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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  33
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.
According to the third paragraph, which of the following statements is NOT true?
选项 A、Tropical grasses are usually C4 plants.
B、C3 and C4 plants used to live in northern Mexico and Guatemala.
C、C3 and C4 plants don’t have the same photosynthetic processes.
D、Tropical trees are all C3 plants.
答案D
解析 本题定位到第三段，注意本题要选错误选项作为正确选项。根据第三段第五句，许多C4植物是热带草类(tropical grasses)，而多数热带树种 (most tropical trees)则是C3植物，因此选项A与原文一致(true)，不可选；选项D过于绝对，原文毕竟是most，而不是all，所以根据题意。本题正确选项为选项D。根据第四句话，通过对叶蜡同位素组成的测定，研究者们可以区分出这些地区(in these areas)的两类主要植物——分别被称为C3和C4植物，它们有着不同的光合作用过程(different photosynthetic processes)，因此选项C与原文是相同含义(true)。这些地区指的是北墨西哥和北危地马拉的两个湖泊(in northern Mexico and Guatemala)，因此选项B也符合原文(true)。第三段：新技术检测得出历史上两个地区有C3和C4两大类植物。
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