The easiest way to have a low-res proxy is to just make something like maybe a cone and stick one where every tree is, then hide the original. This will at least let you work faster when you're not rendering.
As for making renders work, one way (usually the best way) is to divide the trees into layers and render each layer separately, then composite. Another way is to use lower-res trees for those that are in the distance, or even replace faraway trees with a 2D image mapped onto a plane. If you're careful, you'll never be able to tell the difference and it will render much faster!!