2011年托福阅读真题+题目+答案解析:Forest Succession

 Succession is a continuous change in the species composition, structure, and function of a forest through time following 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. 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. 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. The rate of succession is dependent upon the extent of the disturbance and the availability of appropriate seeds for recolonization.

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 succession 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.

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 disturbance in some tropical forests, but may take three to fifty years in evergreen forests.

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.

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.

paragraph 1

题目及答案解析：

1.According to paragraph 1, each of the following can create a site that is likely to be subject to secondary succession EXCEPT

A. large scale forest clearances

B. volcanic eruptions

C. hurricanes

D. major forest fires

答案解析：B

本题为否定事实信息题，这道题的选项很短，所以很有可能对应原文中某一处的举例。我们根据题干中的关键词“secondary succession”定位到本段的第6句话“Secondary succession is the reestablishment of vegetation following a disturbance that killed or removed the vegetation but did not greatly affect the soil.”这里介绍了次生演替的概念。而随后的这句话主要介绍了次生演替发生的场所“……while clear-cutting of forests, wild fires, and hurricanes are examples of sites subject to secondary succession.”这句话说被砍伐的森林、野火和飓风过境之处是次生演替发生的地点。这里注意区分开“primary succession(原生演替)”和“secondary succession(次生演替)”两个概念，不要混淆。选项中只有 B：volcanic eruptions 火山爆发属于原生演替发生的地点，而不是次生演替的发生地，所以这道题选 B。

2.Paragraph 1 supports which of the following statements about disturbances?

A. They occur in a series of stages.

B. They can result from natural causes or human activities.

C. They may contribute to increased volcanic eruptions.

D. They have no impact on the speed with which succession occurs.

答案解析：B

本题为事实信息题，题干关键词“disturbance”在整段中重复出现，无法通过定位词缩小阅读范围，因此保险起见我们需要阅读整段文字，然后用排除法做题。 A 选项说干扰分好几个阶段进行。而文章中只提到“Each stage of succession is referred to as a successional sere.”这句话是说演替的每一阶段都被称为一个演替系列。整段文字完全没有提到 disturbance 分阶段进行，故排除。B 选项说干扰可以由自然因素和人为因素造成。我们定位到这句话“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.”这句话中例举了原生演替和次生演替发生的场所，其中火山爆发、冰川退缩、裸露沙丘、野火、飓风都属于自然因素，而森林砍伐很明显属于人为因素，因此 B 选项正确。C 选项说 disturbance 会加剧火山爆发，而文中只说火山爆发之处是原生演替发生的场所，也就是说火山爆发自身就是一个 disturbance。所以 C 选项逻辑错误，排除。D 选项说 disturbance 对演替速率没有影响，我们定位到本段最后一句“The rate of succession is dependent upon the extent of the disturbance and the availability of appropriate seeds for recolonization.”这句话说演替速率取决于受到干扰的程度以及合适的再生植物种子的有效性。所以 D 选项与原文矛盾，故排除。

3.According to paragraph 1, the main factor that explains why it takes so long for a primary succession to reach climax is the time it takes for

A. glaciers to retreat.

B. dead vegetation to decay.

C. soil to form.

D. seeds to mature after recolonization.

答案解析：C

本题为事实信息题，根据题干中的关键词“climax”定位到这一段倒数第二句话“ 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.”这句话说原生演替需要的时间更长的原因是因为土壤必须先形成。“because”后面的内容往往是解题关键，所以同学们阅读时必须注意文章逻辑层次。因此 C 为正确答案。

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