Onions, garlic or chives would be easy enough, especially the chives...have lots of them and garlic. I would think the chives would like the shade better than the garlic. Some of these trees are 15+ years old, not sure I could dig 6 inches down, but I can see where the hardware cloth would be a good idea...maybe with stones on the top of the soil too. I've never had issues with mice or voles on trees with just grass around them, but have with mulch, especially wood chips or sawdust. Thanks.
