I'm thrilled to have taken the time to look carefully at these magnificent leaves closely enough to be rewarded with a definitive identification - Betula populifolia or gray birch. It's a nurse tree, providing habitat and foliage for other, slower-growing and longer-lasting trees to thrive.
It seems SO easy and obvious once you've looked at it - it can't be mistaken for anything else. Its leaves are exquisite, shapely and glossy. Superb!