I have grown gingers under mine for years and they have suffered no adverse effects. I also grow vines up a couple. Its really pretty with the vine's flowers hanging down from the canopy. One of the myrtles flowers white while the vine flowers sky blue. They look great together :o) Don't use a really agressive vine like the cypress vine.
With that in mind if looking for a smallish plant you may want to think of shallow rooted plants like smaller gingers, cast iron plant, ferns and so forth.
Mike

I have two underplanted crape myrtles. One is underplanted with daylilies, society garlic, bearded iris, daffodils and sedum. A second, the largest, is underplanted with rosemary, sweet alyssum, pelargonium, daffodils, cuphea hyssopifolia and allium triquetrum.

It's possible that the shrubs under your trees have root systems that are competing unsuccessfully. Maybe dwarf shrubs, bulbs or perennials (with low water requirements, similar to crape myrtle) would do better.