I'm trying to render your scene, but all objects look as white although the Exposure parameters seem correct.
However, why are you using separate objects for trees and shadows, instead a single object with "Forest Material 3" and the shadow options of the plugin ?
If you have not done it yet, i suggest you upgrade to Forest 3.05b. In the latest beta releases we have improved the Mental Ray shader, it renders faster and get better results. There must not be differences from using "Architectural" materials or "Forest Material".