Upland forests occur where drainage is sufficient so that soils do not become saturated for extended periods of time. Water can either run off or percolate through the soil.  The upper canopy is 80% to 100% closed, and sub-canopies of younger trees and shrubs typically exist.  The herbaceous (non-woody) ground layer includes forbs, grasses, lichens, and mosses. Particularly distinctive are the "spring ephemerals" such as bloodroot (Sanguinaria canadensis) and trout lilies (Erythronium sp.), which flower in the spring when light is available before the trees leaf out.

Upland forests occur throughout Illinois. They are classified according to soil moisture. They range from xeric (extremely dry) through mesic (moderate soil moisture) to wet-mesic (wet, but not flooded soil conditions). 
 

• Xeric upland forest occurs mainly in the Shawnee Hills in southern Illinois on sites where the soils are even thinner than those of barrens. The trees have a stunted, gnarly growth form on these nutrient poor, droughty soils. 

The ground cover is comprised of a dense, scrubby layer of stunted trees and shrubs, rather than the prairie plants typical of barrens. These forests usually have few herbaceous plants in the understory. Post oak (Quercus stellata) and blackjack oak (Quercus marilandica) dominate the xeric upland forest.

• Dry upland forest (panorama) grows on steep ridges, along rocky cliffs, and on bedrock outcrops along the Mississippi River and in the Shawnee Hills. Many of these sites were barrens in the past when natural fires were not extinguished by humans. The soils are thin and excessively drained, but the trees are not as stunted as those of the xeric upland forest, which have even thinner soils.  

In addition to blackjack oak and post oak, bur oak (Quercus macrocarpa), black oak (Quercus velutina), chestnut oak (Quercus montana), black hickory (Carya texana) and pignut hickory (Carya glabra) are characteristic. The scrubby understory is not so thick, and the herbaceous layer is better developed than in the xeric upland forest.

Barrens or Dry Upland?
Fire frequency determines wheter dry upland forest or barrens occur in otherwise similar areas. If there is more frequent burning, barrens develop and are maintained instead dry upland forest. 

• Dry mesic forest is the most common type of upland forest in Illinois. The canopy is more open than in a typical mesic upland forest but the trees are by no means stunted. 

The dominant species are white oak (Quercus alba), red oak (Quercus rubra), black oak (Quercus velutina), mockernut hickory (Carya tomentosa), and pignut hickory (Carya glabra). Ironwood (Ostrya virginiana) and flowering dogwood (Cornus florida) occur in the sub-canopy.

• Mesic upland forests grow throughout the state on hilly or sloping areas that are protected from fire. They occur on moderately moist soils through which water moves slowly but does not saturate the soil for significant periods of time. Soil nutrient content is high. Funk's Grove, in central Illinois is an example of a mesic upland forest. 

Dominant trees of mesic forests include red oak (Quercus rubra), sugar maple (Acer saccharum), and Beech (Fagus grandifolia).

Other characteristic trees include a variety of oaks, several species of hickory (Carya sp.), slippery elm (Ulmus rubra), basswood (Tilia americana), paw paw (Asimina triloba), hackberry (Celtis occidentalis), white ash (Fraxinus americana), and blue ash (Fraxinus quadrangulata). Southern Red Oak (Quercus falcata) occurs in mesic forests in the southernmost part of the state. The leaves of these trees are rich in nutrients at leaf fall and decompose quickly, adding nutrients to the soil.

• Wet mesic upland forests grow on poorly drained areas in otherwise well drained uplands. They occur along drainage ways and in seepage areas (areas that are wet, but not flooded). They contain floodplain species such as bur oak (Quercus macrocarpa), American elm (Fraxinus americana), slippery elm (Ulmus rubra), and hackberry (Celtis occidentalis).

The changing makeup of the forest
Prior to European settlement, mesic upland forest was more abundant than dry upland forest. Mesic forests, however, were especially appealing to settlers, who came from forested areas. Mesic upland forests were a prime source of wood for fuel and building materials, and were cleared for cultivation. 

Many mesic upland forests in Illinois have large oak trees with spreading branches, which indicate a formerly more open environment that was maintained by frequent ground fires. Whereas oaks are resistant to fires, sugar maple and other mesic trees are quite sensitive to fire. In particular, ground fires kill sugar maple seedlings and saplings. 

In the absence of annual fires, sugar maple becomes increasingly dominant in the forest, replacing native oak trees in many places. Sugar maple can reproduce in the dense shade of a closed-canopy forest. 

Seedlings of sugar maple often carpet the forest floor, (notice the maple saplings and sprouts in the panorama) where they can survive for years growing very slowly. When a gap in the forest canopy forms because a canopy tree dies, the sugar maple seedlings are well positioned to rapidly grow into saplings and to occupy the gap. 

Oak trees do not have shade-tolerant seedlings that can persist in the understory. Oak seedlings survive only for a short time under heavy shade. They do not reach the sapling stage. Oaks must disperse their seed just after or at the same time as a gap appears in the canopy in order to regenerate. Becuase of this, they do not compete as well as species whose seedlings become established and can grow in the shade. These species have a head start on oaks and other shade-intolerant species.

Dilemma for the future
Prior to fire suppression, oaks were able to regenerate in groves and savannas. With active fire suppression, however, the canopies closed, reducing the light available to seedlings below. As a result, oaks failed to regenerate and sugar maple seedlings thrived. In many groves, the young trees are predominantly sugar maples, and in the absence of fires, they will continue to dominate.

Today the conservation of groves presents us with a set of conflicting land management issues. On the one hand, introduction of controlled burning and perhaps removal of old-growth maples can favor the restoration of oak woodland and the continuance of an ecosystem that was widespread in the last century. This ecosystem is becoming increasingly endangered in spite of attempts to protect it in nature preserves.

On the other hand, especially if the change to sugar maple is far along, restoration to oak woodland may be difficult and potentially destructive to the natural biodiversity of the maple forests.