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  3. FOREST SUCCESSION—LAYERS OF A FOREST (1) Succession is a continuous change in the species composition, structure, and functi

FOREST SUCCESSION—LAYERS OF A FOREST (1) Succession is a continuous change in the species composition, structure, and functi

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问题                                         FOREST SUCCESSION—LAYERS OF A FOREST
    (1) Succession is a continuous change in the species composition, structure, and function of a forest through time following a disturbance. Each stage of succession is referred to as a successional sere. The final stage of succession, which is generally self-replacing, is referred to as the climax sere. There are two major types of succession: primary and secondary. Primary succession is the establishment of vegetation on bare rocks or radically disturbed soil. Secondary succession is the reestablishment of vegetation following a disturbance that killed or removed the vegetation but did not greatly affect the soil. Volcanic eruptions, retreating glaciers, and bare sand dunes are examples of sites subject to primary succession, while clear-cutting of forests, wild fires, and hurricanes are examples of sites subject to secondary succession. [A] Hundreds to thousands of years are required for primary succession to reach the climax sere, compared to decades to hundreds of years for it to occur in secondary succession. [B] A longer time is needed to reach the climax sere for primary than secondary succession because soil development must first take place in primary succession. [C] The rate of succession is dependent upon the extent of the disturbance and the availability of appropriate seeds for recolonization. [D]
    (2) What morphological (structural) and ecophysiological characteristics determine the species composition and abundance in succession? In general, nitrogen fixing plants (plants that can make use of atmospheric nitrogen) are important early successional species in primary succession because nitrogen is not derived from the weathering of rock and little or no organic matter is present in the soil. Weedy plants are common early successional species because of their rapid growth and high reproductive rates, while stress-tolerant species are common late successional species.
    (3) The structure of a forest changes as well in secondary succession. Depending on the type and the severity of the disturbance, a moderate to large amount of dead organic matter from the previous forest remains on the site immediately from the disturbance. The leaf area of the forest is at a minimum and slowly increases as new vegetation occupies the site. Following a disturbance, such as a fire, the new canopy (the uppermost spreading and branching layer of a forest) is largely composed of similar-aged, or even-aged, trees. Light, nutrient, and water availability are highest during the early successional sere because the vegetation has not completely occupied the site. Canopy closure, or maximum leaf area, can occur within several years after a disturbance in some tropical forests, but may take three to fifty years in evergreen forests.
    (4) In the second stage of forest development there is tree mortality caused by competition for light, nutrients and water. The intense intraspecies (within a species) and interspecies (between species) competition for light, nutrients and water induces the mortality of plants that are shaded or have one or more life-history characteristics that are not well adapted to the changing environment. The third stage of forest development is characterized by openings in the overstory canopy, caused by tree mortality, and the renewed growth of understory in response to increased light reaching the forest floor. Consequently, the forest canopy becomes more complex, or multilayered. The final stage of forest development, the climax or old-growth stage, is characterized by a species composition that in theory can continue to replace itself unless a catastrophic disturbance occurs. Unique characteristics of old-growth forests include large accumulation of standing and fallen dead trees—referred to as coarse woody debris. Also, the annual input of forest litter is dominated by coarse woody debris compared to the earlier stages of forest development, when leaf and fine root debris were the dominant sources of nutrients and organic matter input into the soil.
     (5) Some ecosystems may never reach the latter stages of succession if natural disturbances (fire, flooding, hurricanes, etc.) are frequent. A pyric climax refers to an ecosystem that never reaches the potential climax vegetation defined by climate because of frequent fires. The ecotone, a boundary, between grassland and forest is a pyric climax, and only with fire suppression have woodlands and forests began to advance into these regions.
Look at the four squares [ ■ ] that indicate where the following sentence can be added to the passage.
Tree species that have small, light seeds that are dispersed by wind or transported by animals recolonize a disturbed area more quickly than species with large seeds.
Where would the sentence best fit?
选项
答案D
解析 本题属于插入句子题，需要插入的题干句子意为“有些树种的种子体积小，重量轻，容易被风吹散，或是依附到动物的身上。因此，这些树种能比种子大的树种更快地转移到受干扰区域继续生长”。D处前一句提到了是否有再繁衍所需要的适宜的种子(the availability of appropriate seeds for recolonization)，而把句子插入D处是对种子与演替速率关系的进一步解释。符合行文逻辑，故选。A处前一句说的是原生演替和次生演替的例子对比。后一句说的是原生演替和次生演替达到演替顶级系列所需要的时间对比。若句子插入A处，它与上下句均没有关系。不符合行文逻辑，故不能插入A处。B处上下句分别说的是，原生演替和次生演替达到顶级系列所需时间和造成这种差异的原因，这两句关系紧密，不宜被打断，故不能插到B处。C处上一句说的是原生演替和次生演替达到顶级系列所需时间不同的原因，下一句说的是影响演替速度的因素，题干中的句子属于影响因素的进一步解释，应当出现在影响因素后面，而不是前面，故不能插入C处。
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