When I bought the Berry Farm, I got a living pharmacy in addition to a garden site.
I'll talk about the trees first, starting with Ulmus rubra, aka Slippery Elm. According to Trees, Shrubs and Woody Vines of the Southwest by Robert A. Vines,
elm bark ... contains a mucilaginous substance forming a jelly when mixed with water. A warm infusion was used as a treatment of coughs and diarrhea by early settlers. It was prepared by stirring an ounce of powdered bark in a pint of hot water. Somtimes the strips of elm bark prevented fat from becoming rancid, and settlers sometimes cooked bear fat with the bark.
I see lots of it for sale on the Internet. It seems to be an ingredient in many cough drops.
Sloan Kettering says,
Derived from the inner bark of the tree. Slippery elm has been used historically for gastrointestinal disorders, skin ulcers or abscesses, cancers, cough, fevers, and inflammation. The primary constituent is mucilage, which is thought to account for the demulcent effects. To date, no human or animal studies have been performed to evaluate the efficacy of any proposed claims. Toxicity of slippery elm is low based upon chemical components. No adverse reactions or drug interactions are reported in the literature. Slippery elm appears to be safe for coughs and minor gastrointestinal complaints, but it should not be used to treat severe conditions such as cancer or bronchitis.
I have never tried it, so I cannot say anything about it from personal experience. I will certainly keep the trees here.