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　　As I mentioned last week, I’ve recently returned from Australia. While I was there, I visited a eucalyptus forest that, in February, was the scene of an appalling wildfire. Perhaps naively, I had expected to find that many trees had been killed. They hadn’t. They had blackened bark, but were otherwise looking rather well, many of them wreathed in new young leaves. This prompted me to consider fire and the role it plays as a force of nature.

　　Fossil charcoals tell us that wildfires have been part of life on Earth for as long as there have been plants on land. That’s more than 400 million years of fire. Fire was here long before arriviste plants like grasses; it pre-dated the first flowers. And without wanting to get mystical about it, fire is, in many respects, a kind of animal, albeit an ethereal one. Like any animal, it consumes oxygen. Like a sheep or a slug, it eats plants. But unlike a normal animal, it’s a shape-shifter. Sometimes, it merely nibbles a few leaves; sometimes it kills grown trees. Sometimes it is more deadly and destructive than a swarm of locusts.

　　The shape-shifting nature of fire makes it hard to study, for it is not a single entity. Some fires are infernally hot; others, relatively cool. Some stay at ground level; others climb trees. Moreover, fire is much more likely to appear in some parts of the world than in others. Satellite images of the Earth show that wildfires are rare in, say, northern Europe, and common in parts of central Africa and Australia. (These days many wildfires are started by humans, either on purpose or by accident. But long before our ancestors began to throw torches or cigarette butts, fires were started by lightning strikes, or by sparks given off when rocks rub together in an avalanche.)

　　Once a fire gets started, many factors contribute to how it will behave. The weather obviously has a huge effect: winds can fan flames, rains can quench them. The lie of the land matters, too: fire runs uphill more readily than it goes down. But another crucial factor is what type of plants the fire has to eat.

　　It’s common knowledge that plants regularly exposed to fire tend to have features that help them cope with it—such as thick bark, or seeds that only grow after being exposed to intense heat or smoke. But what is less often remarked on is that the plants themselves affect the nature and severity of fire.

正确答案及解析

正确答案

解析

上周提起，我刚从澳大利亚回来。在那儿，我参观了一片桉树林，那片树林二月刚刚遭受一次骇人的野火肆虐。也许是因为我还有些天真，曾以为会发现许多树被烧死。但它们却没死。树皮烧黑了，不过看上去还好，有许多树还长出了新的嫩叶。这种现象让我想到了火与它作为一种自然力的作用。

　　木炭化石告诉我们，自陆地上有了植物，野火就一直是地球上生物的一部分，也就是说，火的历史要在4亿年以上。早在草这类生长迅猛的植物存在之前，火就已经存在了；而在第一束花朵出现之前也早已有了火。不必把火想得很神秘，火，在很多方面，就像是一种动物，尽管飘忽不定、难以捉摸。和其他动物一样，火消耗氧气，像一只绵羊或蛞蝓一样吞食植物。但和正常动物又不同，有着捉摸不定的形态。有时仅仅轻咬几片叶子，有时却能吞噬参天大树。有时比蝗灾更致命，破坏性更大。

　　火的这种形态捉摸不定的性质让研究更加困难了，因为它并非单一的整体。有些火炙热可怕；有些火则温度相对较低。有些火在地面上燃烧；另一些则会窜上大树。此外，世界上有些地方更容易出现火灾。地球的卫星图像显示，欧洲北部很少出现野火，而在非洲中部的一些地方和澳大利亚则常有野火。（近年来很多野外火灾是人为的，要么是故意，要么是偶然。但在我们的祖先扔火把或烟蒂之前的很长一段时间，火灾是由闪电或雪崩中乱石摩擦引起的。）

　　一旦起火，就会有许多影响火势的因素。显然，天气对火势有着极大的影响：风助火势、雨灭火威。火势也受地形的影响：火易顺山而上，而非沿坡向下。另一个关键因素就是：火易吞噬哪类植物。

　　有一项常识：经常遭大火肆虐的植物具备助于它们应对火灾的特性——如厚厚的树皮或只有经过烈火和烟熏才会生长的种子。但人们并不太注意，这些植物本身就影响了火的性质及火势。