Black poplars looking brown

Why would black poplar leaves be turning brown just now?

I noticed one clone of white poplar (trembling aspen, Populus tremuloides) just starting to turn yellow out at the farm on Friday, and that seemed a bit early. One out of how many? Good question! I just realized that I have no idea how many clones there would be out there. Hundreds? The farm is three quarter sections (480 acres), mostly a mixture of wooded coulees or north slopes and grassy ridges or south slopes.

Anyway, most white poplar trees around here are still in full summer green. But last week at Coyote Lake (at the west end of Moose Mountain Park) I thought some of the black poplar (Populus balsamifera) trees were looking a little brown. Not a real "dead leaf" brown, just a brown tinge to the leaves. I didn't get a close look, and didn't think anything more of it, until today when we drove past the east end of the park, and saw whole patches of black poplars, through the whole area, looking much browner than their white poplar neighbours. South of White Bear Lake, some of them looked rather sickly. What's up?

- update - talked with Mom - we think it could be drought stress. We've had plenty of moisture through the first part of the summer, but the last few weeks have been very hot and mostly dry, and I've heard reports that crops are under stress now. The black poplars need more moisture than white poplars, so it would make sense that they are the first to show symptoms.