Forest fires in Montana Bitteroot valley

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Related to these issues of forest logging are issues of forest fires, which have recently increased in intensity and extent in some forest types in Montana and throughout the western U.S., with the summers of 1988, 1996, 2000, 2002, and 2003 being especially severe fire years. In the summer of 2000, one-fifth of the Bitterroot Valley's remaining area of forest burned. Whenever I fly back to the Bitterroot nowadays, my first thought on looking out my airplane's window is to count the number of fires or to gauge the amount of smoke on this particular day. (On August 19, 2003, as I was flying to Missoula airport, I counted a dozen fires whose smoke reduced visibility to a few miles.) Each time that John Cook took my sons out fly-fishing in 2000, his choice of which stream to fish depended partly on where the fires were burning that day. Some of my friends in the Bitterroot have had to be evacuated repeatedly from their homes because of approaching fires.

This recent increase in fires has resulted partly from climate change (the recent trend towards 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 effects of logging, which often turns a forest into something approximating a huge pile of kindling: the ground in a logged forest may remain covered with lopped-off 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 U.S. Forest Service in the first decade of the 1900s adopted a policy of fire suppression (attempting to put out forest fires) for the obvious reasons that it didn't want valuable timber to go up in smoke, nor 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 World War II, thanks to the availability of firefighting planes, an expanded road system for sending in fire trucks, and improved firefighting technology. For a few decades after World War II, the annual acreage burnt decreased by 80%.

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 U.S. federal government's fire suppression policy was contributing to those big fires, and that natural fires caused by lightning had previously played an important role in maintaining forest structure. That natural role of fire varies with altitude, tree species, and forest type. To take the Bitterroot'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 of fire-sensitive Douglas Fir seedlings that have grown up since the last fire. But after only a decade's growth until the next fire, those seedlings are still too low for fire to spread from them into the crowns. Hence the fire remains confined to the ground and understory. As a result, many natural Ponderosa Pine forests have a park-like appearance, with low fuel loads, big trees well spaced apart, and a relatively clear understory.

Of course, though, 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 Congress repeatedly failed to appropriate money to thin out the saplings. Another human-related factor, sheep grazing in national forests, may also have played a major role by reducing understory grasses that would otherwise have fueled frequent low-intensity fires. 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 may become a ladder that allows the fire to jump into the crowns. The outcome is sometimes an unstoppable inferno in which flames shoot 400 feet into the air, leap from crown to crown across wide gaps, reach temperatures of 2,000 degrees Fahrenheit, kill the tree seed bank in the soil, and may be followed by mudslides and mass erosion.

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 U.S., 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, acid remove the dense understory by cutting or by controlled small fires. But that would cost over a thousand dollars per acre for the one hundred million acres of western U.S. forests, or a total of about $100 billion. No politician or voter wants to spend that kind of money. Even if the cost were lower, much of the public would be suspicious of such a proposal as just an excuse for resuming logging of their beautiful forest. Instead of a regular program of expenditures for maintaining our western forests in a less fire-susceptible condition, the federal government tolerates flammable forests and is forced to spend money unpredictably whenever a firefighting emergency arises: e.g., about $1.6 billion to fight the summer 2000 forest fires that burned 10,000 square miles.

Montanans themselves hold diverse and often self-contradictory views about forest management and forest fires. On the one hand, the public fears and instinctively dislikes the "let it burn" response that the Forest Service is forced to take towards huge fires that would be dangerous or impossible to try to extinguish. When the 1988 fires in much of Yellowstone National Park were allowed to burn, the public was especially loud in its protests, not understanding that in fact there was nothing that could be done except to pray for rain or snow. On the other hand, the public also dislikes proposals for forest thinning programs that could make the forests less flammable, because people prefer beautiful views of dense forests, they object to "unnatural" interference with nature, they want to leave the forest in a "natural" condition, and they certainly don't want to pay for thinning by increased taxes. They (like most foresters until recently) fail to understand that western forests are already in a highly unnatural condition, as the result of a century of fire suppression, logging, and sheep grazing.

Within the Bitterroot, people build trophy homes next to or surrounded by flammable forests at the urban/wildland interface and then expect the government to protect those homes against fires. In July 2001, when my wife and I went for a hike west of the town of Hamilton through what had been the Blodgett forest, we found ourselves in a landscape of fire-charred dead trees killed in one of the big forest fires whose smoke had filled the valley during our summer 2000 visit. Blodgett-area residents who had previously blocked Forest Service proposals to thin the forest demanded then that the Service hire 12 big firefighting helicopters at a cost of $2,000 per hour to save their homes by dropping water on them, while the Forest Service, obeying a government-imposed mandate to protect lives, people's property, and then the forest in that order, was simultaneously allowing expanses of public timberlands far more valuable than those homes to burn. The Forest Service subsequently announced that it will no longer spend so much money and endanger firefighters' lives just to protect private property. Many homeowners sue the Forest Service if their house burns in a forest fire, or if it burns in a backfire lit by the Forest Service to control a much bigger fire, or if it doesn't burn but if a forest providing a pretty view from the deck of their house does burn. Yet some Montana homeowners are afflicted with such a rabidly anti-government attitude that they don't want to pay taxes towards the costs of firefighting, nor to allow government employees onto their land to carry out fire prevention measures.

Autres références de Jared Diamond

Mis en ligne le 04 juillet 2006 par Pierre Ratcliffe Contact: (pratclif@free.fr) site web: http://pratclif.free.fr