What kind of tree is this?
I have these trees popping up all over my yard. At first I thought they were cool but they are like weeds so now I dig them out. You can view an album of photos at the link but I've also uploaded a few including pics of the whole tree, the bark/trunk, and a branch with fruit. The berries are the size of small blueberries and start out green but turn dark purple when the ripen.
Macomb County Michigan
Looks like common buckthorn Rhamnus cathartica. It is an invasive species.
Here is some more information.