This fall at the edge of some oak woods at a home near Geddes and Arlington, I found a pawpaw plant (Asimina trilolba). It was in the shade and arched over at about 5' -- definitely not big enough to be called a tree. The homeowner had lived there for 10 years and did not plant it and he wasn't even aware of it. So I'm fascinated by the question: where did it come from?
You won't find pawpaws in many yards around here. I'm told there are some wild specimens growing in a hidden spot in Sugarbush Park. I've also seen numerous ones growing under sugar maples and bitternut hickories in the remnant rich beech-maple woodlands around Plymouth Township. And it's possible that pawpaw wasn't all that uncommon in Ann Arbor prior to settlement.
The closest fruiting specimen I am aware of is north of a house on Vinewood, a bit more than 3/8 of a mile away. Could this be the parent? My guess is that Ann Arbor is rich with people who, like me, read garden books by the likes of Michael Dirr and Jack Elliot and who enjoy growing the unusual plant in the corner of the back yard. The more yards I visit, the more I discover. Wouldn't it great if pawpaws started to compete with buckthorn in some of our unmanaged woodlands?