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Paragraph 1: The transition from forest to treeless tundra on a mountain slope is often a dramatic one. Within a vertical distance of just a few tens of meters, trees disappear as a life-form and are replaced by low shrubs, herbs, and grasses. This rapid zone of transition is called the upper timberline or tree line.In many semiarid areas there is also a lower timberline where the forest passes into steppe or desert at its lower edge, usually because of a lack of moisture. 第一个是说this rapid zone of transition 叫做 upper timberline. 第二个是说the forest passes into XXX or XXX at its lower edge. 所以不是没有树的那个zone是timberline,而是过度的那个过程是timberline. 其实这也好想,没有树的那片区域应该就是草原或者沙漠了,就没必要叫什么timberline了。这是我个人的理解
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