I've cut eastern cottonwood (Populus deltoides) in my mill, but it can be really terrible for warping and checking. I've had boards split themselves in half a day off the mill.
But seeing that other thread, whatever poplar that was, made a great bow! I guess as Willie says, it would be important to know just what kind of "poplar" that was in that particular bow.
Though I don't particularly like cottonwood, it is quite a hard wood once dry, and fairly dense too. Least the stuff I have here. Never thought about a bow from it.