Are you using LOD? What is the poly count of each tree? Do you know that adjusting the camera focal angle will also affect how far you can see? Are you using fog and setting clip_far to a realistic range? How dense is the mesh on that chunked terrain? What texture sizes are you using (256*256 is the most efficient, 1024*1024 and higher are a significant performance hit).

I don't think grouping your trees into one model is the answer, simply because they will surround the player, and if any one of them is within the view the whole lot will get sent for rendering.