tree collider doesn't work

I've put the "generate colliders" on the meshes of the trees I'm using. and I've checked the box called "create tree colliders" in the terrain. Then I generate a tree-populated map of 1000 trees. but alas I can still walk through trees.

First answer works well.

http://answers.unity3d.com/questions/5932/i-can-walk-through-trees