Careful not to root around too much, and instead gently tuck stuff into pockets at most. Beeches are usually pretty much solitary souls under which little else takes root because of the beech’s competitive root structure. Epimediums are a good dry shade choice, and there are many species and varieties. Some ferns will also tolerate dry conditions, and even sow in under my large trees on their own from time to time. You won’t be able to make a garden under the tree, however–you can’t do too much digging, and at most some swaths of groundcover will have to do. I love the truck of my biggest beech, so I wouldn’t want to mask it, anyhow.