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。

　　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