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.
Unfortunately, we still don’t know what’s killing the yellow cedar, none of the explanations discussed in the reading is adequate.
First, the cedar bark beetle. Well, the problem with this explanation is that healthy yellow cedars are generally much more resistant to insect infestation than other tree species. For example, the bark and leaves of yellow cedar are saturated with powerful chemicals that are poisonous to insects. So, healthy cedars are unlikely to suffer from insect damage. So, how can we explain those dead cedars that were infested with beetles. In those cases, the beetles attack trees that were already damaged or sick, and would’ve probably died anyway. So, the beetles are not the fundamental cause responsible for the decline of yellow cedars.
Second, although bears damage some trees, there’re not the cause of the overall population decline. Yellow cedar population’s been declining all across the northwestern coast of North America both on the mainland and on islands just off the coast. There were no bears on the islands, yet the islands cedars are still in decline. Since the decline occurs with and without bears, the bears cannot be responsible.
And finally, the theory about roots suffering from frost damage, well, the reading passage forgot to take one factor into account. Many more trees are dying at lower elevations where it is warm than at higher elevations where it is cold. If freezing damage were responsible for the decline, we could expect to see more trees dying in the cold weather of higher elevations. Instead, more trees are dying in the relative warmth of the lower elevations. So, although the climate change may have made the cedar roots more sensitive than it used to be, this isn’t what’s killing them.
Main points: Scientist have some hypotheses to explain the drop of yellow cedar population.
Sub point 1: Decline caused by insect parasites.
Sub point 2: Decline caused by brown bears for food.
Sub point 3: Decline caused by cold winter.
Attitude: Against. None explanations is adequate.
Sub Point 1: Healthy yellow cedars can resist insect infestation. (Poison. Tree already sick)
Sub Point 2: Bears are not the cause of overall decline. (Decline on no bear island)
Sub Point 3: Cold weather is not responsible for the decline. (Die at lower warm elevations)
The lecturer holds that the main reason causing the overall decline in yellow cedar population has not yet been determined. The proposed reasons in the reading, though responsible for the poor health of some individual plants, may not account for the decline of the whole species throughout the North American Continent.
First, healthy yellow cedar trees can secrete a chemical that is poisonous to insects feeding on its bark. Hence, it is unlikely that the cedar bark beetle can ever attack one of these trees before it falls ill or dies. The reading thus finds a misleading causal relationship between the insect and the tree.
Second, bears cannot be blamed for the large-scaled death of yellow cedars across North America, although they might be responsible for the accidental death of some individual trees. Another condition that can absolve bears of this accusation is that yellow cedars growing on bear-free islands are also dying in large numbers. Hereby the reading material fails to identify the primary reason again.
Third, cold climates also cannot be blamed for the general decline in cedar population. The proof can be found in the trees’ wider recession in areas of lower elevation, where the temperature is warmer than at higher elevations. Though cold weather may have made cedars more sensitive, it is not the primary killer as the reading indicates.
全文遵从总分结构，第一段立刻指出L觉得造成黄杉数量下降的原因 not yet been determined，所以对R的态度主要是，R的解释都不充分(not adequate).
1. …..(论点A)……。The proof can be found…..(证据A).
The defendant is not guilty. The proof can be found when new evidence is processed.
2. absolve (某物，某人，某事件) of this accusation
The newly found evidence can absolve the defendant of his accusation of murder.