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.
黄杉是北美西北部常见的树木。但是从1880年开始的一个多世纪以来，黄杉的数量在逐渐减少。目前，科学家提出了几个假说来解释黄杉数量的下降。
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.
其中一个假说认为黄杉数量下降是由以衫皮甲虫为代表的寄生害虫导致。这种甲虫据了解是
以杉树皮为食，而其幼虫则以木质部分为食。史上不乏对于甲虫侵蚀和导致黄杉大量死亡的
记录。因为，这种甲虫很可能是黄杉数量减少的原因。#p#副标题#e#
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.
第二种假说认为是棕熊导致了黄杉数量的减少。熊有时会扒下黄杉的树皮用以食用，因为黄杉树皮还有很高的糖分。事实上，黄杉树皮的含糖量同棕熊主要使用的野生浆果相差无几。虽然棕熊扒树皮的行为不会直接导致黄杉的死亡，但是它们这种破坏性的觅食行为却因为降低了树木的生命力，而间接导致黄杉数量的减少。
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.
第三个假说是气候的缓慢变化导致了黄杉数量的减少。在过去的一百年里，在北美西北部，无论是季节性的气候更替还是每天的气温调节都发生了变化。这些变化对黄杉根系产生了影响。以前，黄杉的表面细根是初春开始生长的;而现在却在晚冬就开始生长。根系生长时间的变化可能会造成非常严重的后果。新生的根系都非常敏感，因而非常容易因为冬季夜晚部分结冰的温度受到损害。这种根系冻伤现象可能会导致整个树木健康状况下降，并逐渐导致死亡。
托福综合写作阅读部分需要提取的观点是：
- 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
托福综合写作听力材料：
Unfortunately，we still do not knowwhat is killing the yellow cedar. None of the explanations discussed in thereading is adequate.
很遗憾，我们还是不知道是什么导致了黄杉的死亡。阅读部分没有一个假说可以解释其数量减少。
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.
首先是杉皮甲虫假说。这个解释的问题在于黄杉相对于其他树种来说其实有更强的抵御害虫侵袭的能力。比如，黄杉的树皮和叶子上聚集着一些对于昆虫来说有毒的的强力化学物质。所以，健康的黄杉是不会受到害虫损害的。那么，我们该如何解释那些因为甲虫侵袭而死亡的黄杉呢?这种情况是因为甲虫侵袭的是已经受损或者患病的黄杉，这些黄杉没有甲虫也会死去。所以，甲虫的入侵不是黄杉数量下降的重要因素。
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.
第二，虽然熊破坏一些树木，但是他们却不是整个黄杉种群数量下降的原因。在北美西北部整个海岸线上的黄杉数量都在下降，不管是在大陆还是在离岸的岛屿上。而在那些岛屿上是没有熊的，而黄杉的数量却依然减少。因为无论有无熊黄杉数量都在减少，所以熊不回事数量减少的原因。
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.
最后是根系因低温损害假说。恩，阅读部分忘掉了一个事实，那就是在温暖的海拔低处，黄杉死亡数量比相对寒冷的高海拔地区要多。如果低温损害是数量减少的原因，那么我们可以推理在寒冷的高海拔地区树木死亡的要多。然而事实相反，温暖的低海拔地区死亡的数量更多。所以，虽然气候的改变让黄杉根系变得比过去更加铭感，但是这不是他们死亡的原因。
托福综合写作听力笔记：
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。
托福综合写作范文(仅供参考)：
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.