Difficult to say, partly because the pic is a bit blurred, but more importantly, because the leaves on young plants like this are often different to the leaves on mature trees. My suspicions (which may be off the mark) are that it is an aspen (presumably either American Aspen Populus tremuloides or Bigtooth Aspen P. grandidentata)
Thanks, Michael. These are seedlings that came from a tree that my dad propagated. He died before I could ask him about it. It may be simply the very species you suggest !
