Forest fires have recently increased in intensity and extent in some forest types throughout the western United States. This recent increase in fires has resulted partly from climate change (the recent trend toward hot, dry summers) and partly from human activities, for complicated reasons that foresters came increasingly to understand about 30 years ago but whose relative importance is still debated. One factor is the direct effect of logging, which often turns a forest into something approximating a huge pile of kindling (wood for burning): the ground in a logged forest may remain covered with branches and treetops, left behind when the valuable trunks are carted away; a dense growth of new vegetation springs up, further increasing the forest’s fuel loads; and the trees logged and removed are of course the biggest and most fire-resistant individuals, leaving behind smaller and more flammable trees.
Another factor is that the United States Forest Service in the first decade of the 1900s adopted the policy of fire suppression (attempting to put out forest fires) for the obvious reason that it did not want valuable timber to go up in smoke, or people’s homes and lives to be threatened. The Forest Service’s announced goal became “Put out every forest fire by 10:00 A.M on the morning after the day when it is first reported.” Firefighters became much more successful at achieving that goal after 1945, thanks to improved firefighting technology. For a few decades the amount of land burnt annually decreased by 80 percent. That happy situation began to change in the 1980s, due to the increasing frequency of large forest fires that were essentially impossible to extinguish unless rain and low winds combined to help. People began to realize that the United States federal government’s fire-suppression policy was contributing to those big fires and that natural fires caused by lighting had previously played an important role in maintaining forest structure.
The natural role of fire varies with altitude, tree species, and forest type. To make Montana’s low-altitude ponderosa pine forest as an example, historical records, plus counts of annual tree rings and datable fire scars on tree stumps, demonstrated that a ponderosa pine forest experiences a lightning-lit fire about once a decade under natural conditions (i.e.., before fire suppression began around 1910 and became effective after 1945). The mature ponderosa trees have bark two inches thick and are relatively resistant to fire, which instead burns out the understory—the lower layer—of fire-sensitive Douglas fir seedlings that have grown up since the previous fire. But after only a decade’s growth until the next fire, those young seedling plants are still too low for fire to spread from them into the crowns of the ponderosa pine trees. Hence the fire remains confined to ground and understory. As a result, many natural ponderosa pine forests have a parklike appearance, with low fuel loads, big trees spaced apart, and a relatively clear understory.
However, loggers concentrated on removing those big, old, valuable, fire-resistant ponderosa pines, while fire suppression for decades let the understory fill up with Douglas fir saplings that would in turn become valuable when full-grown. Tree densities increased from 30 to 200 trees per acre, the forest’s fuel load increased by a factor of 6, and the government repeatedly failed to appropriate money to thin out the saplings. When a fire finally does start in a sapling-choked forest, whether due to lightning or human carelessness or (regrettably often) intentional arson, the dense, tall saplings young trees may become a ladder that allows the fire to jump into the crowns of the trees. The outcome is sometimes an unstoppable inferno.
Foresters now identify the biggest problem in managing Western forests as what to do with those increased fuel loads that built up during the previous half century of effective fire suppression. In the wetter eastern United States, dead trees rot away more quickly than in the drier West, where more dead trees persist like giant matchsticks. In an ideal world, the Forest Service would manage and restore the forests, thin them out, and remove the dense understory by cutting or by controlled small fires. But no politician or voter wants to spend what it would cost to do that.
最近，在美国西部的一些森林类型，森林大火的强度和范围都有所增加。大火的增加一部分是因为气候变化（最近夏天又热又干的趋势），还有一部分原因是人类活动，其复杂原因林业工人早在30年前就已经知晓但其重要性仍有争议。一个因素是森林砍伐的直接影响——通常将森林变为一大堆引火物（用于燃烧的木头）：森林被砍伐之后，当有价值的树干被运走，留下满地的树枝和树冠。稠密的新植被萌芽生长，进一步增加了森林的可燃物负荷量；那些被砍伐运走的树自然是最大而且最防火的个体，留下的只是更小且更容易燃烧的小树。 另一个因素则是美国林业局在20世纪初的十年间对扑灭林火（尝试扑灭森林大火）所采取的政策措施，这样做的理由很明显：不想让有价值的木材化为乌有，或者不想人们的家园或生命遭受威胁。林业局宣传他们的目标是“火灾首次被报导那天后，早上10点就将它们扑灭”。1945年之后，由于消防技术的改善，消防员在达成目标方面更加成功。数十年间，被烧的土地数量下降了80%。这种喜闻乐见的情景从20世纪80年代开始发生改变，因为那些除非有雨和微风的帮助否则基本不可能被完全扑灭的大面积森林火灾越来越频繁。人们开始意识到美国联邦政府的灭火政策会促发更多大火，而以前由闪电所造成的自然火灾在维持森林结构方面一直起着重要作用。 火的自然作用随着海拔、树木种类和森林种类的变化而变化。以蒙大纳州低海拔地区的黄松林为例，历史记录加上树木的年轮数量和树桩上可确定年代的火烧痕迹，都证明了在自然条件下每10年黄松林都要经历一次由闪电击中而引发的大火（比如，大约在1910年扑灭林火政策开始之前和1945年生效之后）。成年黄松的树皮有两英寸厚，比较防火，相反在之前大火之后生长出来的易燃的花旗松苗，其林下叶层（较低层）则会被烧尽。但是，在下一次大火之前，因为仅仅只生长了10年，这些幼苗还太矮，不能将火引向黄松的树冠。因此，火还是被限制在地面和下层植被。所以许多天然黄松林有着公园般的景象——低可燃物负荷量，大树之间相隔一定距离，以及相对干净的下层植被。 但是，伐木工人致力于移除那些又大又老又有价值且防火的黄松，而数十年的林火扑灭措施又使得下层植被充满了花旗松幼苗，幼苗完全长大后也是有价值的。树木密度从每公顷30棵树增加到每公顷200棵，森林可燃物负荷量增大了6倍，政府也多次未能拨款以减少幼苗数量。当火最终从一个充满了幼苗的森林中开始蔓延时，不管起因是闪电还是人类粗心还是（很遗憾通常就是）故意纵火，密集而茂密的树木幼苗很可能就是一个梯子，使火苗延伸到树冠。结局有时候就是无法停止的地狱。 现在，守林人意识到管理西部森林的最大问题是如何处理那些在过去半个世纪的有效林火扑灭过程中日益增加的可燃物负荷量。在多雨的美国东部，死亡树木的腐烂速度比西部干燥树木的腐烂速度要快得多，在西部已经死亡的树木更多地是像巨大的火柴棍一样被留存下来。理想情况下，林业局可以管理和恢复森林，通过砍伐或可控的小火来减少并移除茂密的下层植被。但是，没有政客或选民想要投入这笔应该会很昂贵的费用。