At the upper timberline the trees begin to become twisted and deformed. This is particularly true for trees in the middle and upper latitudes, which tend to attain greater heights on ridges, whereas in the tropics the trees reach their greater heights in the valleys. This is because middle- and upper- latitude timberlines are strongly influenced by the duration and depth of the snow cover. 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.
树带界限上界开始变得卷曲变形。对于中高维度的树来说正是这样，但是低纬度树变得更高。这是因为中高纬度树被大雪长时间和深度覆盖。树木在山脊有更高的高度。
tropics热带
“At the upper timberline the trees begin to become twisted and deformed”is particularly true for trees in the middle and upper latitudes, which tend to attain greater heights on ridges, whereas in the tropics the trees reach their greater heights in the valleys. This is because middle- and upper- latitude timberlines are strongly influenced by the duration and depth of the snow cover.
逻辑：trees in the middle and upper latitudes tend to greater heights on ridges, whereas in the tropics greater heights in the valleys.because middle- and upper- latitude timberlines are strongly influenced by the duration and depth of the snow cover.
trees tend to attain greater heights on the ridges, even though trees are more exposed to high-velocity winds and poor, thin soils there.
逻辑：
middle and upper latitudes < tropics
middle and upper latitudes 中 ridges>valley
tropics 中 ridges<valley
- According to paragraph 3, which of the following is true of trees in the middle and upper latitudes?
===========================================================================
Tree growth is negatively affected by the snow cover in valley
B. Tree growth is greater in valleys than on ridges.---错误，不是山谷比山顶好，而是山顶比山谷好
C. Tree growth on ridges is not affected by high-velocity winds.-错误，不是没有影响，而是影响没有的deep snow大
“trees tend to attain greater heights on the ridges, even though they are more exposed to high-velocity winds and poor, thin soils there. ”
D. Tree growth lasts longer in those latitudes than it does in the tropics.--错，没有提及growth lasts
