Groundpounders and I worked on the Torque method of making large numbers of trees, "automatically". Clumps of forests make up a large part of the Moonshine Run scenery. Pictured here is a frame-grab of Deborah Marshall's video tutorials on creating the forests on the Oregon coast..

Here is a snapshot, the thing at the base of this tree the collision is wider than the trunk. Collision can not be convex, of course you can make invisible collision boxes and align the roadway were you don't want players to go.
groundpounders

http://vimeo.com/2762649