I agree with John's assessment. As a group right now, the spaces between the trees are not som much "negative space" as they are simply "empty."
I find John's solution to be a wonderful idea. I also propose another approach. Instead of adding more trees, one could group the existing trees closer, creating an effective clump instead of an ineffective group.
As to the p[ot, IMHO a forest should not have feet, since it looks really unstable. But I think the clump idea work alright in this pot, since the feet are spread enough beyond the center to give it a stable feeling.
I did not take the time to add the ramification that this approach would need, just a basic idea.
humbly submitted,
pootsie