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［D］ 矮木构成的下层植被和木藤的藤边，或是藤蔓植物，形成了一片具有相当复杂的内部体系的森林。在这片森林中，很多动物，包括体积很大的一些动物，很少或者根本不能到达地面。
［F］ 然而，由于更多的阳光穿透森林冠层，下层植被和森林地表植被比在热带地区生长得更好。
4、Directions:
You are going to read a list of headings and a text about preparing in the academic community. Choose the most suitable heading from the list A-F for each numbered paragraph (41-45). The first and last paragraphs of the text are not numbered. There is one extra heading which you do not need to use. Mark your answers on ANSWER SHEET 1. (10 points)
Until about two million years ago Africa’s vegetation had always been controlled by the interactions of climate; geology, soil, and groundwater conditions; and the activities of animals. The addition of humans to the latter group, however, has increasingly rendered unreal the concept of a fully developed “natural” vegetation—i.e., one approximating the ideal of a vegetational climax. (41). Early attempts at mapping and classifying Africa’s vegetation stressed this relationship: sometimes the names of plant zones were derived directly from climates. In this discussion the idea of zones is retained only in a broad descriptive sense.
(42). In addition, over time more floral regions of varying shape and size have been recognized. Many schemes have arisen successively, all of which have had to take views on two important aspects: the general scale of treatment to be adopted, and the degree to which human modification is to be comprehended or discounted.
(43). Quite the opposite assumption is now frequently advanced. An intimate combination of many species—in complex associations and related to localized soils, slopes, and drainage—has been detailed in many studies of the African tropics. In a few square miles there may be a visible succession from swamp with papyrus, the grass of which the ancient Egyptians made paper and from which the word “paper” originated, through swampy grassland and broad-leaved woodland and grass to a patch of forest on richer hillside soil, and finally to juicy fleshy plants on a nearly naked rock summit.
(44). Correspondingly, classifications have differed greatly in their principles for naming, grouping, and describing formations: some have chosen terms such as forest, woodland, thorn-bush, thicket, and shrub for much of the same broad tracts that others have grouped as wooded savanna (treeless grassy plain) and steppe (grassy plain with few trees). This is best seen in the nomenclature, naming of plants, adopted by two of the most comprehensive and authoritative maps of Africa’s vegetation that have been published: R. W. J. Keay’s Vegetation Map of Africa South of the Tropic of Cancer and its more widely based successor, The Vegetation Map of Africa, compiled by Frank White. In the Keay map the terms “savanna” and “steppe” were adopted as precise definition of formations, based on the herb layer and the coverage of woody vegetation; the White map, however, discarded these two categories as specific classifications. Yet any rapid absence of savanna as in its popular and more general sense is doubtful.
(45). However, some 100 specific types of vegetation identified on the source map have been compressed into 14 broader classifications.
［A］ As more has become known of the many thousands of African plant species and their complex ecology, naming, classification, and mapping have also become more particular, stressing what was actually present rather than postulating about climatic potential.
［B］ In regions of higher rainfall, such as eastern Africa, savanna vegetation is maintained by periodic fires. Consuming dry grass at the end of the rainy season, the fires burn back the forest vegetation, check the invasion of trees and shrubs, and stimulate new grass growth.
［C］ Once, as with the scientific treatment of African soils, a much greater uniformity was attributed to the vegetation than would have been generally accepted in the same period for treatments of the lands of western Europe or the United States.
［D］ The vegetational map of Africa and general vegetation groupings used here follow the White map and its extensive annotations.
［E］ African vegetation zones are closely linked to climatic zones, with the same zones occurring both north and south of the equator in broadly similar patterns. As with climatic zones, differences in the amount and seasonal distribution of precipitation constitute the most important influence on the development of vegetation.
［F］ Nevertheless, in broad terms, climate remains the dominant control over vegetation. Zonal belts of precipitation, reflection latitude and contrasting exposure to the Atlantic and Indian oceans and their currents, give some reality to related belts of vegetation.
［G］ The span of human occupation in Africa is believed to exceed that of any other continent. All the resultant activities have tended, on balance, to reduce tree cover and increase grassland; but there has been considerable dispute among scholars concerning the natural versus human-caused development of most African grasslands at the regional level.
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