The solution will depend on the render engine used. In case of mental ray
, Forest Pack includes a custom shader that enhances the rendering of billboards - Forest Shader
, which requires the use of Forest Material
If that's your case, I would suggest you to lower the Self-ilum factor
value (by default set to 50%) under Material - Forest Shader
. That should eliminate that undesired effect.
Anyway, as you've already tested, exclusion affected items from light source would be an another suitable option.