no sizeable, high-quality remnant savannas in Iowa, we are left with spotty historical accounts, degraded remnants, restoration efforts and our imagination to piece together the great complexity of these savanna communities.
Importance of Fire
Oaks were the predominant tree species of Iowa savannas. Bur oak was probably the most common savanna tree due to its ability to tolerate fire and a wide variety of soil and moisture conditions. The bur oak has several adaptations to fire including an extensive root system; thick, corky bark; and rot resistance even when scarred by fire. A thick, woolly cap protects its acorns. Oaks not only tolerate fire, but their flammable leaves promote it. While less tolerant of fire than bur oaks, white oaks were also a major component of oak savannas. Red oaks, swamp-white oaks, black oaks, hickories and walnuts were present on a more limited basis.
Disappearance of Savannas
Within a 100 years of European settlement savannas were completely wiped out across the state. Many savannas were chosen as ideal sites for homesteads and towns. Savannas flat and rich enough to be farmed were cleared and plowed. Areas too poor for row cropping were often grazed, logged or both.