Each triangle has a front and a back, determined by order in which vertices are appear in the vertex list. The fact that you can walk into a tree suggests that the fronts are on the inner surface of the tree. Try using FlipMesh and see if that fixes the problem. I know that was suggested before. But maybe you have muliple meshes in a model and didn't flip the right thing. That's just a guess.
You might think that with "backward" triangles you couldn't see the tree trunk at all. But in fact you can see through the near side of the tree to the inner surface of the far side.
You might think that with "backward" triangles you couldn't see the tree trunk at all. But in fact you can see through the near side of the tree to the inner surface of the far side.