Why is the timberline different on different mountains?

You can get different timberlines on the same mountain. Not only can they be different on different sides of the mountain, but on the same side of the mountain! Look carefully and you will see that the timberline is not a straight, horizontal line, it varies depending upon rain, sun, soil, wind circulation patterns, and general climatological conditions. For the most part, a "timberlines" is the line above which trees DO NOT grow, not where the do grow.