Originally Posted by luthier

It's called LODs At that distance our airplanes lose their gear legs and canopy framework and flaps, buildings lose their chimneys, tanks lose their gun barrels, and so why should the trees keep their trunks?

We still live in the age where computers have limited resources. Some day PCs will become powerful enough to render a fully modeled tree with every twig and every leaf all the way to the horizon, and that'll be a very happy day for everyone in game development, but the way things are, if we were to splatter a bunch of tree trunks everywhere around the player, you'd be looking at an extremely pretty picture that runs at about 1 frame per minute.