Populations of the yellow cedar, a speciesof tree that is common in northwestern North America, have been steadilydeclining for more than a century now, since about 1880. Scientists have advancedseveral hypotheses to explain this decline.
One hypothesis is that the yellow cedardecline 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 andkilling yellow cedars, so this insect is a good candidate for the cause of thetree’s decline.
A second hypothesis attributes the declineto brown bears. Bears sometimes claw at the cedars in order to eat the treebark, which has a high sugar content. In fact, the cedar bark can contain as muchsugar as the wild berries that are a staple ofthe bears’ diet. Although thebears’ clawing is unlikely to destroy trees by itself, their aggressive feedinghabits may critically weaken enough trees to be responsible for the decline.
The third hypothesis states that gradualchanges of climate may be to blame. Over the last hundred years, the patternsof seasonal as well as day-to-day temperatures have changed in northwestern NorthAmerica. These changes have affected the root systems of the yellow cedartrees: the fine surface roots now start growing in the late winter rather thanin the early spring. The change in the timing of root growth may havesignificant consequences. Growing roots are sensitive and are therefore likelyto suffer damage from partial freezing on cold winter nights. This frozen root damagemay be capable of undermining the health of the whole tree, eventually killingit.
- Main point: 有三个假说解释黄杉数量减少
- Sub point 1: 杉皮甲虫的侵蚀
- Sub point 2: 棕熊的破坏
- Sub point 3: 气候变化对根系的影响
Unfortunately，we still do not knowwhat is killing the yellow cedar. None of the explanations discussed in thereading is adequate.
First, the cedar bark beetle. Well, theproblem with this explanation is these healthy yellow cedars are generally muchmore resistant to insect infection than other tree species. For example, thebark and leaves of the yellow cedars are concentrated with powerful chemicalsthat are poisonous to insects. So, healthy cedars are unlikely to suffer fromthe insect damage. So, how can we explain those dead cedars that were infectedwith beetles? In those cases, the beetles attacked trees that were alreadydamaged or sick and what probably dead any way. So, the beetles are not the fundamentalcause responsible for the decline of the yellow cedars.
Second, although bears damage some trees, theyare not the cause of the overall population decline. Yellow cedars population hasbeen declining all across the northwestern coast of North America, both on themainland 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 andwithout bears, the bears cannot be responsible.
And finally, the theory about rootsuffering from frozen damage. Well, the reading passage forgot to take one factinto account. Many more treesare dying in the lower elevations where it is warmerthan in the higher elevations where it is cold. If freezing damage wereresponsible for the decline, we would expect to see more trees die in the coldweather of the high elevations. Instead, more trees die in the relative warm ofthe low elevations. So, although the climate change may have made the cedarroots more sensitive than they used to be, this is not what killed them.
Main point: 三个假说都不成立
Sub point one: 杉皮甲虫不能侵蚀健康树木，可以侵害的树木本身就已经有问题
Sub point two: 在没有棕熊的岛屿上黄杉数量也减少了，所以不是棕熊造成的
Sub point three: 因为气温高的地海拔地区死亡的多，气温低的高海拔地区死亡的少，因此低温损害说不成立。
In the lecture, the professor points outthat the hypothesis made by the reading passage is not adequate to explain whythe population of Yellow Cedar is on the decline. He points out several weaknessesof the statements made by the reading passage.
On one hand, the professor claims thatinsect parasite may not be the fundamental cause of the decline. In fact, heargues that healthy yellow cedar can secrete chemicals which can kill the cedarbark beetles so that they are unlikely to suffer from insect parasite. Thetrees that suffers insect parasite have already been sick or on the edge ofdeath.
On the other hand, the professor assertsthat brown bears cannot cause overall decline in yellow cedar population. Thereis decline both on the mainland of North America and islands along the coast.However, there is no brown bear on the islands at all.
What’s more, the professor doubts thehypothesis that the change of climate is to blame for the declination. Headmits that the climate has changed rapidly recently. And, if the climatechange is the real cause, the number of trees died on the higher elevationshould be larger than that on a lower elevation. The fact is, however, on theopposite.