Giant trees done in Quark or Modeling app?

I am in the process of creating a forest of giant trees where the roots of the trees are partially sticking out of the ground at the base of the tree. I am trying to figure out what would be the best way to do the collision for these. I don't want to use one convex shape because I want the player to be able to walk in between the roots and not collide way out from the trunk. When I say giant trees I am meaning 30 - 50 feet in diameter.

I don't require a ton of polygons to get the look I am after so would it be best to do this in Quark or Hammer or in a modeling program. Would the trees be to irregular for dif? I am only worried about the base and trunk of the tree right now. I will do the branches and leaves in a modeling program for sure. My biggest concern with doing them in a modeling program is collision. Would a combination of the 2 approaches work? ie modeling the tree bases and trunk and using some type of very low poly dif shell for collision of course this would create it own challenges getting the 2 to match up. I would appreciate any ideas.