In video games like The Elder Scrolls: Morrowind and Halo, they usually use transparent images (probably targas) for the leaves and branches. The detail is all in the texture. The actual planes the leaves and limbs are placed on are usually no more than 2 or 3 faces. just do a few good ones, then start duplicating around a simple trunk model with a good bark texture and you've got it. Believe it or not, the tree trunks in Halo (the first level btw) are actually quite huge to help take up space to prevent the player from seeing through it too much, but they look very realistic. With Morrowind, the trees are much much bigger in size, so I believe they probably used bigger texture maps for those.

Anyway, if you have absolutely no idea what I'm saying, either ask me to explain with actual images or a self-writen tutorial or rent one of the two games and see for yourself. I consider this a good technique for thick trees like wintergreens.