Oak Savanna

Oak Savanna

You can get a close-up view of a savanna that’s currently being restored by taking a walk on the savanna trail.

Oak savanna is a fire-dependent ecosystem distinguished by widely spaced, spreading trees with a unique group of plants growing below. At Neal Smith National Wildlife Refuge, bur oak is the most common type of tree. Other common savanna trees include red oak, black oak, shagbark hickory and bitternut hickory. These trees provide a dappled shade, so the shorter plants are adapted to a mosaic of sun, shade and partial shade. The savanna understory consists of both sun-loving plants typical of the prairie and plants adapted to the dappled or heavy shade of a forest. In addition, a set of very unique savanna plants live only in these areas. Penn sedge, creamy gentian, purple milkweed and leather flower are some representative savanna plants, which all do best in filtered light.

The low-growing plants under the trees, along with fallen oak leaves, provide fuel for fires that are often slow and creeping, compared to the raging prairie fires. Historically, the grazing, browsing, and rubbing of bison and elk also provided disturbance in oak savannas and influenced the plants’ growth. Deer are the only large, grazing animals living in our oak savannas today. Fire is an important management tool to keep these areas open and to keep out non-native plants.

In the absence of fire over the past 100 years or more, many oak savannas on the refuge have become overgrown with dense stands of fire-intolerant trees. Since fire has not been used in most savannas since settlement times, species such as honey locust, hackberry, and black cherry have grown up. These trees cannot withstand fire when they are young, so even though they are native to Iowa, they did not occur in savannas that were repeatedly burned. Seedlings of these trees are shade-tolerant, meaning they can grow under the canopy of other trees. Oak seedlings require more light to grow, so they are not usually found in oak savannas that have become overgrown with dense trees. Our savannas have many large, old oaks, but few young oak trees ready to replace the old ones as they die.

Restoring oak savannas often begins with removing fire-intolerant trees. This allows light to reach the ground, so that native savanna plants including grasses and sedges can grow. Grasses and sedges burn easily, so their growth makes prescribed burning possible – our most important savanna restoration tool. Oak leaves are also specifically suited to burning, since they curl when they dry, leaving air pockets that help them burn. Thinning the trees also allows more air circulation, which makes burning easier. Fire in turn promotes the growth of grasses, flowers and other plants. Although most oak seedlings will not survive repeated fires, a small number will be able to reach maturity. Oak trees are long-lived, surviving 200 years or more. A healthy savanna will have trees of varying ages so that younger trees will be ready to fill in when old trees die. Even after death, oak trees provide important habitat for many wildlife species, including cavity-nesting birds and bats.

Our oak savannas at Neal Smith National Wildlife Refuge are mainly in the southern half of the refuge (south of the visitor center). You can get a close-up view of a savanna that’s currently being restored by taking a walk on the Savanna Trail. Look closely at the different variety of plants, and try to spot which area has been restored already. See the maps page for more information, or pick up a map at the visitor center.