I have been thinking about various ways this could be done, without having too large an impact on performance, and have also gotten a few Ideas from other games. I recently played a game called Dear Esther, and instead of animating all of the plants, it had animated "detail" plants mixed in with non animated plants. The movement was enough that it looked like all the plants were animated. So for 0ad, animations could be assigned to a percentage of the trees, instead of all of them. It could even change which tree is being animated, so that every tree is animated, but never at the same time.