[SAT课外阅读30解析]科学：山火带来的好处

Burning benefits: Controlled fires can both help prevent combustion and reduce insect attacks

The Economist, May 21,

The giant wildfire that raged recently through the Canadian province of Alberta, forcing more than 80,000 people to flee their homes, was caused in part by global warming producing drier conditions. Yet conservation efforts to prevent burning have not helped either, say some experts. Forests can regenerate after being burnt, with much of the tinder-like underbrush being cleared away and dense tree canopies broken up so that young trees can get the sunlight they need to grow. Now a new study finds that fires, whether started naturally or under controlled conditions, can alsothwart（阻止）nasty（严重的） insect infestations.

段1: 通过最近的火灾引入话题和新的研究发现（火灾有助于阻止害虫泛滥）。

Like many useful discoveries, this one came about somewhat by accident. Sharon Hood of the US Forest Service was working with the University of Montana and colleagues on the ecology of a forest in western Montana that had been managed in a number of ways to make it more resistant to fire. They were monitoring areas that had been thinned to open up the canopy, exposed to controlled burns to remove ground growth, or both. To provide a control area for comparison, the team also kept an eye on part of the forest that had not seen a blaze since the 1800s. They were planning to monitor the forest for many years to come, but then the beetles arrived.

段2: 解释研究设计（实验组vs对照组）及意外情况（虫灾）。

These were mountain pine beetles, a pest in North America. The beetles lay their eggs in the phloem（韧皮部） layer inside the trees’ bark, whichconducts（传递） nutrients from the leaves down to the roots. When the eggs hatch, the larvae feed off the phloem, draining the tree of its vital resources. If the number of beetles is low, trees are able to defend themselves effectively by flooding the areas under attack with an insecticidal resin. However, in great numbers the beetles tend to use chemical signalling compounds called pheromones（弗洛蒙/信息素） to concentrate their invasion on one tree at a time, which exhausts resin levels and ultimately kills the tree. Beetle outbreaks have taken place for millennia, but recently have been getting much larger and lasting a lot longer.

段3: 解释害虫如何对树木造成损害。

Dr Hood knew the beetles were particularly problematic in forests where fires had been suppressed for a long time, although no one was sure why. The arrival of the beetles in , just five years after the team began their monitoring, provided a golden opportunity to find out. So the researchers compared a survey of the forest with the findings from . They found 720 of 2,189 trees had died during that time. Obvious evidence in the form of larval tunnels and bore holes showed that the beetles were responsible for theirdemise（死亡）. However, the distribution of the dead trees was notevenly（均匀地） spread among the various zones.

段4: 说明研究过程及结果（发生虫灾，之后7年里的变化说明了虫灾的危害，并且地区分布不均匀）。

As Dr Hood reports in Ecological Applications, the death toll was 50% in the control zone, 39% in the area intentionally burned, 14% in the one both thinned and burned, and nearly zero where it was merely thinned. There are two main reasons for this, she argues. First, the trees in the burn-only zone had particularly low levels of chemical compounds known as monoterpenes（单萜） that the beetles use to make their pheromones. Without these chemicals, Dr Hood speculates that the beetles found it more difficult tocoordinate（协调） their attacks. Second, trees living in the thinned-only forests were healthier overall due to reduced competition for resources and had larger supplies of resin available to them. Thus they were in a much better position tofend off（抵挡） the beetles. Thinning and controlled burning may well be a good way to reduce wildfires and insect infestations, but organising this over the vast swathes of land that have beensubject to（受…影响） fire suppression for over a century will not be easy.

段5: 研究结果（被人为削减的森林遭受的损失小）及解释原因（1. 燃烧使某种化合物减少，进一步影响害虫发送信息素；2. 降低森林密度使树木能产生更多树脂来抵御害虫）。