I prefer not to have the weight of the rock adding to the compression of the soil. Rocks can also absorb heat causing the premature drying of top layer of soil as well as potentially refecting heat towards the trunk. Breakdown of organic mulch and humus as a topdressing can add nutrients to the soil and do a more superior job with moisture retention. While the practice of topdressing with stones is common for decorative purposes, I don't think that it is the best choice for a potted maple, but it can work just fine with the above "potential" drawbacks being of little consequense. One functional problem is that it is more difficult to access the soil to check moisture content to determine watering needs. A water meter can circumvent that problem as it is easy to probe the soil.

mjh, thank you for your detailed answer; very helpful.

maro