Wouldn't be simple to calculate tree collisions only on aircrafts little area?
I mean:
- very lazy update on tree groups (not the single bounding box), just saying, once per second in respect to the aircraft until 500 or less feet from them;
- increase updates progressively as distance reduces;
- when very near ( < 10 feets), start to calculate distance of the tree bounding box;
I don't think that should be so hard on CPU an algorithm like this :D