I put literally hundreds of tropical plants either on the ground or hanging from branches of four large oak trees. Many are still small plants, and thus might be in 3-4" pocket trays (24, 30 or 36 compartments) or in 3-4" pots, and thus don't take up a lot of room. The plants I have under my oaks include orchids, bromeliads, tillandsias, staghorn ferns, and fiddle leaf ficus. There are more varieties, but those five families of plants are the bulk of what I have. All the areas around my oaks are mulched, and I use cypress mulch exclusively now. When you water your plants the ground (mulch) will naturally get the run-off, so that certainly will be good for your trees and for raising the humidity. Also, the trees will supply needed shade between the hours of 10:00 AM and 4:00 PM. For most tropical plants, mid-day sun is a real killer. Trees also "perspire" greatly during the hottest part of the day, and this alone will supply added cooling and humidity for those tropical plants.
