The word "severity" in the passage is closest in meaning to -小站备考

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.

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