TPO 2 PASSAGE 1 TIMBERLINE VEGETATION ON MOUNTAINS 树木分界线 【TPO原文】

　　1) Timberline trees are normallyevergreens, suggesting that these have some advantage over deciduous trees (those that lose their leaves) in the extreme environments of the upper timberline. (A比B有优势---两者比较，容易出题)

　　2) This is particularly true fortrees in the middle and upper latitudes, which tend to attain greater heightson ridges, whereas in the tropics the trees reach their greater heights in the valleys.(whereas 的转折信息，成为很多同学做题失分的原因)

　　3) As the snow is deeper and lasts longer in the valleys, trees tend to attain greater heights on the ridges, even though they are more exposed to high-velocity winds and poor, thin soils there. In the tropics, the valleys appear to be more favorable because they are less prone to dry out, they have less frost, and they have deeper soils. (寒冷地区和热带地区比较信息成为出题点。)

　　4) Late-lying snow reduces the effective growing season to the point where seedlings cannot establish themselves. (late-lying 积雪， establish扎根)

　　5) Some scientists have proposed that the presence of increasing levels of ultraviolet light with elevation may play a role, while browsing and grazing animals like the ibex may be another contributing factor. (原因类的学术表达)

　　6) This enables them to avoid the worst rigors of high winds and permits them to make use of the highertemperatures immediately adjacent to the ground surface.