Collapse_ How Societies Choose to Fail or Succeed - Jared Diamond [26]
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