A method that approximates the vegetation, in this case the trees, with billboards when viewed from far away. This seems to work like a more advanced impostor as it uses more than one billboard per tree/object. The image below shows to the right how the tree is split up into multiple billboards.

Link where you could find the paper:
http://graphics.uni-konstanz.de/publikationen/2005/hybrid_level_of_detail_approaches/Colditz%20et%20al.%20–%20Real-Time%20Rendering%20of%20Complex%20Photorealistic%20Landscapes%20Using%20Hybrid%20Level-of-Detail%20Approaches.pdf