Thank you very much Mike 
, at last I understand what the aspens are. I began some years ago thinking the aspens are betula, more recently concluded they could be populus tremula but I too saw the difference: the trunk of my populus isn't white...
No,
Betula sp. is birch, and not all birch have white bark.
Populus tremuloides have a creamy-white/green bark, but
Populus grandidentata have more of of an orangey/green bark. Other poplars like the cottonwoods can vary greatly. One thing all
Populus sp. in North America have in common (except balsam poplar - there's one in every crowd) is a flat leaf stalk or petiole. This is the stem that goes from the base of the leaf to the branch. The flat petiole makes the leaves rustle in the breeze, hence names like 'trembling aspen' or 'quaking aspen'.
Okay, that's enough dendrology for the day.
Mike.
Mike.
