Looks like Prunus mahaleb

Wow-you've thrown me for a loop. This is growing right next to P serotina and I found three occurrences of it so far. The Wisconsin natural resources site states that the leaves are rounded, almost as wide as long. On these, the leaves appear almost twice as long as they are wide, but the leaves are small, less than two inches long.

Being non-native, I will be destroying them.

Just throwing this in, leaves don't look right, but flowers & bark look similar to Prunus pensylvanica. But there are alot of prunus species -- I've got one adjacent to my lot that I just can't nail down the ID. Below are pin cherry images:

https://duckduckgo.com/?t=ffnt&q=pin+cherry&iax=images&ia=images

I am with being that this looks a lot like Prunus Pennsylvania to me.

A different look with it sides-by-side with P serotina on the left. They are growing two feet apart at this location.

Prunus species can be very similar, but I'm "near" certain that the flowers on your mystery prunus are not those of black cherry -- black cherry flower-racemes (below) are quite distinctive, and I think the flower-timing is different.

Below is a prunus that I simply cannot ID -- one problem is the difficulty getting across the stream. But it ain't black or pin cherry -- blooms way too early. Isn't serviceberry either -- leaves are coming out w/the flowers, unlike serviceberry. I wish it would produce fruit, but it doesn't -- not a single berry. Alittle over 15 ft tall in fairly deep shade when the forest leafs out: