well, most other applications offer passes for such stuff and also offer the option to limit light to certain objects. Thats not possible in TG at the moment - but there is still the old way of doing this. Actually sometimes I still do it this way in other applications too, as it can give you more creative freedom.
The only thing that needs light in this case are the trees - and so only the trees should be visible. If atmosphere still is in the scene, then the sky and probably complete scene will get brighter.
I would have to check the options, but I think i would:
-remove all objects/populations except the trees here in the foreground
-turn off atmosphere
-give the terrain a black surface
-play with the light settings until you get a nice effect on the trees
-in Photoshop add it on top of the regular render. As everything else is black you simply can add/screen, no need of an alpha or object channel
