Chicago boasts a diverse variety of trees. After the widespread invasive Buckthorn, the top ten hold only a few percent each, followed by dozens with one percent or less. Some of the most common Chicago tree species are: European Buckthorn >25%, Green Ash 6%, Boxelder Maple 6%, Black Cherry 5%, American Elm 3%, Sugar Maple 3%, Other 52%|
European Buckthorn (Rhamnus Cathartica): Elliptical 1-4in leaves, grey-brown bark and spiny branches, shrubby up to 33ft, tiny flowers, clusters of small fruits, toxic, the most common Chicago tree.
Green Ash (Fraxinus Pennsylvanica): Large oval leaves with several per twig, smooth gray bark, 40-80ft, leaves turn yellow early, many small seed pods.
Boxelder Maple (Acer Negundo): Serrated 2-4in leaflets 3-5 per twig, smooth bark and scaly trunk, 33ft, winged seed pods.
Black Cherry (Prunus Serotina): Long shiny leaves, broken dark grey to black bark, 50-80ft, tiny white flowers, cherry-like fruit, long lived.
American Elm (Ulmus Americana): Large serrated leaves, small flying saucer seeds, scaly bark, can reach over 100ft, hardy.
Sugar Maple (Acer Saccharum): Notched lobed leaves, grooved bark, 82-115ft, winged seed pods, bright fall foliage, edible syrup, long lived.
Reader feedback of other common species to expand later: Tree of Heaven, Willow, Pine, Catalpa, Bradford Pear, Ginko, ...
Printable PDF of the above.
Images from PlantGenera.org
U.S. Forest Service Study - Urban Trees and Forests of the Chicago Region (in association with Morton Arboretuem)
City of Chicago quick reference page for a few species.
City Guide to tree selection for planting with criteria matrix.
WTTW Story on Chicago Trees.