First, check the bottleneck of your level. When pressing [F11] you're getting the CPU time for different tasks, like rendering and script execution.

If model rendering time is the bottleneck, try to simplify the LOD2 and LOD3 stages of the trees. Group 4 to 8 trees together in one model. Make sure that the tree models don't have animation.

If script execution is the bottleneck, check whether you can speed up your scripts. Of course, you should definitely not assign any action to your trees!