Not sure if this is a bug or a feature... trees spawn in these rather ugly groups, causing them to collide constantly. There's no way to "ungroup" them. Basically there's not a single tree that isn't colliding with another tree when I turn the tree frequency up.

Trees ignore other trees when spawning. When there is a suitable spot for trees to spawn, a lot of trees will spawn there.

You can transform the trees into BO3s, which have a nice block check system to make sure that the trees don't spawn too close together.