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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  11
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 was the reason for the different growth trends of C3 and C4 plants in the two places?
选项 A、The soil composition greatly changed.
B、The bottom of one lake was elevated.
C、The moisture availability was different.
D、The carbon dioxide levels were different.
答案C
解析 本题关键词是C3，C4 plants，问题是C3和C4植物在两地的生长趋势不一致的原因是什么。可以定位到第四段。根据第四段第二、三句话，但是事实上，研究者发现两个地区的两类植物的变化是不同的。其变化并不与二氧化碳相关(not with carbon dioxide levels)。而是取决于降雨量的变化(shifts in rainfall)。选项C的moisture和原文rainfall是相同含义，是正确选项。选项D与二氧化碳相关，属于正反混淆。选项A、B无中生有，在文中找不到相关支持信息。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。
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