Autumn has always been my favorite season. Oaks are my favorite trees. So, it seems fitting in "Oaktober" to write about oaks and oak woodlands
At Nachusa Grasslands and other places in the Midwest, we are trying to restore the health of our oak woodlands. And really, our region is defined by the oaks. The Nature Conservancy calls our eco-region “The Prairie-Forest Border” between the grand prairie to the south and the mixed woodlands in neighboring states to the north. The area was historically maintained as prairie intermixed with oak savanna and woodlands by Native American nations.
Natural plant and animal communities of the region were a direct result of Native Peoples’ use of fire on the land. Without fire, in “The Prairie-Forest Border,” the amount of rain we receive would have yielded dense forests.
Many botanists have defined the various intergrades between savanna, open woodland, and closed woodland by the level of sunlight and density and species of trees. (See https://oaksavannas.org/)
Today, much of our Illinois woodlands have become too shady to allow sun-loving oaks to grow. Shade is the enemy of oaks. Acorns in shade will not thrive after germination and the limbs of oaks will also die if shaded.
How can we tell if the “woods” we are looking at was historically savanna or naturally shady? The presence of large, old oaks with limbs that stick straight out (or used to, but are dead now) indicate the area was likely savanna. An oak growing in full sun without other trees close by will grow limbs horizontally. Oaks that grow close together grow their limbs vertically to reach the sun.
To give the oaks a fighting chance modern prairie restorers make use of controlled burns, as well as actual removal of invasive trees growing under and up into the limbs of the old sentinel oaks. We also remove the non-native bush honeysuckle (and other non-native shrubs) from the understory.
Our region’s historic mixture of prairie interspersed with woodland types — from dry to wet, from open to somewhat shady — had an enormous species diversity of understory flowers, grasses, and native shrubs.
If action is not taken by restorers, the result is a mud forest floor, impenetrable understory with honeysuckle bushes, and an overstory of dead oaks. Elms, maples, and other shade-loving fire intolerant trees move in to take the place of oaks and hickories.
This combination of a solid thicket of invasive honeysuckle and loss of oaks gives hardly any habitat for animals. For example, deer and turkey dislike maple and elm, but they love acorns!
Native oak-hickory savannas and woodlands in the region are disappearing along with bird species that depend on the open structure: red-headed woodpeckers, great crested flycatchers, and eastern bluebirds. Native wildflowers such as kitten tails, wild hyacinth, prairie lily, and starry campion along with grasses such as bottle brush rye, woodland brome, and long-awned wood grass will not grow in heavy shade. And there are the native shrubs: hazelnut, American plum, hawthorn, and Iowa crabapple! None of these tolerate shade either.
I encourage you to read up on oak woodlands, then take a hike. Explore the Stone Barn Savanna and see how we are doing. It’s a work in progress and sometimes messy, but we see the native oak savanna dependent plants and animals are thriving.
Here are some great websites that discuss oak savannas:
Pleasant Valley Conservancy—Oak Savannas
Last of the Oak Savannas Survive in Minnesota
What is an Oak Savanna?
Written by Susan Kleiman, a Nachusa volunteer.