Illinois Natural History Survey - University of Illinois

Bur Oak

Susan Post

 

                            

When the first settlers gazed westward across the vast prairies of Illinois, bur oaks were the burly trees on knolls and ridges which stood like ships in a sea of grass.

Roberts Mann 

Nature Bulletin No. 708 

Cook County Forest
Preserve, 1963

 

The bur oak is a tree primarily of the midwestern and Great Plains states. Found further north than any other native American oak,  it ranges north to central Manitoba and south to central Tennessee and southern Texas. It is present in all Illinois counties. Bur oaks grow on sites with deep rich soils that are moist but well drained,  as well as dry upland sites. In the heart of its range the bur oak is often found growing by itself, standing alone in a farmer’s plowed field or in a flat bottomland. When grown in the open, the straight trunk supports a broad,  irregular crown of stiff, gnarled branches, with the lower ones decidedly drooping. Under forest conditions, the crown is rather small and the trunk long and clear.

The bur oak even played a role in the Lewis and Clark Corps of Discovery. The Council Oak at Sioux City, Iowa, was a bur oak. Here during the week of August 13–20 in 1804, the tree shaded a meeting with Native Americans.

Its scientific name (Quercus macrocarpa) means oak with a large seed and refers to the large acorns. Its common names include prairie, bur, and mossy-cup, with the latter two referring to the seed, which is enclosed in a knobby cup with a fringed edge. 

Bur oaks have simple,  rounded, lobed leaves with a deep sinus (space between two lobes of the leaf) near the center of the leaf that appears to split the leaf in half. The leaf will have 5 to 7 lobes and is 15 to 30 cm long and 7 to 15 cm wide. The leaves are arranged alternately on the twigs and are dark green and shiny from above. Below, they appear pale and somewhat hairy. Not much fall color is present as the leaves turn dull yellow or yellow green to yellow brown. The twigs are stout and yellowish brown and smooth, and as they age they develop corky ridges. Its acorn is 1.9 to 5.0 cm long and enclosed half or more in a deep cup adorned with conspicuous scales forming a gray-fringed margin. The kernel of the acorn is sweet and provides excellent mast.

The bur oak’s trunk bark is dark brown to gray, rough and deeply ridged. The bark is also thick and resistant to fire, allowing the bur oak to be the most abundant tree in many groves where the forest met the fire-prone prairie.

Paul Strode, a recent University of Illinois doctoral student, used a prairie grove to look at the effects of global climate change on warblers and their food source. An interesting observation resulted. According to Paul, “For centuries, songbirds migrating through east-central Illinois from South and Central America to Canada depended on oaks like the bur oak to supply them with caterpillars as a food resource. By observing the foraging behavior of wood warblers and vireos at Trelease Woods outside Urbana in spring, I found that bur oaks are still a critical tree species for these long-distance migrant songbirds.  Warblers and vireos preferred bur oaks disproportionately to their availability in the forest, and bur oaks were found to contain over twice as many caterpillars per leaf than sugar maple and hackberry, the two most dominant tree species in the forest.  My results are a strong indication that conserving bur oaks in our Illinois forests would help to conserve populations of wood warblers and vireos.”

 

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