Mountains: Trees

In Minecraft Live, it was mentioned that tree generation would stop at a certain level on mountains. To take it a step farther, and make it more realistic, I believe it would be a cool game mechanic to stunt the growth of trees/saplings the higher up they are. In real life, trees that grow on the very tips of mountains can look very young, but be thousands of years old. Just making it so a sapling placed at the top of the mountain takes longer to grow because of the high elevation would be an extra detail that I think would fit in.