http://www.ogre3d.org/tikiwiki/tiki-index.php?page=PagedGeometry+Engine
"Real-time rendering of massive, dense forests, with not only trees, but bushes, grass, rocks, and other "clutter". Supports dynamic transitioned -LOD between batched geometry and static impostors (extendable). "
MOGRE might already have this...
http://www.ogre3d.org/addonforums/viewtopic.php?f=8&t=29456&p=99352&hilit=PagedGeometry#p99352