I'm guessing american for the slender leaf. Does it have a furry midrib or hairs along the lower surface? What colour is the new growth after it hardens off...greenish or reddish-brown? here is a known american leafy seedling

bottom leaf w stipule

top w stipule

twig is kind of brownish-not furry.

these are branches from a 3 inch dia trunk.

The stipules seem a little big for a pure american...perhaps a hybrid backcross to american from chinese? American stipules often fall off too, not remaining till fall. The lower surface of the leaf seems a bit silvery, and americans are often more dark green than that. I'll guess a hybrid, meaning a keeper, as it may have the best of both.

Generally, shiny means Chinese, matte (not shiny) means American. Somewhere between is prb'ly a hybrid.

The first pic looks like Chinese (left) & American (right).

European also more glossy than American. Notice in the first picture the American is also starting to turn and drop, around here it is consistently earlier than European.

Of course August is so early the one could also be turning due to a problem, maybe blight (if that is how it manifests) or drought. But the previous point about the relative timings between the two remains true.