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.
逻辑：trees disappear and are replaced is called upper timberline.the forest passes into steppe or desert at lower timberline’s lower edge.
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.
因为缺水，在许多雨量少的地区有一个树带界限下界，在那里树木变成干草原或者沙漠。
timberline 树带界限
Shrubs灌木
moisture水分
semiarid 雨量少
steppe 干草原
2. Where is the lower timberline mentioned in paragraph 1 likely to be found?
○In an area that has little water
○In an area that has little sunlight---错，不是sunlight，而是water
○Above a transition area---错，upper timberline是transition area
○On a mountain that has on upper timberline.----错，未提及mountain