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 to 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 do not know what is 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 these healthy yellow cedars are generally much more resistant to insect infection than other tree species. For example, the bark and leaves of the yellow cedars are concentrated with powerful chemicals that are poisonous to insects. So, healthy cedars are unlikely to suffer from the insect damage. So, how can we explain those dead cedars that were infected with beetles? In those cases, the beetles attacked trees that were already damaged or sick and what probably dead any way. So, the beetles are not the fundamental cause responsible for the decline of the yellow cedars.
Second, although bears damage some trees, they are not the cause of the over all population decline. Yellow cedars population has been declining all across the northwestern coast of North America, both on the mainland and on the islands just off the coast. There are no bears on the islands, yet the islands cedars are still in decline. Since the decline occurs with and without bears, the bears can not be responsible.
And finally, the theory about root suffering from frozen damage. Well, the reading passage forgot to take one fact into account. Many more trees are dying in the lower elevations where it is warmer than in the higher elevations where it is cold. If freezing damage were responsible for the decline, we would expect to see more trees die in the cold weather of the high elevations. Instead, more trees die in the relative warm of the low elevations. So, although the climate change may have made the cedar roots more sensitive then they used to be, this is not what killed them.
The reading passage is about three hypotheses explaining why the yellow cedars in America decline. However, the lecturer points out that none of these hypotheses is adequate. She puts forward three reasons of her own to defend her views.
First, the first hypothesis mentioned in the reading passage is that the decline of the yellow cedar is caused by insect parasites, to be more specific, the cedar bark beetle. However, the lecturer disagrees with this because of the twofold reasons. On the one hand, she states that healthy trees are not likely to be attacked because their barks and leaves are saturated with poisonous chemicals and thus they are more resistant against the insect parasites than other trees. On the other hand, what beetles attack are actually sick or damaged trees, which will probably die soon. Thus, the lecturer thinks the beetles are not the fundamental cause of the decline of the yellow cedar.
Second, as for the hypothesis discussed in the reading that the brown bears cause the decline of the trees, the lecturer also opposes this because she says if the brown bears are the cause, only the number of trees located in the places with brown bears should decline. Actually, there are decline on the mainland of the North America and the islands along the coast, however, there is no brown bear on the islands at all.
Third, even though the third hypothesis discussed in the reading passage suggests that gradual changes of climate leading to the damage of the trees’ roots is the reason why the yellow cedars decline, the lecturer says this hypothesis does not consider one important factor so that it is not accurate. Her reason is that if the root systems of the yellow cedar trees are damaged from the cold winter nights, trees at high elevation with colder climate should suffer more than that at lower elevation with warmer climate. However, the fact is, on the opposite, that trees that are at lower elevation die more than that at higher elevation. Thus, though growing roots may have been more sensitive because of the cold weather, the cold winter is not killing the trees.
TOEFL listening lectures: Why does the professor mention New York Harbor?