By Chris Steeples

In the 1800s, Illinois’ oak forests once accounted for 60 percent of the state’s tree population. Today, they comprise only 5 percent and are being supplanted by native maple trees and several invasive species.

There are 20 species of quercus, or oak tree, native to the state; Quercus Alba, the white oak, is Illinois’ state tree. Lindsay Darling, a researcher at The Morton Arboretum in Lisle, says those species filled forests in the 1830s.

While the oak population may be shrinking, Illinois’ forests are not. A study by the Illinois Department of Natural Resources found that the state has been slowly reforesting, rather than deforesting, a consistent trend since 1926. Illinois’ forest stock increased from 3 million acres at its lowest level to 4.75 million acres in 2012, according to the Illinois Forestry Development Council.

But how that reforestation is occurring troubles experts in the field. For successful oak reproduction,  oak seedling needs more space to grow than present conditions allow and more light than is often available.

Oak and hickory species used to dominate Illinois’ forest canopy, but now it is mostly maples. The understory is filled with three invasive species: the European buckthorn, the amur honeysuckle and garlic mustard. Illinois’ aging oaks are still producing a bounty of acorns. But the lower light conditions of a thicker maple canopy combined with the overcrowding of invasive woody bush and thorny tree species prevent oak seedlings from growing to their full potential, says Henry Eilers, botanist and retired plant nurseryman.

Read more and listen to Chris Steeples talk with Charles Cannon, director of The Morton Arboretum's Center for Tree Science, about the state of oaks in Illinois.