Very nice start! I like the growth rings quite a lot. The roots look very flat--a little dodge and burn to make them look round might help. Once you get it finished, save it out with transparency in place of the grass so you can use it anywhere easily (and so we can, too!)

On the subject of battlemats with trees, there were a couple efforts that should be mentioned, particularly one by JFJohnny5 that was simply gorgeous. It was a bandit camp, but the trees and their shadows were gorgeous.

It seems like someone was experimenting with tree canopies with reduced opacity, so the ground and trunk could be seen clearly, but the canopy information was still there in case of climbers and flyers.