The Reading.

The Reading.

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.

The Lecture.

TASK RESPONSE 1.

The reading passage propose three hypothesis for the decline of the yellow cedar in north-western North America, while the listening material casts doubt on all the explanations made in the reading.

Firstly, the reading puts forward that the cedar bark beetle should be held responsible for the decline of the yellow cedar and there are recorded tree deaths caused by beetle attacks. However, the lecturer refutes this theory by pointing out that healthy yellow cedars are generally much more resistant to insect infestation as their bark and leaves are saturated with powerful chemicals that are poisonous to insects. Due to the fact that healthy cedars are unlikely to suffer from insect damage, this explanation is not quite reasonable.

Secondly, the author claims that brown bears may have caused the decline of the yellow cedar. Because the tree bark contains a large amount of sugar, the bears will claw the trees to feed themselves, which could severely weaken the trees. But according to the lecture, the point in the reading cannot explain the overall population decline. Yellow cedar population’s been declining on islands just off the coast where no bear is found. Since the decline occurs with and without bears, the bears cannot be responsible.

Finally, the reading suggests that the yellow cedar decline should be attributed to climate change, which makes the roots now start growing in late winter instead of early spring. As a result, the roots are likely to get frozen and the trees may suffer from this root damage. The lecturer, conversely, disagrees with this explanation. He argues that if this theory is valid, then more trees are supposed to die in higher elevations because it is colder there, but on the contrary, it is in the lower elevations that more trees are found dying.

TASK RESPONSE 2.