Originally posted by Kev
You can remove trees in the terraformer, but it's pretty much an all or nothing type of thing. Once you remove your trees, you'll have to go back in the editor and add the ones you want.
To remove the trees in the terraformer, click on "terrain" and then "place trees". Select "place trees of randomly selected type" and set the amount to 0. Click OK.
Hope this is helpful.
I think he means the trees in the far background, between the land and the skybox images. You can remove those with the environmental tool. But i can't find it anywhere, maybe you can try google.