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  3. Forest is a complex ecosystem in which trees are the dominant life form. Tree dominated forests can occur wherever the temperatu

Forest is a complex ecosystem in which trees are the dominant life form. Tree dominated forests can occur wherever the temperatu

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问题     Forest is a complex ecosystem in which trees are the dominant life form. Tree dominated forests can occur wherever the temperatures rise above 50°F(10℃) in the warmest months and the annual【C1】______  is more than 8 in. (200mm). They can develop under various conditions within these limits, and the kind of soil, plant, and animal life differs according to the extremes of environmental influences. In cool, high-latitude subpolar regions, taiga forests are dominated by【C2】______  conifers (针叶松). In more temperate high-latitude climates, mixed forests of both conifers and broad-leaved deciduous trees【C3】______ . Broad-leaved deciduous forests develop in midlatitude climates. In humid equatorial climates, tropical rain forests develop. There heavy rainfall supports evergreens that have broad leaves instead of the needle leaves of cooler evergreen forests. Forests are among the most complex ecosystems in the world, with extensive vertical layering. Conifer forests have the simplest structure: a tree layer, a shrub layer that is【C4】______ or even absent, and a ground layer covered with lichens, mosses, and liverworts. Deciduous forests are more complex (the tree canopy is divided into an upper and lower story), and rain-forest canopies are divided into at least three layers. The forest is nature’s most efficient ecosystem, with a high rate of【C5】______ affecting both plant and animal systems in complex organic relationships.
    Rain forest is lush forest, generally composed of tall, broad-leaved trees and usually found in wet tropical regions around the equator. Depending on the amount of annual rainfall, the trees may be evergreens or mainly deciduous. The former require more water. Soil conditions vary with location and climate, though most rain-forest soils tend to be permanently moist and not very【C6】______ , because the hot, humid weather causes organic matter to decompose rapidly and to be absorbed quickly by tree roots and fungi. Rain forests have many vertical layers of plant and animal development. The highest plant layer, the tree canopy,【C7】______ to heights of 100-165 ft (30-50 m). Animals there mostly feed and escape from predators among leaves and branches. The next-highest layer is filled with small trees. Above the ground surface the space is occupied by tree branches, twigs, and  【C8】______ . Many species of animals live in the undergrowth. Contrary to popular belief, the rain forest floor is not impassable, being bare except for a thin layer of humus and fallen leaves. Animals【C9】______ this layer are adapted to walking and climbing short distances Burrowing animals are found below the soil surface, as are microorganisms that help decompose and free much of the organic litter accumulated by other plants and animals from all layers. The climate of the ground layer is unusually【C10】______ because the upper stories of tree canopies and the lower branches filter sunlight and heat radiation and reduce wind speeds, keeping the temperatures fairly even.
   A. resort           I. extends
   B. stable           J. inhabiting
   C. conform          K. predominate
   D. foliage          L. biology
   E. fertile          M. spotty
   F. apprentice       N. precipitation
   G. photosynthesis   O. perishing
   H. hardy
【C4】
选项
答案M
解析 spotty形容词，意为“零星的”。本句的意思是针叶林的构造最简单，即由树层，零星的灌木层或是没有灌木层以及长有地衣、苔藓和叶苔的地面层构成。
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