Origins of the Deciduous Forests
Forests dominated by deciduous trees (trees with leaves that fall off at the end of the growing season) are found around the world. But they were not always so widespread, and there are unanswered questions about their evolution. The ancestors of deciduous trees can be traced back to the Cretaceous Period, 113-65 million years ago, when dinosaurs ruled the land and the continents occupied different positions from today. At this time, extensive deciduous forests existed only at very high latitudes, near the north and south poles.
Alaska was closer to the North Pole during the late Cretaceous than it is today, but the climate was milder, reflecting higher temperatures for the entire planet. This can be deduced from the shape of fossil leaves. There is a remarkably consistent relationship between leaf shape and mean annual temperature for modern plant species. For example, the proportion of species with toothed leaves consistently increases as one moves from warm to cold climates. Based on statistical analyses of a large number of such leaf characteristics, temperatures in the Cretaceous Arctic generally hovered above freezing during the winter. This is confirmed by the lack of geological evidence of permafrost, a layer of earth that is permanently frozen. However, fossilized tree trunks at Ellesmere Island in northern Canada display damaged sections (growth interruptions) consistent with damage caused by severe frost indicating that occasional frosts occurred. Also, although the winters were generally mild, they were-like modern Arctic winters-very dark. Because of the tilt of Earth’s axis, very little sunlight reaches high northern latitudes for several months each year, so trees and shrubs of the ancient Arctic could not photosynthesize(produce chemical energy from sunlight) for several weeks or even months depending on the latitude. This is apparent from growth rings from fossilized wood of the period; the annual growth rings end abruptly, evidence of a rapid interruption of photosynthesis and plant growth with the onset of winter darkness.
The combination of mild temperatures and dim light during the winter may explain why so many Cretaceous trees shed their leaves at the end of the growing season. This would help reduce water loss and energy use during an extended period of darkness. Because the winters were relatively warm, leaves on an evergreen tree, which does not lose its leaves, would continue to undergo respiration (breathing), resulting in a steady loss of energy during a period when photosynthesis was not possible and energy stores could not be replenished. Thus, the food reserves of the plant would be expended to keep the leaves alive. In contrast, deciduous trees dropped their leaves and entered a period of deep dormancy during which they lost relatively little energy. When the Sun reappeared in the spring, new leaves sprang out to take advantage of the short- but almost continuously sunny-growing season.
This explanation for the prevalence of deciduous trees in the ancient Arctic is so logical that it was not rigorously tested for many years. In the early 2000s, however, this hypothesis was undermined by experiments with both evergreen and deciduous species belonging to plant groups that grew in the Arctic or Antarctic forests of the Cretaceous. A team of researchers compared growth rates of young trees for two types of deciduous trees-ginkgo and dawn redwood-and two types of evergreen trees-southern beech and coast redwood-in greenhouses with a simulated (imitated) Cretaceous Arctic winter (dim light and mild temperatures). Surprisingly, the deciduous trees lost more of their food reserves(as measured by the amount of carbon in the trees)during the simulated winter than did the evergreen trees. This is due to the immediate and massive loss of carbon as leaves are shed, which offsets the carbon saved by winter dormancy. If these modern trees responded in a similar way to their ancient relatives, then it is perplexing that evergreen trees did not predominate in Arctic forests. Deciduous trees may have an advantage during the summer rather than during the winter, however, because their leaves could grow and photosynthesize more rapidly than evergreen leaves during the brief growing season.
1
Forests dominated by deciduous trees (trees with leaves that fall off at the end of the growing season) are found around the world. But they were not always so widespread, and there are unanswered questions about their evolution. The ancestors of deciduous trees can be traced back to the Cretaceous Period, 113-65 million years ago, when dinosaurs ruled the land and the continents occupied different positions from today. At this time, extensive deciduous forests existed only at very high latitudes, near the north and south poles.
The word “extensive” in the passage is closest in meaning to
Awidespread
Bearly
Cdeveloping
Ddense
2
Alaska was closer to the North Pole during the late Cretaceous than it is today, but the climate was milder, reflecting higher temperatures for the entire planet. This can be deduced from the shape of fossil leaves. There is a remarkably consistent relationship between leaf shape and mean annual temperature for modern plant species. For example, the proportion of species with toothed leaves consistently increases as one moves from warm to cold climates. Based on statistical analyses of a large number of such leaf characteristics, temperatures in the Cretaceous Arctic generally hovered above freezing during the winter. This is confirmed by the lack of geological evidence of permafrost, a layer of earth that is permanently frozen. However, fossilized tree trunks at Ellesmere Island in northern Canada display damaged sections (growth interruptions) consistent with damage caused by severe frost indicating that occasional frosts occurred. Also, although the winters were generally mild, they were-like modern Arctic winters-very dark. Because of the tilt of Earth’s axis, very little sunlight reaches high northern latitudes for several months each year, so trees and shrubs of the ancient Arctic could not photosynthesize(produce chemical energy from sunlight) for several weeks or even months depending on the latitude. This is apparent from growth rings from fossilized wood of the period; the annual growth rings end abruptly, evidence of a rapid interruption of photosynthesis and plant growth with the onset of winter darkness.
Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
ATrees and shrubs in the ancient Arctic could not photosynthesize during those months of the year when little sunlight reaches high latitudes due to the tilt of Earth’s axis.
BBecause of the tilt of Earth’s axis. very little sunlight was needed for trees and shrubs to produce the chemical energy necessary for photosynthesis at high northern latitudes.
CArctic trees and shrubs photosynthesized between several weeks and many months each year, depending on the latitude and the tilt of Earth’s axis.
DHigh northern latitudes receive little sunlight because of the tilt of Earth’s axis, which required ancient Arctic trees and shrubs to produce all the energy they needed in only a few months.
3
Alaska was closer to the North Pole during the late Cretaceous than it is today, but the climate was milder, reflecting higher temperatures for the entire planet. This can be deduced from the shape of fossil leaves. There is a remarkably consistent relationship between leaf shape and mean annual temperature for modern plant species. For example, the proportion of species with toothed leaves consistently increases as one moves from warm to cold climates. Based on statistical analyses of a large number of such leaf characteristics, temperatures in the Cretaceous Arctic generally hovered above freezing during the winter. This is confirmed by the lack of geological evidence of permafrost, a layer of earth that is permanently frozen. However, fossilized tree trunks at Ellesmere Island in northern Canada display damaged sections (growth interruptions) consistent with damage caused by severe frost indicating that occasional frosts occurred. Also, although the winters were generally mild, they were-like modern Arctic winters-very dark. Because of the tilt of Earth’s axis, very little sunlight reaches high northern latitudes for several months each year, so trees and shrubs of the ancient Arctic could not photosynthesize(produce chemical energy from sunlight) for several weeks or even months depending on the latitude. This is apparent from growth rings from fossilized wood of the period; the annual growth rings end abruptly, evidence of a rapid interruption of photosynthesis and plant growth with the onset of winter darkness.
Which of the following can be inferred from paragraph 2 about deciduous-tree species with toothed leaves?
AThey were more likely to thrive during the Cretaceous Period than they are in modern times.
BThey are most commonly found in regions with low average temperatures.
CThey are particularly susceptible to damage by frost.
DThey cannot survive dark winters.
4
Alaska was closer to the North Pole during the late Cretaceous than it is today, but the climate was milder, reflecting higher temperatures for the entire planet. This can be deduced from the shape of fossil leaves. There is a remarkably consistent relationship between leaf shape and mean annual temperature for modern plant species. For example, the proportion of species with toothed leaves consistently increases as one moves from warm to cold climates. Based on statistical analyses of a large number of such leaf characteristics, temperatures in the Cretaceous Arctic generally hovered above freezing during the winter. This is confirmed by the lack of geological evidence of permafrost, a layer of earth that is permanently frozen. However, fossilized tree trunks at Ellesmere Island in northern Canada display damaged sections (growth interruptions) consistent with damage caused by severe frost indicating that occasional frosts occurred. Also, although the winters were generally mild, they were-like modern Arctic winters-very dark. Because of the tilt of Earth’s axis, very little sunlight reaches high northern latitudes for several months each year, so trees and shrubs of the ancient Arctic could not photosynthesize(produce chemical energy from sunlight) for several weeks or even months depending on the latitude. This is apparent from growth rings from fossilized wood of the period; the annual growth rings end abruptly, evidence of a rapid interruption of photosynthesis and plant growth with the onset of winter darkness.
According to paragraph 2, the lack of wealth geological evidence of permafrost in the Arctic during the Cretaceous period indicates that
Awinters in the Arctic were shorter during the Cretaceous than they are today
Bthe occasional frosts during the Cretaceous did not cause major damage to the Arctic’s deciduous trees
Cwinter temperatures in the Arctic usually did not fall below freezing during the Cretaceous
Dthe Arctic was colder during the Cretaceous than it was during later periods
5
The combination of mild temperatures and dim light during the winter may explain why so many Cretaceous trees shed their leaves at the end of the growing season. This would help reduce water loss and energy use during an extended period of darkness. Because the winters were relatively warm, leaves on an evergreen tree, which does not lose its leaves, would continue to undergo respiration (breathing), resulting in a steady loss of energy during a period when photosynthesis was not possible and energy stores could not be replenished. Thus, the food reserves of the plant would be expended to keep the leaves alive. In contrast, deciduous trees dropped their leaves and entered a period of deep dormancy during which they lost relatively little energy. When the Sun reappeared in the spring, new leaves sprang out to take advantage of the short- but almost continuously sunny-growing season.
The word “expended” in the passage is closest in meaning to
Arequired
Bstored
Creplaced
Dused up
6
The combination of mild temperatures and dim light during the winter may explain why so many Cretaceous trees shed their leaves at the end of the growing season. This would help reduce water loss and energy use during an extended period of darkness. Because the winters were relatively warm, leaves on an evergreen tree, which does not lose its leaves, would continue to undergo respiration (breathing), resulting in a steady loss of energy during a period when photosynthesis was not possible and energy stores could not be replenished. Thus, the food reserves of the plant would be expended to keep the leaves alive. In contrast, deciduous trees dropped their leaves and entered a period of deep dormancy during which they lost relatively little energy. When the Sun reappeared in the spring, new leaves sprang out to take advantage of the short- but almost continuously sunny-growing season.
Which of the following best describes the effect of Arctic winters on evergreen trees during the Cretaceous, as discussed in paragraph 3?
AEvergreen trees were unaffected by Arctic winters because of the relatively warm temperatures.
BThe lack of light in winter prevented evergreen trees from photosynthesizing, which required them to use stored energy to maintain their leaves.
CThe mild temperatures in winter caused evergreen trees to undergo respiration in order to prevent water loss.
DThe winter caused evergreen trees to enter a period of dormancy in order to conserve energy until the Sun reappeared in the spring.
7
The combination of mild temperatures and dim light during the winter may explain why so many Cretaceous trees shed their leaves at the end of the growing season. This would help reduce water loss and energy use during an extended period of darkness. Because the winters were relatively warm, leaves on an evergreen tree, which does not lose its leaves, would continue to undergo respiration (breathing), resulting in a steady loss of energy during a period when photosynthesis was not possible and energy stores could not be replenished. Thus, the food reserves of the plant would be expended to keep the leaves alive. In contrast, deciduous trees dropped their leaves and entered a period of deep dormancy during which they lost relatively little energy. When the Sun reappeared in the spring, new leaves sprang out to take advantage of the short- but almost continuously sunny-growing season.
This explanation for the prevalence of deciduous trees in the ancient Arctic is so logical that it was not rigorously tested for many years. In the early 2000s, however, this hypothesis was undermined by experiments with both evergreen and deciduous species belonging to plant groups that grew in the Arctic or Antarctic forests of the Cretaceous. A team of researchers compared growth rates of young trees for two types of deciduous trees-ginkgo and dawn redwood-and two types of evergreen trees-southern beech and coast redwood-in greenhouses with a simulated (imitated) Cretaceous Arctic winter (dim light and mild temperatures). Surprisingly, the deciduous trees lost more of their food reserves(as measured by the amount of carbon in the trees)during the simulated winter than did the evergreen trees. This is due to the immediate and massive loss of carbon as leaves are shed, which offsets the carbon saved by winter dormancy. If these modern trees responded in a similar way to their ancient relatives, then it is perplexing that evergreen trees did not predominate in Arctic forests. Deciduous trees may have an advantage during the summer rather than during the winter, however, because their leaves could grow and photosynthesize more rapidly than evergreen leaves during the brief growing season.
Which of the following best describes the relationship between paragraphs 3 and 4?
AParagraph 4 challenges the idea from paragraph 3 that deciduous trees were common in the Arctic because they could save energy in the winter
BParagraph 4 elaborates on paragraph 3’s description of how deciduous trees entered dormancy in the winter in order to save energy.
CParagraph 4 contrasts modern examples of Arctic deciduous tree growth with those of ancient trees described in paragraph 3.
DParagraph 4 supports the idea presented in paragraph 3 that mild temperatures were an important factor in the shedding of leaves by Cretaceous trees.
8
This explanation for the prevalence of deciduous trees in the ancient Arctic is so logical that it was not rigorously tested for many years. In the early 2000s, however, this hypothesis was undermined by experiments with both evergreen and deciduous species belonging to plant groups that grew in the Arctic or Antarctic forests of the Cretaceous. A team of researchers compared growth rates of young trees for two types of deciduous trees-ginkgo and dawn redwood-and two types of evergreen trees-southern beech and coast redwood-in greenhouses with a simulated (imitated) Cretaceous Arctic winter (dim light and mild temperatures). Surprisingly, the deciduous trees lost more of their food reserves(as measured by the amount of carbon in the trees)during the simulated winter than did the evergreen trees. This is due to the immediate and massive loss of carbon as leaves are shed, which offsets the carbon saved by winter dormancy. If these modern trees responded in a similar way to their ancient relatives, then it is perplexing that evergreen trees did not predominate in Arctic forests. Deciduous trees may have an advantage during the summer rather than during the winter, however, because their leaves could grow and photosynthesize more rapidly than evergreen leaves during the brief growing season.
According to paragraph 4, what was an unexpected finding of the experiments conducted in the early 2000s?
AModern deciduous trees are remarkably similar to those that grew in the Cretaceous Period.
BDeciduous trees were not as prevalent in the ancient Arctic as once believed.
CDeciduous trees lost more energy during the simulated winter than during recent actual winters.
DEvergreen trees were better than deciduous trees at retaining food reserves during the simulated winter.
9
Forests dominated by deciduous trees (trees with leaves that fall off at the end of the growing season) are found around the world. [■]But they were not always so widespread, and there are unanswered questions about their evolution. [■]The ancestors of deciduous trees can be traced back to the Cretaceous Period, 113-65 million years ago, when dinosaurs ruled the land and the continents occupied different positions from today. [■] At this time, extensive deciduous forests existed only at very high latitudes, near the north and south poles. [■]
Look at the four squaresthat indicate where the following sentence could be added to the passage
However, we do know with some certainty when and where they originated
Where would the sentence best fit？Click on a square sentence to the passage.
10
During the Cretaceous Period, the ancestors of deciduous forests were located mainly near the north and south poles.
AThe high latitudes where the earliest deciduous forests existed get very little sunlight at certain times of the year and had a relatively mild climate during the Cretaceous
BAn early explanation for the success of deciduous trees was that they were able to save energy during the dark winters, which evergreen trees could not do
CModern deciduous trees have evolved to lose more energy during the winter than their ancient counterparts
DThe location of deciduous forests in the north during the Cretaceous Period was due to the gradual movement of the continents over millions of years.
EGrowing seasons during the Cretaceous Period were longer and sunnier than modern growing seasons in the Arctic, which allowed for the success of deciduous and evergreen trees alike.
FRecent studies suggest a need to reconsider the reasons why deciduous trees were successful in the Arctic during the Cretaceous Period
答案：
