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.
1. According to paragraph 1 each of the following can create a site that is likely to be subject to secondary succession EXCEPT large scale forest clearances volcanic eruptions hurricanes major forest fires
2. Paragraph 1 supports which of the following statements about disturbances They occur in a series of stages. They can result from natural causes or human activities. They may contribute to increased volcanic eruptions. They have no impact on the speed with which succession occurs.
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 glaciers to retreat dead vegetation to decay soil to form seeds to mature after recolonization
Paragraph 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 succession species in primary succession because nitrogen is not derived from the weathering of the 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.
4. Paragraph 2 supports the idea that nitrogen-fixing plant species are heavily represented in the early stages of a primary succession because they do not require soil that contains nitrogen. generally grow more rapidly than other plants. tend to be more stress tolerant than other plants. tend to be less weedy than other plants.
Paragraph 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 disturbance in some tropical forests, but may take three to fifty years in evergreen forests.
5. The word "severity" in the passage is closest in meaning to suddenness seriousness location timing 6. According to paragraph 3 the earliest stage of a secondary succession is characterized by each of the following EXCEPT an incomplete covering of vegetation a canopy made up of trees that vary widely in age relatively ample nutrients available in the soil relatively plentiful light reaching growing plants
Paragraph 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.
7. The word "induces" in the passage is closest in meaning to
explains
increases
disappears with
brings about
8. The word "consequently" in the passage is closest in meaning to
in other words
nevertheless
as a result
basically
9. The word "catastrophic" in the passage is closest in meaning to
extremely harmful
very complex
different
long-lasting
10. According to paragraph 4 which of the following is a characteristic of old growth forests
improved ability to withstand major disturbances
increasing amounts of woody debris
a decline in the mortality rate of tree species
increasing reliance on leaf and fine root deatritus as a source of nutrients
11. According to paragraph 4 one difference between the second and the third stage in forest
development is that in the third stage
there is more intraspecific competition than interspecific competition.
the growth of understory and suppressed substory trees slows down.
sudden changes in the environment are less frequent.
the canopy becomes more complex
Paragraph 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.
12. Why does the author discuss pyric climax in the passage?
To help explain the idea that a forest may not necessarily reach the old growth stage.
To suggest that fire is a natural disturbance least likely to prevent an ecosystem from reaching its potential climax stage.
To acknowledge that the pattern of succession following a disturbance is a very different in ecosystems such as grasslands than it is in forests.
To help explain how the boundary areas of an ecosystem affect the way its structure changes during succession
托福考试栏目为您推荐：
