I've only known of poplar as a type of hard wood that my dad used to make our hutch with. But apparently the bark was used for medicinal purposes. This comes from "The Indian Household Medicine Guide." ©1883
This is a very valuable remedy, and should be used more than it is, and would be if everybody knew of its valuable properties. It is a plant common to this country, and is best gathered in the fall of the year, and is within the reach of everybody.
Medical properties and uses.—There are two kinds of barks, white and yellow; one is as good as the other. It is a very valuable remedy in all stomach troubles. It is a fine tonic, and should be used in cases of general debility with feeble digestion. It is good for convalescents when the appetite is deficient. My brother, some few years ago had a severe spell of continued fever. After the fever broke his convalescence was very slow; he had no appetite, and was swarthy, weak, and melancholy; the smell of victuals was that of disgust rather than a pleasure. Our family physician, and a good one, gave him tonics, but without the desired effect. I chanced to be at home at the time, and my mother being alarmed about his condition, asked me if I could recommend anything in our line of practice that would be good for him, give him an appetite and build him up. I recommended equal parts of the inner barks of poplar and dogwood and sarsaparilla root, cut up fine and put in a quart bottle until it was half full, then add whisky till full, and take a large tablespoonful, or a common swallow, before each meal. She did so, he took it, and in four weeks gained fifteen pounds. It immediately increased his appetite, strengthened his nerves, and restored his complexion to its natural color. He now lives twenty miles east of Cincinnati, Clermont county, Ohio. I will give you an Indian formula still better than the above: