This is a problem of Forest Material with Mental Ray when Final Gathering is enabled. Unfortunately, there is no way to fix it without disabling FG or patching the plugin.
Please, try the attached Forest shader. It's patched to solve this issue, so you may finish up your scene. However, we must do extensive tests yet, before including it in Forest 3.04b.
To install it, unzip the file, and copy the FShader3.dll file to <maxdir>\mentalray\shaders_autoload\shaders, where <maxdir> is your 3DS main folder.
With this shader and using Final Gather, i suggest not to use self-illum. You may set in "Material" rollout, "self-Illum factor" parameter to 0% for each Forest object, instead modifying the materials.
Please, post here the results, to know if the patch solves the issue.