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  3. Rising global carbon dioxide levels tied to global warming may not be as crucial in determining the composition of plant communi

Rising global carbon dioxide levels tied to global warming may not be as crucial in determining the composition of plant communi

admin2022-06-29  25
问题     Rising global carbon dioxide levels tied to global warming may not be as crucial in determining the composition of plant communities as other, localized climate shifts, such as droughts or temperature changes, an international team of scientists reports this week.
    "Nobody really knows what the increases in carbon dioxide are going to entail in terms of future changes in vegetation types," said study co-author Mark Brenner, a University of Florida assistant professor of paleolimnology, the study of ancient lakes. Brenner investigates climate change and human impacts on lakes and regional watersheds. "It looks like climate changes in different areas may be more important than carbon dioxide, at least carbon dioxide by itself," he said.
    Brenner’s research team, composed of six geologists and geographers, based their conclusions on an analysis of sediment from two lake bottoms, one in northern Mexico and one in northern Guatemala. The researchers used new techniques that allowed them to analyze only the remains of land plants, specifically their leaf waxes. "The technology has come on line to allow us to do studies that we couldn’t do at the time we collected these samples," Brenner said. By measuring the isotope composition of the leaf waxes, the researchers were able to distinguish two broad categories of plants living in these areas—so-called C3 and C4 plants, which have different photosynthetic processes. Many C4 plants are tropical grasses, while most tropical trees are C3 plants. The researchers analyzed sediments deposited over the last 27,000 years, from the last ice age to the current geological period. Over this period, there was a worldwide, relatively uniform increase in atmospheric carbon dioxide concentrations.
    Brenner said that if carbon dioxide played the major role in determining plant composition, one would assume that analysis of the sediments would reveal very similar changes in relative abundance of C3 and C4 plants in the two places over the study period. But, in fact, the researchers found that trends in the two types of plants were different at the two locations. The changes were related not with carbon dioxide levels, but with shifts in rainfall. Over the past 27,000 years, the climate shifted from wet to dry in Mexico and dry to wet in Guatemala, with corresponding shifts in the plant communities. At Lake Alta Babicora in Mexico, abundant trees and shrubs shifted to grasses. At Lake Quexil in Guatemala, the abundance of trees and shrubs increased. "The result appears to be that climate factors, especially moisture availability, determine whether C4 or C3 plants dominate in an area, not carbon dioxide," Brenner said. Researchers have found pollen, sediment and isotopic data at Lake Quexil indicating a much drier and much cooler climate 20,000 years ago. The vegetation was a sparse dry temperate shrub and grassland of a type that now grows above 5,000 feet.
    Many scientists believe global warming will cause major variation in local climates worldwide, with some wet areas becoming dry and dry areas becoming wet. If that happens, it could have more impact on relative C3 versus C4 plant distribution than the rising carbon dioxide levels.
What’s the main idea of the text?
选项 A、Global warming will cause major variation of vegetation distribution.
B、Climates factors determine the composition of plant communities.
C、How Brenner’s research team proved a truth.
D、C3 and C4 plants can decide the composition of plant communities.
答案B
解析 本文属于主旨大意题，需总结各段主题：第一段：在植物群落的组成方面，局部气候变化比二氧化碳浓度影响大。第二段：古湖学助理教授也认为气候变化对植被的影响比二氧化碳浓度增长大。第三段：新技术检测得出历史上这两个地区有C3和C4两大类植物。第四段：实验证明，二氧化碳浓度变化对植被的影响不如降雨量的变化大。第五段：局部地区气候变化对植物分布的影响比二氧化碳增多更大。从以上各段总结得出，第一段、第二段、第四段和最后一段都鲜明地指出了文章主题，即：局部地区气候的变化对植物相对分布的影响大于二氧化碳增多所带来的影响，因此选项B属于全面概括，为正确答案。选项A原文没有提及，属于无中生有；选项C是原文细节；选项D曲解文中细节。
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