TPO 第23套 综合写作题目重新选题
Populations of the yellow cedar, a species of tree that is common in northwestern North America, have been steadily declining for more than a century now, since about 1880. Scientists have advanced several hypotheses explain this decline.
One hypothesis is that the yellow cedar decline may be caused by insect parasites, specifically the cedar bark beetle. This beetle is known to attack cedar trees; the beetle larvae eat the wood. There have been recorded instances of sustained beetle attacks overwhelming and killing yellow cedars, so this insect is a good candidate for the cause of the tree’s decline.
A second hypothesis attributes the decline to brown bears. Bears sometimes claw at the cedars in order to eat the tree bark, which has a high sugar content. In fact, the cedar bark can contain as much sugar as the wild berries that are a staple of the bears’ diet. Although the bears’ clawing is unlikely to destroy trees by itself, their aggressive feeding habits may critically weaken enough trees to be responsible for the decline.
The third hypothesis states that gradual changes of climate may be to blame. Over the last hundred years, the patterns of seasonal as well as day-to-day temperatures have changed in northwestern North America. These changes have affected the root systems of the yellow cedar trees: the fine surface roots now start growing in the late winter rather than in the early spring. The change in the timing of root growth may have significant consequences. Growing roots are sensitive and are therefore likely to suffer damage from partial freezing on cold winter nights. This frozen root damage may be capable of undermining the health of the whole tree, eventually killing it.
Summarize the points made in the lecture, being sure to explain how they support/contradict specific points made in the reading passage.
Main point: Scientists have advanced several hypotheses to explain this decline.
Sub point 1: The yellow cedar decline maybe caused by insect parasites, specifically the cedar bark beetle.
Sub point 2: A second hypothesis attributes the decline to brown bears.
Sub point 3: Gradual changes of climate maybe to blame.
Main point: We still do not know what is killing the yellow cedar. None of the explanations discussed in the reading isadequate.
Sub point 1: These healthy yellow cedars are generally much more resistant to insect infection than other tree species.
Sub point 2: Bears cannot be the reason forthe decline, because islands cedars are decreasing without bears.
Sub point 3: Although the climate change may have made the cedar roots more sensitive than they used to be, this is not what killed them.