Illinois Forests

From The Changing Illinois Environment: Critical Trends, Summary Report of the Critical Trends Assessment Project

Because it sits at the junction of several continental climate regions, Illinois has a rich diversity of wooded lands. In all, Illinois counts 14 subcategories of upland and floodplain forest plus the less common sand and flatwoods forests; Illinois forests include eight major species associations that encompass more than 30 forest types in all.


In 1820 an estimated 38% of the Prairie State--some 13.8 million acres--was wooded. Within a century only slightly more than 8% of this original forest remained, and today only 11,600 acres, or 0.9%, of the presettlement forest, is left. (Figure 1) Large plots of rarer forest types, such as sand forests, have virtually vanished from Illinois. The removal of the Illinois forest rivaled in pace, if not in scale, the cutting of the tropical rainforests today.

The process continues in some places wherever tillable or buildable land bears trees; for example, 17 of 21 counties lost more than 5,000 acres of forest land between 1962 and 1985 in the farming districts of south central Illinois between Shelbyville and the Shawnee Hills. Ancient Illinois trees generally survive on private land that is too hilly or wet for farming or located too inconveniently for housing; Beall Woods, at 329 acres the largest tract of original deciduous forest in Illinois, stands in the Wabash River bottom.

Hundreds of thousands of acres of pastures and forage cropland were converted to row crops or abandoned beginning in the 1940s as Illinois farmers switched from animal husbandry to row-crop production. Woody plants quickly re-established themselves on abandoned pastures. This secondary growth forest is a patchwork, geographically and ecologically. The shift of land back into forest has been uneven, proceeding briskly in the north, while forests continue to dwindle in the south. Overall, the increase has been estimated to be 41% (1.24 million acres) compared to 1926. The current Illinois forest is about 31% as large as the state's original wooded acreage.

Wooded parcels everywhere in Illinois tend to be small. Only 11% of the 214 "Grade A" and "Grade B" forest sites cataloged by the Illinois Natural Areas Inventory are greater than 100 acres in size. Each of the more than 169,000 private forest owners is estimated to hold only 21.5 acres on average. An analysis of 13 counties in south central Illinois found that the vast majority of "forests" in this region were smaller than one acre in size, the equivalent of backyards with trees.


While the southern Illinois forests still contain commercially attractive populations of oak and hickory suitable for lumber, Illinois imports virtually all of the wood it needs from other states. Nearly two million cords (43% of the annual removal of wood material of recent years) is firewood. Three-fourths of the firewood harvest today is from dead trees, so the harvest poses no particular threat to the resource; it does threaten animal species that depend on snags for breeding and roosting, however, and thus forest biodiversity.

Stands of trees in Illinois are in fairly good health overall, at least compared to those in the southern and eastern U.S. (No direct damage from acid precipitation has been noted in Illinois.) Atmospheric deposition is the primary route of nutrient supply to forest ecosystems over geological time scales. (Illinois' rich loess soils were deposited by wind, for example.) The total amount and proportions of this nutrient chemical "rain" is certainly different today than it was 10,000 years ago, and the impact of this change is as yet unknown, but the potential to affect forest ecosystems exists.

Recent field surveys found a relatively high incidence of crown dieback in white oak and sugar maples--a potential sign of stress for which researchers have no explanation. Mortality rates since 1965 have increased, but the increase is thought not to signal general ill health among forests trees; instead, it reflects catastrophic losses among one species--the American elm--due to Dutch elm disease and the general aging of the forest. (Figure 5-2)

Figure 2. Forest Growing Characteristics, All Species

Source: The Changing Illinois Environment: Critical Trends. Volume 3: Ecological Resources. Illinois Natural History Survey, 1994.

While growth rates overall are slowing, Illinois forests still are growing faster than they are being depleted. "Growing stock" in 1985 amounted to 4.8 billion cubic feet (or 21.6 billion board-feet of lumber), which is 40% more than in 1962. Overall annual growth exceeds timber removals by 40%, so that (barring other changes) the volume of wood in Illinois forests will continue to increase. The historically dominant oak-hickory forest still accounts for half the commercial acreage, but it is giving way to the maple-beech (mainly sugar maple) forest that is less biologically diverse and has less value as a timber resource. The acreage dominated by maples has increased 40-fold since 1962, while oak acreage has decreased by 14% and elm acreage has shrunk by half (mainly as a result of disease and bottomland conversion). (Figure 3)


Notwithstanding the shift in acreage toward the maple-beech forest, the single most numerous tree in Illinois forests is an elm--the slippery or red elm, a smallish (and thus not commercially valuable) tree that thrives in the understory, untouched by Dutch elm disease. Of the 1.93 billion trees estimated to stand in Illinois forests, roughly 18% are slippery elms.

A total of 508 taxa of woody plants (284 of them shrubs) are found in Illinois forests. Nearly half (49%) of the plant species rare to Illinois are found in its woods, from grasses to orchids. Twelve Illinois native forest plant species are thought to be extirpated.

Perturbations, natural or human, often provide growth opportunities for exotic or non-native weedy shrubs. When trees are downed by winds or disease, certain woody vines such as the Japanese honeysuckle quickly exploit the opening and crowd out any trees that would otherwise repopulate the opening. Defoliation (a common disease symptom) increases the amount of sunlight that falls on the forest floor; this benefits sun-loving species (many of which, unfortunately, are aggressive exotics) and thus changes the species composition of that level of the system.

Most of these plants, such as the amur honeysuckle and the autumn olive, were introduced to Illinois as ornamentals or wildlife habitat. Unfortunately, they thrive in environments lacking the pressures from predators and competitors that keep their spread under control in their own native habitats. Common buckthorn is a pest in northern Illinois woods while multiflora rose is a problem everywhere in Illinois; garlic mustard now is recorded in 41 counties, and probably is present in many more. There are even weed trees--the amur maple, white mulberry, golden rain tree, and tree of heaven. In areas where these aliens thrive, the natural succession of forest plants may be altered and the structure of the forests themselves thus drastically changed.


Illinois forests are the state's unofficial wildlife refuge system. As islands amid the ocean of grain, forests comprise more than 75% of Illinois wildlife habitat, according to one index. Four of five mammals and amphibians and three of five birds need forested land for at least part of their lifecycles; in all, the woods are home to more than 420 birds and other vertebrates.

The gradual loss of biological diversity observed in Illinois forests in recent decades is reflected in the adaptation to--one could almost say appropriation of--the forest by adaptable, "generalist" plant and animal species such as starlings and brown-headed cowbirds. (Some generalist animals of the forest, such as opossum and raccoon, have made themselves at home in towns and villages, too, as have woodland birds like the blue jay.)

Forest edges are rich habitats for plants and animals like deer able to exploit them, but they leave the forest itself vulnerable to aggressive weeds, non-native animal competitors for nest sites and food, and predators. Hunters surveyed in 1991 reported seeing half as many housecats as raccoons in woods in most regions of the state.

Mammals. Many commercially important furbearers dwell mainly or exclusively in forests, including the red and gray foxes, coyote, and raccoon. A 1991 survey of hunter sightings found that the red fox was more common in the north, the coyote and the badger in the south.

Although small rodents are numerically dominant, squirrels and deer are the most conspicuous among mammals in Illinois woods. Their large numbers are less a matter of these species adapting to the Illinois environment as the Illinois environment having been modified--inadvertently--for them. Fox squirrels prefer mature oak-hickory forests near corn and bean fields. Similarly, the white tailed deer, Illinois' largest and most coveted game animal, was so victimized by habitat change and hunting that by 1901 it was considered extirpated in Illinois. The reversion of farm fields to woods, and the recovery of understory in woods where it had been suppressed by grazing livestock, provided deer with a perfect habitat. That change in habitat plus the lack of predators led to a population explosion. The forests of Illinois (including its urban and suburban woods) are now home to more deer than were thought to have been present at settlement; in many parts of the state, deer are a road hazard and garden pest.

Birds. According to a 1991 survey of hunter sightings, turkeys were common where the forests are most dense (mainly in the western counties), while pheasants were common where habitat favors them, mainly farming areas that still have hedgerows.

Some of Illinois' forest songbirds--like warblers and vireos--are neotropical migrants that breed in North America and winter in Central or South America. Surveys of two forests in central Illinois that have been monitored since 1927 and 1949 respectively--one nearly 60 acres in size and the other 1,500 acres--confirm findings from other studies. Larger tracts of woods harbor more different kinds of birds than do small ones, and annual fluctuations in abundance are pronounced. The Illinois data also suggest that while the number of species found in these particular woods is not in decline, the relative abundance of migrants has declined. Neotropical migrants once accounted for more than 75% of breeding birds but as of 1992 make up less than half those numbers on even large wood lots. (Figures 5-4 and 5-5)

Figure 4. Numbers of Bird Species Breeding in Trelease Woods

Figure 5. Relative Abundances of Migrants in Trelease Woods

Source: The Changing Illinois Environment: Critical Trends. Volume 3: Ecological Resources. Illinois Natural History Survey, 1994.

Insects. The number of insects harbored by the virgin forests is unknowable. No counts were done, although reminiscences of travelers and settlers give the impression that greenhead flies, katydids, grasshoppers, and (especially) mosquitoes made their presence known to nonscientists.

It is known that when mature forest is destroyed and is replaced by second-growth forest, dramatically different insect communities move in. Non-native trees provide habitat for insect species not indigenous to Illinois; for example, pine plantations that were planted as part of soil erosion programs in the 1930s, are now home to such pests as the northern pine weevil, pales weevil, and Nantucket pinetip moth. In 1914 the European pine shoot moth was found in Illinois; in its borer form it stunts and disfigures Austrian, Scotch, and red pines in the northern half of the state. In 1979 pine wilt disease, caused by the pine wilt nematode, was discovered in Illinois. The Caroline pine sawyer (a native beetle) serves as the nematode vector in Illinois. The disease has devastated red and Scotch pine plantations, many of which will probably be eliminated from the state by the late 1990s. The larger pine shoot beetle was found in Illinois in 1992; an exotic, it bores into new pine twigs, killing them.

As noted, many of these infestations are exotic in origin. One of historic Illinois' ecological catastrophes--the decimation, beginning in the 1950s, of the American elm forest--was caused by a fungus borne by the smaller European elm bark beetle. The gypsy moth, which has already decimated the forests in the East, has been caught in traps in Illinois (mostly around Chicago) since the mid-1970s. Entomologists worry that, as Illinois is exposed to more international trade, the unintended importation of exotic insect pests will increase in spite of restrictions on imported plant materials.


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