yep i know the reason for it. when you use a copy(duplicate) of the same tree in the scene and they are close together it tends to cause this problem. try going into the tree editor and adjusting the "Randomness" up by 1 until it fixes the problem. You will need to do this for every copy(duplication) of the tree model you use.

I have opinions of my own -- strong opinions -- but I don't always agree with them.