Other forms: tree lines
The tree line is the point on a mountain above which trees can't grow. Once you pass the tree line, it's too dry, cold, and windy to support the growth of anything larger than small shrubs.
The terms tree line and timberline are commonly used interchangeably. Technically, however, trees at and below the timberline grow densely and form a canopy, with sparse tree growth continuing up to the tree line. Higher than that, there are no trees at all. Climb as high as the tree line and you might find it harder to breathe!