@sPIKe: Thanks for the tip, that would be a quick and easy way to place them by hand.
I think this won't be a good idea because you can't use correct-LOD for the trees (one model) anymore! Also coll. dectection, shaders, events etc. will be very slow. This is not a good scene-managment!
Good point.
There is never going to be a perfect way to do something, it always depends on the situation.
Why would the shaders/events slow down?
My example may be a little misleading, rather than doing this for trees, (usually collision detection is needed as well as lod) you could just do it for grass/brush that has no collision detection. You may even be able to group dense regions of trees into meshes and still have collision/lod working. There is always a balance that depends on your game though.
