Unity terrain trees on layers

Hi, my game is high performance (more of a visual than a game necessarily).

I am wondering if there is a way for me to add trees to the terrain on specific layers?

In example: Birch trees drawn on the terrain would be added to the birchTree layer.

The current method I've thought of would be to create the terrain first (raised and lowered) and copy it (x) amount of times for (x) amount of "tree layers" and then apply a tree to each layer, but this seems very counter intuitive.

The map is static (forest should be the same every time it is played) and the issue I currently face is that Unity (4.6) slows to a crawl when it's trying to process all the trees (both in Scene and Game viewports).

P.S. Billboarding, fog and camera clipping planes are already in effect to reduce processing

Any suggestions regarding this are greatly appreciated,

Thank you.