I was wondering how to make trees placed from terrain solid i know with custom ones you add a mesh collider thx
As lampshade said, you need to add a collider.
However, I think you mean on the terrain. With the terrain trees, you get the original trees (The prefab), add a collider (I don't recommend a mesh collider, use a capsule), and then put it where the original was with a new prefab. Now use this on the terrain.
If you don't mean with the terrain, again, I don't recommend using the mesh collider, but it's your choice.
Since this post was updated not long ago, figured I would add that the Unity doc explains this
Scroll to the bottom
Select all your trees in your scene then go to Component->Physics and choose Mesh Collider. This should attach a mesh collider to all your trees.
Go to the prefab and open it in the inspector and add the componenet Phisics > Mesh collider. As simple as that.