White oak trunk (Quercus alba); photo by Phillip Merritt on Flickr (noncommercial use permitted with attribution / share alike).
Official State Tree of Illinois
Illinois designated white oak (Quercus alba) as the official state tree in 1973, the result of a special poll of 900,000 schoolchildren. All State Trees
The state tree had previously been native oak (adopted in 1908), but there are many species of "native oak" in the state of Illinois, and no particular one had been specified.