Ancient trees adorn many towns in Europe and South America: broad old Sycamores in England, venerable, twisty olive trees in the Iberian peninsula, straight, majestic Ceibas in Manaus, Brazil.
Evanston still has some old trees, too, despite sometimes unnecessary removals in the name of public safety. Residents can look out for these oldsters and fight for their preservation, if necessary.
How to recognize an old tree? Robert Leverett's introduction to the book Eastern Old-Growth Forests, edited by Mary Byrd Davis (1996, Island Press, Washington, DC) lists the hints. The tree is very tall and asymmetrical, with few lower branches and stout upper ones. It leans a bit from avoiding a too-near neighbor, and its trunk may have a twist. It bears the many marks of insults endured and survived: deeply corrugated and plaited bark, large, protruding roots, cavities in the trunk, and round, crusty growths where insect or other creature made an irritation.
Some of the old trees in our Evanston yards may have germinated even before our old houses were built. I have an ancient but healthy hornbeam in my back yard. Its stout trunk is twisted like taffy, and its bark is patchy, both species-specific traits in old individuals. Like many old trees, it has some large, broken-off branches in the crown. They were the reason I asked an arborist to evaluate the tree, a few years ago, in case it was sick and needed to be removed. "No, no", he exclaimed, quite alarmed at the idea. "It's fine; it's just old! You are fortunate to still have one of these rare trees of our indigenous forests." He said it was likely to live another couple of hundred years, if left in peace.
I'm glad I asked a knowledgeable person and kept the tree. Its fine, drooping branches are richly draped with small celadon green pendants each Spring.