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  3. FOREST SUCCESSION—LAYERS OF A FOREST

FOREST SUCCESSION—LAYERS OF A FOREST

游客2024-01-02  8
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. [br] An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Succession is a continuous change in the species composition, structure, and function of a forest through time following a disturbance.
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Answer Choices
(A) Primary succession occurs at sites where soil must be developed and thus takes a far longer to complete than secondary succession, which occurs where relatively undisturbed soil already exists.
(B) The second stage of forest development is much shorter in boreal forests than it is in tropical forests.
(C) Old-growth forest differs from earlier succession stages in that most soil nutrients come from leaves and fine root debris rather than from dead trees and other coarse woody debris.
(D) Early in secondary succession resources are relatively abundant and vegetation increases until canopy closure, after which competition for resources brings about increased plant mortality.
(E) With a rising tree mortality, openings in the canopy develop, leading to layered plant growth beneath the overstory canopy, and eventually a climax stage is reached.
(F) The effective suppression of grassland wild fires has provided opportunities for the ecotone to proceed into a forest.
选项
答案 A,D,E
解析 A项“原生演替发生的地方土壤必须充分发育，次生演替发生在未被干扰的土壤上。因此，原生演替所需时间远比次生演替的长”符合原文第1段第6句和倒数第2句话的表达，是解释前文描述的原生演替比次生演替所需时间长的重要原因，故选此项。D项“次生演替早期阶段的资源相对丰富，直到冠层封闭前，植被都在成长；冠层郁闭后，资源竞争导致植物死亡率上升”符合原文第3段第5句和第4段第1句所述，分别是对第一阶段和第二阶段的描述，也符合文章主题——论述演替阶段，故选。E项“随着树木的死亡数量增加，冠层缝隙扩大，上层林冠以下的多层植被得以生长。最终，演替到达了顶级阶段”，根据原文第4段第3句，在第三阶段，上层林冠的打开使下层植被得以生长；根据第4段第5句，森林演替的最后阶段是顶级阶段。因此，E项符合题意。B项“与热带森林相比，北方森林的森林演替第二阶段更短”，原文未提到第二阶段演替的时间和纬度的关系，无从得出此结论，故排除B项。C项“原始森林阶段与森林的早期演替阶段的区别在于原始森林的土壤成分来源于叶子和细根碎片，而不是死树和粗糙的木质残体”与原文第4段倒数两句的描述相悖，故排除C项。F项“草地野火的有效扑灭，使交错群落有机会演变成森林”符合原文第5段最后一句的表达，但该野火的例子只是细节信息，而不是核心观点，故排除此项。
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