I'm thinking that with those trees you need to show a darker center with the start of branches going out in every direction, maybe even some leaves showing until the branches reach the edge of the trunk area. It looks like a stump cut off which works well for a place holder but it's going to make people think they can jump straight over or attack through it. Come to think of it, this could be hilarious from a Game Master's point of view:

GM:"So, you want to jump straight ahead and attempt to land on the opposite side?"
Player: "Yes, I have high strength and excellent jumping skills."
GM: "Well, if you are sure, give me a roll."
moments later....
GM: "Belgar crouches down and with a mighty surge of his powerful legs vaults himself face first into a two foot diameter tree trunk. He might have made it if only his head was a nail. Too bad he wasn't wearing a helmet."



The other thing you can do to sell it is add the full shadow of the trees upon the ground. This would define the shape of the tree above. It's a tough one to pull off though so keep at it. I'm sure it's going to be great when you are done.