I've never seen a tree like this before I bought this house.. neither has anyone else I know! It's driving me crazy. I'm in zone 7a I believe, maybe 7b. there's a picture of a medium sized leaf, some of the bigger ones have four lobes. there's one mature tree and three young ones.
I just identified one in my own yard and the leaf looks like that one. Red Mulberry, I believe. http://sfrc.ufl.edu/4h/RedMulberry/RedMulberry.htm
It could be a Mulberry, the leaves certainly look right, but the green trunks and height of the saplings is throwing me off a bit... Maybe they're root suckers rather than saplings?
If it is one of the Mulberries, it could be either M. rubra or M. alba, hard to tell at this point. Morus rubra is native to North America, while Morus alba was introduced from China as food for silkworms over 200 years ago, but both are very similar and can easily hybridize (and have for over 200 years now). If a tree is not a hybrid, the best way to differentiate between the two species is that Morus alba blooms white (hence the name) and Morus rubra blooms are green and barely noticable. Hybrids may bloom in either color. Also, the leaves of M. alba are rather shiny, while the leaves of M. rubra are somewhat dull. Again, hybrids can go in either direction though. Here are images of both:
M. alba: http://www.duke.edu/~cwcook/trees/moal.html
M. rubra: http://www.duke.edu/~cwcook/trees/moru.html
Yes, Firmiana simplex does seem to be a better match for the bark of your plants :).
You are correct! I should have taken a closer look, but I was looking at the shape of the leaf. I should know better than that by now. The veins in my mulberry leaf and your parosol leaf are very different.