Thought it might seem so, it's not a Forest related issue. As you can see on the first attached image where on the left side are Forest objects and on the right side mesh objects, the result is exactly the same. This halo artefact comes from anti-aliased alpha channel.
The issue is tight to premultiplied alpha channel rendered against non-black background. For post-production purposes in Photoshop would be easier to not use the Environment Map for Background (and render the image just with black background).
This tip is mentioned also on the Autodesk's site
regarding this topic:"If you plan to composite objects in another program such as Combustion or Photoshop, render the objects against a black background. Otherwise, a fringe of environment or background color can appear around the objects. "
Let me mention, that for these purposes, there is a handy tool in Photoshop: Layer > Matting > Remove Black Matte or Layer > Matting > Remove White Matte
Hope that helps.