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

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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.

The reading states the declining of the yellow cedar population in northwestern North America and provides three possible hypotheses. However, the professor explains that there is no proof about what is actually the cause of the reduction of these trees and refutes each of the author's claims.

First, the article claims that the bark beetle is responsible for the reduction of the yellow cedar trees. The professor refutes this point by saying that the healthy cedars have several mechanisms of defense against any kind of insect parasites. Thus, if a beetle tries to attack this species of trees, then the tree will release a powerful chemical to kill it. Moreover, the beetle only could attack sick trees, so insects are not the primary cause of the reduction of these trees.

Second, the reading posits that brown bears have caused the declining of yellow cedars because they can obtain a good source of food rich in sugar from them. However the professor says that bears cannot cause the overall reduction of the population of trees. According to the professor, this specie of tree has been depleting also on the coast of the country, in which bears do not inhabit the areas. Therefore, the brown bears are not responsible.

Third, the article supports that changes in the weather could trigger a reduction of trees. As a result, the trees' roots are more sensible and likely to suffer injuries. The professor opposes to this point by mentioning that the trees more affected are those at lower elevation with warmer temperatures. Moreover, by this theory the trees at higher elevation should be damaged even more. Nevertheless, this is not the case, so the climate changes are not killing the yellow cedar trees.

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Sentence-Text Coherence: 0.362 0.35
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