Hi, i can confirm there is a memory leak with Mental Ray.
We are working to isolate the problem, but it seems to happen on the Mental Ray side, so probably we have to contact with Autodesk (that may take some time).
As far as we can see, the instances created by Forest are not deleted from the Mental Ray scene, even after resetting Max. Problem is worse when using Camera clipping, because Forest need to rebuild all instances at each frame, so the memory leak grows with each rendered frame.
By now, the only workaround would be to not use Camera clipping. In this way, Forest only creates the instances one time by render. It must be disabled in the following way:
1) Uncheck Camera->Auto assign to active view.
2) Uncheck Camera->Limit to visibility.
3) Click Camera->Clear, to remove the link the camera.
While checking this issue, we've found a way to avoid the memory errors that happen randomly with Mental Ray (this is a completely different issue from above). We'll release an update in the next days, but until then we suggest to keep disableMRTintByElement=1 in ForestPack.ini. That fixes this specific problem.
We apologise for any inconvenience.