I have textured my surface with a wire brush, going up and down on some. Going left to right on others. I think it gives a neat texture, almost like sedimentary rock. Maybe the up and down would work with the wire brush, along with deeper cuts with a chisel or file to seem more tree-ish. I am no artist, but I would say work on a little one and see if you can get the result you want. ( Of course, doing this when "fresh", after the first 24-48 hrs of curing, then re-bag for a week or so.

The technique is called "faux bois". You draw the bark, branch stumps, tree rings, etc using screwdrivers, forks, awls, etc. You need to work while the mix is still damp and carveable. Depending on the size of the piece you can unmold in a few hours to 24 hours. Work in the shade and keep damp. If you do a search on this forum for "faux bois" you'll find more information.

Thanks for the tips. Will give them a try