Players lay black and white pebbles on a gridded board, occasionally surrounding an unprotected line of their opponent's markers -- prompting a victory swap to the engulfing color. After numerous reversals of fortune, the tide turns and one will eventually engulf the other, triumphant. This can take hours. Imagine every square on the board is an acre of forest. Black pebbles are recent wildfires which completely burned the stand; white ones are stands of healthy trees spared from flames by fire-suppression efforts. Surrounding a fire keeps it from spreading, and eventually burned acres grow back to trees. A square may switch between white and black several times.

"Go" in the Real Woods

This game has played out for nearly a hundred years starting in 1900. Lightning and periodic droughts made for a relatively constant number of fires ignited, but the Forest Service's technology and political power to fight fires increased. Slowly, the board turned white with fire-free foliage; only a few pockets turned black each decade.

Today we are very nearly at what looks like the end of the game, except that Nature has hidden the equivalent of a doomsday device in that lovely blanket of white pebbles. The protected woods have built up an enormous fuel load of downed and standing dead trees and limbs, flammable underbrush and grass. With the right weather conditions -- high heat, low humidity and heavy winds -- a single lightning strike or cigarette butt can explode into mass fire that burns thousands of acres. The most valiant and well-equipped firefighters are helpless when embers blow as much as a mile ahead of the main fire, setting spot fires far beyond the widest fire lines.

Yellowstone's Inferno

This is what happened in 1988 when 1.5 million acres burned in the Yellowstone National Park area. David Kovacic at the University of Illinois Urbana-Champaign reconstructed the real-world "go" game of the park's fire history by locating fire scars in tree cores. Any given stand used to burn about once every 300 years, so the simulation begins in 1690 with the birth year of the oldest trees found inside the study area. And instead of just white and black pebbles, Kovacic divided the "white," fire-free acres into four shades of green representing trees in different age groups.

For almost 200 years, until about 1890, the pebbles of different-aged trees jittered around as fire regularly burned some stands. Then fire-fighting programs took hold and the entire forest progressively aged. By 1988 the mosaic of high-fuel, fire-feeding areas and low-fuel, fire-stopping areas had become homogeneous. The stage was set for the ensuing inferno -- and the flames spread seamlessly through the woods.

Small Blazes Prevent Huge Ones

Kovacic's reconstruction of Yellowstone's fire history yields two surprises. First, it turns out the most important boundaries to wildfire are not geographical features like roads, canyons or even rivers. Fire easily crossed them all. Rather, the boundaries are biological. The crazy quilt produced by random fires turns out to be the forest's best protection from fire.

The second surprise refutes the traditional ecological notion that stands of old trees mark the end of the wilderness game of "go." Instead it's the dancing cycle of growth -- life and death -- that is truly the steady state. The ultimate paradox is that constant disturbance from fire on a local scale maintains the stability of the larger ecosystem.