(This is Part 7 of a series. Go back to Part 6.)
In 1988 there was a catastrophic forest fire of 1.5 million acres in Yellowstone National Park. And in recent years there has been a spate of such large conflagrations. Why?
Let’s back up a little. In 1998, just six years ago, the geologist Bruce Malamud and associates gathered and analyzed 100 years of data on forest fires in the United States. They measured the sizes of fires versus how frequently they occurred.
The researchers found, as in so many other areas of natural and social phenomena, that the forests has tuned themselves to a critical state and follow a power law. In this case, if the size of a forest fire is doubled it will occur 2.48 times less frequently—and this relationship holds across fires varying in size by a factor of a million.
This means, with the usual scale invariance of power laws, that there is no typical size for a forest fire. In other words, large, catastrophic fires do not need special causes; they will occur from time to time in the natural order of things. In the critical state a small cause can create a small, medium or large event.
So far, so good. But the researchers also found something else that has profound implications. To see what that is, let's follow their next step:
They created a computer simulation of forests and forest fires. On a grid they allowed the computer to plant a random tree here and there. And once in a while the computer would drop a match at a particular square. If a tree was there, the tree would catch fire. And if the tree had any close neighbors they would also catch fire.
That was the total set of rules. Though this simulation is extremely simple, it nevertheless captues the essential feature of actual forest fires. The researchers found that this computer "forest" tuned itself to the critical state and followed a power law similar to real forests, where fires of all sizes could occur.
But something else happened too. The researchers could vary the rate at which matches dropped onto the forest and started fires. For instance, the computer could drop a match for every 100 trees that grew. In this case the forest tuned itself to a normal critical state.
But the computer could also drop matches at a much slower rate, say one match for every 1000 trees that grew. This is the equivalent of putting out or "suppressing" forest fires. And what they found astonished them:
In the latter case, they found that the forest tuned itself to a "supercritical" state, where extremely large, catastropic fires became the norm.
Funny thing about that. Because the U.S. Forest Service recently made an interesting discovery. You see, beginning in 1890 and for over a century the Forest Service had a "zero tolerance" policy towards forest fires. As soon as a fire started anywhere, of any size, it was immediately stamped out.
Because of this policy of absolute fire suppression, the forests lost their natural "culling" by fire and over the years became denser and older and filled with underbrush. This led the U.S. forests to enter a "supercritical state" where large, catastrophic conflations became the norm.
Now the Forest Service has a new policy: They let the smaller fires burn naturally now, knowing that to interfere with the forest's natural process of correction is to invite catastrophic, system-wide conflagrations. They know that nature's various fires are part of the natural "self-tuning" process of the forest organism.
We see once again the unseen hand of nature's wisdom. And we might wonder: Where else could we apply this principle of respect for the natural process?
(This is the end of Part 7. Go to Part 8.)
—jim sloman, 2.23.04 for Dec 1
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