I attached a scene file that I stripped everything out of except the lighting and the trees. Basically I am using mental ray and for some reason I cant get the trees to light up. Even if I turn up the self illumination the material is still dark. I am also noticing that no shadows are being cast by the trees and that some trees look like there just black squares, which seems odd since all the trees are using the same planes configuration and texture. Any help would be much appreciated, thanks.
After playing for a while with your scene, i have found the problem (a compatibility issue with HDR). We must write a patch for Planes Material, but this may take some time. As workaround, you may increase the 'RGB level' in the Bitmap to get a brighter texure (use a value of 20 or so).
This is only needed with lights that use HDR (as 'Daylight' or 'mr Sun' types).
To get the shadows, you must Pick the light in the 'Shadow' parameters of Forest. This parameter was empty in your scene.
Tip: if you get some black holes between trees when using Mental Ray, try increasing the 'Max.Depth' and 'Max.Refractions' values in the Renderer->Ray Tracing parameters. The default values are too low for scenes that use a lot of overlapped opacity textures (as Forest)