You did notice that those trees in there were made with at least four (4) triangles each (if it wasn't subdivided into segments) with a texture map right? If you want some interactivity the number of polygons would need to be at least doubled if not quadrupled for each tree. Still a big problem if you want collision detection with planes and trees. The additional geometry load in the game is really much better being used for the planes and interactivity between planes in the game.
If you are so concerned about trees then I think you are flying way too low and getting damaged too often