I’d like to put something decorative in the circle area of dirt between the mulch ring and the base of a tree. I realize this is generally verboten, but that stricture is usually for traditional mulches and other ill-considered materials. It seems that something that allows air and water to move freely into the soil without trapping moisture against the trunk/root flare, or that tends to create an environment that might promote other undesirable conditions, should be OK.
I’m just wondering if there is an accepted/acceptable approach. A thin layer of decomposed granite? Same thing, pea gravel? White stone? Large bark chips that will still allow moisture and air to exchange or dry away? What options are there?