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Illinois Issues: State’s Oaks Squeezed Out By Other Species

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.

Credit The Morton Arboretum
The Morton Arboretum
White oak leaves

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. 

Credit The Morton Arboretum
The Morton Arboretum
Bur oak acorns

Two further challenges beset struggling oak seedlings: forest fragmentation and an overpopulation of herbivores. Except during hunting season, deer no longer have predators to thin their numbers and so are free to graze on acorns, says Dan Schmoker, a retired IDNR state forester. In some seasons, few if any acorns survive long enough to take root.

Of course, new oak trees do still grow on their own. But sometimes they need a little help from humans to create the conditions necessary for them to thrive. According to the IDNR, 82 percent of Illinois’ forestland is privately owned. These forest fragments are not always well managed, leading to the predominance of invasive plant species in the understory and the crowding out of new oak seedlings, says IDNR. Many of these plots are broken up among a variety of local landowners and uses — sometimes for harvesting hardwood trees such as maple or oak, and some for partial conversion into agricultural or residential property. Another share exists for the aesthetic appeal of having a small, private forest. 

Credit The Morton Arboretum
The Morton Arboretum
Burr oak

Forestry experts agree that oak trees are keystone species — which are species that alter the environment in ways that make their surrounds more habitable for other living things around them. Darling says oak trees provide a habitat for over 500 species of butterfly and moth. Those insects are an important food source for the birds and bats that find shelter in the boughs of the same oak trees. Deer, rabbits, squirrels and birds feed on the abundant acorns oaks produce. Those rabbits, squirrels and occasionally birds are important food sources for foxes and coyotes. Experts believe that oak forests support the most diverse flower populations outside of the jungle rainforests of the tropics. Many of these species evolved to live in the shade of oak trees.

Guy Sternberg, a retired Illinois landscape architect for the IDNR and the founder and director of the Starhill Forest Arboretum near Petersburg, says oak tree roots help prevent erosion and — in conjunction with several symbiotic species of mushrooms —enrich soil and make it more habitable for other plants. 

Credit Guy Sternberg
Guy Sternberg, retired Illinois landscape architect for the IDNR and founder and director of the Starhill Forest Arboretum, stands next to a cherrybark oak at the Cache River State Natural Area in southern Illinois.

According to a 2012 report by IDNR, Southern Illinois University and the University of Illinois, oaks also enrich rivers, streams and lakes. Leaf litter and acorns provide food for aquatic reptiles, fish and insects. Fallen branches provide shelter for fish spawn that might otherwise be devoured by predators. The shade provided by their boughs cools the water and prevents algae and aquatic grasses from making them too warm and murky for other species to survive in. Additionally, oak trees sequester more atmospheric carbon and break down more atmospheric pollution than any other genus of tree.

Oaks are drought and fire resistant, and fire was a fairly common occurrence before major settlement began in 1820s and 1830s, Eilers says. Nearly two centuries of fire prevention has changed the fundamental makeup of Illinois’ forests. Fire not only killed off less-resilient invasive species, it burned through leaf litter that might otherwise prevent acorns from taking root and growing into trees.

Schmoker has mixed feelings about fire as a tool for thinning the understory. He says fire produces a better seedbed for oak seedlings by recycling the nutrients in leaf litter back into the soil and allowing acorns access to that soil. But, he says: “Fire, as a control method for invasive species has very mixed results and sometimes just does not work. It’ll only work with, as an example, bush honeysuckle, if the plants are one-year-old and very tender.” 

If not fire, then what? Schmoker says that most people won’t have the patience for the mechanical route, which is to cut down or dig up the invasive plants repeatedly over several years. Part of the reason oaks are so easily out-competed is that they build extensive root networks in their first few years of life rather than putting that energy into vertical growth, developing the support mechanism that will hold aloft their wide boughs and 80-to-110-foot average height.

The other option, Schmoker says, is herbicide. Used at the right time of year, early spring and early winter, he believes that herbicides can be safely and selectively used to kill invasive plant species and give oak trees the time and space they need to grow.

A comprehensive forest management policy is difficult to achieve, Schmoker says. Under the Illinois Forestry Development Act, state foresters can inspect a private forest and issue a management plan to the owner. However, both the inspection and the plan are voluntary and opinions differ on how to deal with invasive plant species. 

Credit The Morton Arboretum
The Morton Arboretum
White oak

The trend is troubling enough that in August Gov. Bruce Rauner declared that October would be Oak Awareness Month — Oaktober for short. But a statewide oak regeneration policy is, as of yet, not forthcoming. Under former Gov. Pat Quinn, IDNR produced a study on the current state of Illinois’ forests and recommended the revision of property tax rates and zoning laws to encourage better forest management. Making such potentially controversial changes would take manpower, and IDNR reports that over the past decade it has lost 39 percent of its professional staff, 67 percent of technical staff and 86 percent of clerical staff to budget cuts and retirement. 

Meanwhile, the Chicago Region Trees Initiative, a collaboration between The Morton Arboretum, the U.S. Forest and Fish and Wildlife Service, the Chicago Metropolitan Agency for Planning — and several more local, state, and national trusts and nature preservation groups — plans to educate Illinoisans on oaks. Activities include sponsored camping trips, talks, beer or wine festivals and guided hikes. Morton will also host the 8th International Oak Society conference later this month.

“What do we want out of the forest of the future and how do we get it to that point?”  Asks Cannon. “At this moment, we may really need to intervene.”

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