American Elm Bonsai

I am still not completely sure what exact type of elm this is but I am leaning towards American Elm. Here it is today, the original leader has died and now there are several leaders that make up the apex. I just love the bark on this one and the branches are coming along nicely. The tree was collected near some railroad tracks behind a McDonald's a few years ago. I'm letting the tree do what it wants to do for now while I figure out the first branch. I keep it outside all winter here in Cincinnati with zero protection (could be why I lost the leader branch).

I don't have a garage or a shed or a yard to bury the trees in. I live in the city and all my plants survive on a balcony facing east. These conditions are less than ideal for gardening but I am somehow able to keep them alive. My mulberry died last winter but I attribute that to over-working the roots. City/balcony gardening can be annoying but it beats the alternatives; giving up bonsai or moving out of the city, both things I would never do at this point. Anyhow, the elm is perfect for such a situation as it is very cold hardy and tolerates receiving only morning sun. Pests are still a problem.