just to note: Collision Detection can be maintained without rendering if objects are set to Alpha 0.
Perhaps, also, some optimisation may be done on your trees. If this is what seems to be causing the majority of the slowdown.
Are they meshes or sprites? can you texture them with less surfaces? can you replace meshes with sprites at greater distances (L.O.D stuff)?
This is kinda tricky stuff, and Im not the best to advise on it, but it may help a lot.
Perhaps, also, some optimisation may be done on your trees. If this is what seems to be causing the majority of the slowdown.
Are they meshes or sprites? can you texture them with less surfaces? can you replace meshes with sprites at greater distances (L.O.D stuff)?
This is kinda tricky stuff, and Im not the best to advise on it, but it may help a lot.