Another great tip from EVERMOTION, showing how to create V-Ray Proxies from high resolution models, in this case, showing how to convert high polygon trees to V-Ray Proxy Objects.
Do You have hi-polygon trees in Your scene? Learn how to handle them with Vray proxies
VRayProxy will allow you to use geometry from an external mesh resource at render time only, and you can see here that using trees that are over 1 million polygons, the tree will use over 3 gigs of memory, and creating a V-Ray Proxy will significantly reduce those resources. EVERMOTION will also show a bit of propagation or distribution strategies for creating a scene with the trees.