ok, so i used the terrain tree creator and tested my game but the player walks straight through all of the trees. if there is a way how can i prevent that without adding barriers or deleting all of the trees.
find your tree model.
drag it into your scene. press component-Physics-Capsule Collider.
adjust it so that it fits to your tree.
now make a new prefab and call it tree(or the name of your tree model) and add collider to the name(just so that you can see that it’s this tree you want)
drag the tree with the collider down onto the prefab then in your terrain window add a new tree and choose the prefab that you just made(the one with the collider)
i had the same problem and this makes it work 
Add a collider to every tree 
There is no other way
Just add a BOX COLLIDER to your prefab, then you can set trees with the terrain tree tool.
enjoy