If you get lost in a poplar forest, you are in luck! There are two likely ways to find north. And then, as the commercial says, you’ll be lost in the woods facing north.
First, check the moss. The mossier sides are the north sides. Be sure to check more than one tree, though, and don’t expect it to be due north.
Second, and more reliably, look at the bark. One side is bright white and chalky and the other is greener. If you look at a whole stand of poplar this will be more apparent. The difference isn’t distinct, but it is definite. The whiter side is the south side. Poplars don’t have very thick bark, and no dead layer to protect them. The surface bark is living and active in making food from light, just like the leaves. In winter, when there is no leafy canopy to shade the trunks, the southern sunlight might get strong enough to warm the bark and trick it into thinking spring had sprung. That would be disastrous by the next frost. So, the tree has that extra whiteness on the south side to reflect the winter rays and protect it from false starts. If you are facing the whitest side, you are facing north, so just keep walking until you are out of the woods!
Aspen poplars, populous tremuloides, are named for their leaves, which tremble together in one unified mass at the slightest breeze. Doesn’t it make you think of those crowds anticipating the birth of Prince George? Populous Tremuloides: a headline you’ll likely never see!