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  3. Rain forest structure is distinct from most other forest types because of its many layers of vegetation, referred to as strata.

Rain forest structure is distinct from most other forest types because of its many layers of vegetation, referred to as strata.

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问题    Rain forest structure is distinct from most other forest types because of its many layers of vegetation, referred to as strata. The lowest stratum is the understory, composed of palms, herbaceous plants(such as wild ginger), and tree seedlings and saplings. 【R1】______Many have deep red coloring on the underside of their leaves to capture some of the scarce light that does manage to reach the forest understory. This red coloring enables understory plants to absorb light of different wavelengths than do the plants with rich, green-foliaged canopy, the umbrella-shaped upper structure of trees. Above the forest floor but below the canopy are one or more midstory strata, made up of woody plants, such as large shrubs and midsized trees.
   The overstory is the canopy, in which the tree crowns form a continuous layer that captures the major part of the rainwater and sunlight hitting the forest. The height of the canopy varies from region to region and forest to forest, ranging from 20 to 50 m(65 to 165 ft). 【R2】______Researchers use hot air balloons, cables, catwalks, towers, sophisticated tree-climbing gear, and even robots to study the millions of plants and animals that make their home high up in the forest canopy. Canopy researchers also use huge cranes that are dropped into the heart of the forest by helicopters. Suspended from the crane’ s long, movable arm is a large cabin that functions as a mobile treetop laboratory. Moving from tree to tree, forest researchers collect specimens, conduct experiments, and observe life in the canopy frontier.
   The highest stratum of the rain forest is made up of the emergent trees, those individuals that stick up above the forest canopy. Emergents, which do not form a continuous layer, are usually the giants of the forest , reaching heights of 35 to 70 m(115 to 230 ft)or more, and trunk sizes of over 2 m(6.6 ft)in diameter.
   【R3】______However, these trees tend to be so large that they collectively account for the vast majority of the woody mass, or biomass, of the forest.
   The nicely ordered strata of the rain forest, including the continuous layer of the canopy, are regularly disturbed by naturally occurring events, such as falling trees. Trees in a rain forest canopy are often interconnected by vines, and a falling tree may pull as well as push other trees down with it, producing a domino effect of falling trees. The resulting opening in the forest canopy enables light to pour onto the forest floor. 【R4】______
   Other natural disturbances create even larger openings in the forest canopies. For example, along the hurricane belt in the Caribbean and the typhoon belt along the western Pacific, some forests are substantially altered when high winds and storms blow down hundreds of trees every few decades. 【R5】______
   Scientists have found that these natural disturbances and the subsequent forest regeneration are a vital process that leads to healthy and diverse forests.
   [A]New plants and animals then move into the area and begin to grow.
   [B]Just 2 percent of the sunlight goes through the many layers of leaves and branches above, so understory plant species have developed special traits to cope with low light levels.
   [C]On a smaller scale, large mammals, such as elephants, regularly destroy rain forest vegetation in the Congo River Basin in Africa.
   [D]An understory of shorter trees and a lacework of woody vines, or lianas, produce a forest of such complex internal architecture that many animals, including some sizable ones, rarely or never descend to the ground
   [E]Less than one percent of the trees in the forest reside in the canopy and emergent layers.
   [F]Because more light penetrates the canopy, however, the vegetation of the understory and forest floor is better developed than in the tropics.
   [G]The rich, green canopy is teeming with life, and forest researchers have developed ingenious methods for accessing this mysterious ecosystem.
【R1】
选项
答案B
解析 本题空格出现在第一段中。上文提到热带雨林结构中的最低层是下层植被。下文提到，下层植被的植物叶子呈深红色，从而能够获取到达下层植被的少量的光。由此可见，上下文都围绕“下层植被”展开论述。出现关键词“understory”的选项有B、D和F。但是，D项出现了上下文没有的新信息“藤蔓植物(vines)”。F项中出现了“冠层(canopy)”，但从下文即第一段倒数第二句对canopy一词的定义可知，它应在下文中第一次出现。由此排除D项和F项。B项强调下层植被适应低光强的特性，其中“低光强”与下文“获取少量的光”“吸收不同波长的光”等内容相呼应，因此B项为正确答案。
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