This was one of the first issues encountered for a fly-through done of Seletar Airport. It is pretty much like an architecture shot with hardly any animation.
We initially couldn't render the files with the huge mess of trees but later learnt BSP2 with Ray-tracing as the render mode combined with Binary Proxy was the way to go. It allowed the computers to flush their memory and keep chugging.
BSP2 and Ray Tracing for rendering mode are settings to be used when combined with Binary Proxy in Mental Ray.
Pretty well documented discussion about the creation of forests.CGTalk
Other related info