• Prairie woodlands occupy regions where soil moisture is high enough to sustain trees.

Prairie woodland trees include

• Bur Oak

• Green Ash

• Bur Oak (Quercus macrocarpa) has an extensive root system and thick bark that enable to survive long droughts and prairie fires.

Oaks are monecious: male and female reproductive organs reside on the same tree, but in separate flowers.

Male flowers hang in clusters called catkins; pollen is carried by wind to female flowers.

After fertilization, fruits called acorns are produced.

• Green Ash (Fraxinus pennsylvanica) are dioceous: male and female reproductive organs reside on separate plants.

Male flowers are often infested by parasitic mites, forming ash flower galls.

After fertilization, female flowers produce fruits called samaras that spin in the wind.

Other uninvited feeders include the Periodical Cicada, whose nymphs feed on tree roots for 17 years before emerging through the soil and molting into adults. Females then oviposit in a tree branch.

• The eastern portion of the tallgrass prairie grades into Northern and Southern Prairie-and-Oak Transition zones.

Here oaks and other hardwoods (flowering trees) can form widely spaced woodlands called savannas (or groves).

• White Oak

• Shagbark Hickory

• American Linden

• White Oak (Quercus alba) is the Illinois state tree.

Several species of insects parasitize oaks, producing galls on the leaves.

The Acorn Weevil is a beetle that lays eggs in acorns; after the larvae finish their development in the nut, they exit through the shell and burrow into soil to pupate.

The half-eaten acorn is often then parasitized by the Acorn Moth caterpillar.

• Riparian woods form along the banks of rivers and streams.

• Cottonwood

• Box Elder

• Silver Maple

• Black Walnut

• Kentucky Coffeetree

• Cottonwood (Populus deltoides) is dioceous: male and female reproductive organs reside on separate plants.

Males produce pollen; females produce eggs that, if pollinated (fertilized), yield fruits.

Sometimes a "delta"-shaped leaf may host a parasitic leafminer who spends its larva stage within the epidermal layers, feeding on the mesophyll cells.

Juicy buds may be nibbled by Ruffed Grouse, perching precariously on the branches.

• As trees encroach prairie, many grassland birds such as Western Meadowlark are declining.

Woodland birds such as the following are becoming more common.

• American Robin

• Tree Swallow

• House Wren

• Mourning Dove

• The increase of trees in grasslands (as well as fragmentation of existing woodlands) has created more boundaries (ecotones) between ecosystems.

Some species are edge specialists; they thrive along such ecotones.

• Brown-headed Cowbird

• Fox Squirrel

• White-tailed Deer

• Brown-headed Cowbirds are brood parasites and thrive in the ecotones between grasslands and woodlands.

Females lay their eggs in the nests of other birds; the young are reared by the host birds.

The increase of these brood parasites may be a factor in the decline of grassland birds as well as songbirds that need deep woods.

• Fox Squirrel is an open woodlands dweller, equally at ease on trees and on the ground.

They feed on tree seeds such as Black Walnut fruits and pine cones.

• White-tailed Deer like to feed out in open meadows.

When danger is sensed, they raise their white, flaglike tail as a warning signal and dart into a nearby woods for shelter.

The decrease of grasslands and increase in woodlands in the Great Plains has been accompanied by rising populations of White-tailed Deer and declining numbers of the Mule Deer, which prefer more open country.