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At the timberline, whether upper or lower, there is a profound change in the growth of trees and other plants. A. Birch is one of the few species of tree that can survive in the extreme environments of the upper timberline. B. There is no agreement among scientists as to exactly why plant growth is sharply different above and below the upper timberline. C. The temperature at the upper timberline is probably more important in preventing tree growth than factors such as the amount of snowfall or the force of winds. D. The geographical location of an upper timberline has an impact on both the types of trees found there and their physical characteristics. E. High levels of ultraviolet light most likely play a greater role in determining tree growth at the upper timberline than do grazing animals such as the ibex. F. Despite being adjacent to the timberline, the alpine tundra is an area where certain kinds of low trees can endure high winds and very low temperatures.