In games, they are mainly textures with alphas mapped to planes.

So if you can imagine the tree as geometry, (or a plane also in some cases), then 2 or more planes with branch textures applied on top in a cross-pattern.

Like so:
https://www.datenform.de/blog/uploade...000-703606.jpg


Then when rendered, and/or seen from a distance, they look pretty convincing in a game situation:
https://img.photobucket.com/albums/v5...H/WIP/tree.jpg