“In areas with extremely strong prevailing winds, such as the tops of mountains or sea cliffs, trees receive forces predominantly from one direction. The result is an involuntary growth response called flagging. The leaves on the windward side are killed by wind-borne particles, and the windward branches are bent gradually leeward by the constant force.
The result is that the foliage points mostly downwind of the trunk, which itself leans away from the wind. This makes the tree much more streamlined, reducing the wind forces to which it is subjected. In the most exposed areas, the wind also tends to kill off the leading shoot at the top of the tree, so that the only living shoots are the ones that point downwind.” (source)