terrain.AddTreeInstance() does not generate collider

When I use the following code I get random tree instances on my terrain, but the trees do not have colliders. I need them to have colliders (the prefabs have them attached).

 /****
 * code which creates the tree prototypes
 ****/
 m_treeProtoTypes = new TreePrototype[2];
 m_treeProtoTypes[0] = new TreePrototype();
 m_treeProtoTypes [0].prefab = m_tree0;
 m_treeProtoTypes [1] = new TreePrototype ();
 m_treeProtoTypes[1].prefab = m_tree1;
 terrain.terrainData.treePrototypes = m_treeProtoTypes;
 /***
 * looped code to calculate normative coordiantes and pseudo-random locations
 ***/
  TreeInstance temp = new TreeInstance();
  temp.position = new Vector3(normX,ht,normZ);
  temp.prototypeIndex = Random.Range (0, 2);
  temp.widthScale = 1f;
  temp.heightScale = 1f;
  temp.color = Color.white;
  temp.lightmapColor = Color.white;
  terrain.AddTreeInstance(temp);

If I substitute the AddTreeInstance() with Instantiate() code instead I get the colliders, but it is harder to control location because it is not normalized to the terrain. Am I missing something in regard to AddTreeInstance() in order to get the colliders to work?