http://www.ogre3d.org/tikiwiki/PagedGeometry+Engine
What is PagedGeometry?
The PagedGeometry engine is an add-on to Ogre which provides highly optimized methods for rendering massive amounts of small meshes, covering a possibly infinite area. This is especially well suited for dense forests and outdoor scenes, with millions of trees, bushes, grass, rocks, etc., etc.
Paged geometry gives you many advantages over plain entities, the main one being speed: With proper usage of detail levels, outdoor scenes managed by PagedGeometry can be >100x faster than plain entities. Another advantage is that the geometry is paged; in other words, only entities which are immediately needed (to be displayed) are loaded. This allows you to expand the boundaries of your virtual world almost infinitely (only limited by floating point precision), providing the player with a more realistically scaled game area.