As the snow melts off the millions of acres of Western forest and prairie that were scorched by the wildfires of 2000, federal land managers are preparing for the sweeping ecological changes that the fires will bring.

Last year was one of the most severe fire years on record in the West. More than 92,000 separate fires blackened an estimated 7.4 million acres and destroyed 861 buildings. Among the hardest-hit states were Montana, New Mexico and Colorado.

Many of the lessons on what to expect have been learned here in Yellowstone, where in 1988 fires blackened 739,000 of the park's 2.2 million acres, and nearly twice that if the entire Yellowstone ecosystem is included.

For the past 13 years more than 250 research experiments in the park have chronicled changes brought by the fires, and the researchers have looked at consequences as diverse as the disappearance of old growth forests, the decline of moose and the increase in the size and number of trout in some streams.

The 1988 fires burned with an unparalleled ferocity, fed on fuel that had accumulated under a century-long policy, abandoned in the 1970's, of suppressing fires. An unusual series of nine fronts passed over, bringing high winds and lightning but no moisture.

Videos show the forest was so dry that mature trees burned completely in 15 to 20 seconds.

The short lesson, says John Varley, director of the Yellowstone Center for Research, is this: "Mother Nature does not destroy herself. It's that simple. Instead it is a giant reapportionment of species."

The shifts in habitat fall along a spectrum, from major to subtle, depending on factors including the fire's intensity of the burn and the soil conditions. The changes favor some species and increase their distribution and abundance, while hampering others, sometimes for the short term, and sometimes for much longer. Park researchers detail these effects in a new 118-page book, "Yellowstone in the Afterglow: Lessons From the Fire."

While the effects of fire vary in different ecosystems, some general principles apply.

Depending on wind and available fuel, a wildfire burns in a mosaic, destroying some areas, while singeing other areas and leaving some untouched.

These "patch dynamics" are critical to recovery. Lightly burned or unburned areas provide a reservoir of seeds that revitalize more heavily burned acreage.

Immediately after the 1988 fires, grasses and forbs, which burn on top but regenerate from roots below ground, surged back, stimulated by nitrogen and phosphorous loosed from dead plants. Meadows were filled with the brilliant red bloom of fireweed. And tiny, rare plants like blue-eyed mary and the delicate ground smoke thrived.

Three years after the fire, however, the boost from additional nutrients was largely over. "It's been a remarkably rapid regrowth," said Dr. Bill Romme, a fire ecologist at Fort Lewis College in Durango, Colo., who has studied park fires for 25 years.

A return of the forests, predominantly lodgepole pine in Yellowstone, takes far longer. Stands of spindly black trees that look like burned matches still sprawl over vast acreage. Over the years many have been toppled by the wind. But some remain standing for decades, and their roots continue to hold the soil and moderate the climate as a new generation rises beneath them.

Meanwhile, new pines are anywhere from knee high to 15 feet, depending on soil and moisture. Lodgepole pine cones wrap the seeds for a new generation in a waxy coating, and the wax is melted and seed released only when the temperature reaches 113 degrees.

In areas that burned last year, at lower elevations than the park, much more of the forests were in drier ecosystems that are less resilient. In the Bitterroot Valley in southwest Montana, for example, 25 percent of the forest is ponderosa pine and Douglas fir, and they take much longer to regenerate. Rather than being released from the trees that are burned, the seeds are produced by neighboring trees, and not every year. Regeneration happens fastest from bumper crops of cones - called pulse events - and when they will occur is unpredictable.

"The seed gradually makes its way in with gravity and wind," said Glenda Scott, a forester with the United States Forest Service in Missoula, Mont., and a specialist in reforestation. "It takes from 8 to 15 years. In areas that won't naturally regenerate, we'll plant trees."

But in burned areas, the fires are expected to give rise to a wider variety of different aged stands, and with them a greater variety of habitats.

Before 1988, Yellowstone was about 70 percent old growth or mature forest, and the fires burned off about a third of that. The removal of so much canopy shuffled the deck of forest ecosystems below. Moose were first among the losers. They feed in old growth in the winter, eating the new growth of fir and spruce. Those that had occupied burned areas had to expand their range. After the fires nearly half of the collared moose in one study had died of starvation or at the hand of hunters. Pine martens suffered as well, for they favor tunnels created by deadfall and snow in old growth forests to hide from predators and stalk prey.

Winners included hawks, which prospered even as the fire raged. Dozens of raptors were observed hunting over burning fields, as rodents scurried to find new cover.

Nesting birds, including those who make their homes in the cavities of trees, flourish immediately and in the first few decades after a fire. Boring beetles and other insects feast on dead trees, then become the feast as woodpeckers, mountain bluebirds and tree swallows swarm to burned areas.

These and other nesting birds will thrive until the canopy begins to close again. One Yellowstone fire study in the 1960's showed that 29 years after a fire, there were 72 breeding pairs of nesting birds per 100 acres. Fifty-seven years after the fire all nesting birds were gone.

Large numbers of toppling dead trees alter species composition on the forest floor as well, providing cover for snowshoe hare, voles and deer mice, a fully stocked grocery for a variety of predators. Biologists expect to see an increase in the number of lynx, for example, which feed on hare.

It has been difficult to measure the effects of the fire on the park's "charismatic megafauna" - grizzly bears, bison and elk. (Wolves were not present in the park during the fires.) During the fire biologists discovered a herd of 246 elk that was asphyxiated, but in the words of Roy Renkin, an ecologist in the park, "Direct mortality was minimal."

The winter immediately after the fires was extremely harsh, and some 5,000 elk died, about 25 to 30 percent of the largest herd, in the north. How much of that was fire related and how much was caused by drought the summer before is not known. The northern herd is now about 14,000, substantially less than the peak of 20,000 before the fire.

The fires devastated parts of many streams and rivers - in the short term. However, said Dr. Wayne Minshall, a professor of ecology at Idaho State University, who has studied streams for 35 years, "Two to 15 years after the fire, conditions recover and often get better than they were before the fire."

Right after a fire, ash, burned trees and mud flow create a deadly slurry that flows into a stream and can kill fish, move channels and widen the stream by a factor of 10 to 100 times. After this pulse of burned material has washed out, the stream starts to recover and often becomes more heterogeneous.

With the canopy gone, sunlight causes algae to grow, which feeds insects. More woody debris in the stream increases pools where fish can spawn. Nutrients unlocked by the fire stimulate the growth of riparian vegetation, which improves insect habitat and food. As the size and number of insects increase, so do the size and number of fish.

Yellowstone Park has a natural regulation policy that allows nature, no matter the consequences, to take its course. That should be a guide for people outside the park who would like to hasten restoration of fire- ravaged rivers and streams, Dr. Minshall said. "Human efforts to mitigate exacerbate the impacts," he said. "The worst is salvage logging. Roading intercepts the flow of water, roads blow out and wash into the stream, and wood is important over 20 years as it washes into the stream."

The Yellowstone report suggests that one of the major complications after last year's fires throughout the West will be dealing with exotic species like spotted knapweed or dalmatian toadflax, an escaped ornamental. Some already exist and capitalize on the burned areas, while others have been brought in on the tires of firefighting equipment from other areas. They are minimal in places like Yellowstone, where the landscape is undisturbed, but pose serious problems on disturbed ground that has burned.

"These aggressive exotics push out the native plant communities," said Jim Olivarez, a noxious weed program leader for the Forest Service. "They can push out elk because what they prefer isn't there and cause erosion because they have a different root structure."

The variety of ecosystems burned last year will respond in different ways, Dr. Romme said. Ponderosa pine forests at low elevations evolved with frequent low intensity fire, and the very hot fires may have destroyed pine cones in the fires near Los Alamos, N.M., and parts of Montana. It may take a long time for seeds to blow in.

At Mesa Verde, Colo., where intense fires burned, Gamble oak forests sprout new trees from the roots of old ones. They have done well. On the other hand, pygmy woodlands in the same area, with pinyon and juniper trees, burned in the drought last year and are not fire adapted. They are being invaded by exotic weeds, and their fate hangs in the balance.

While fuel loading from a century of fire suppression is a serious problem in the West, some experts think something else is at the root of catastrophic fires. "The climate is getting warmer and drier and more extreme," said Mr. Varley, the director of the Yellowstone research center. "In the 1880's the annual average temperature here was 34 degrees. Now it's 38 degrees. That doesn't sound like much, but it causes big changes ecologically."

Whatever the case, Yellowstone has played a critical role in demonstrating the changes brought by fires. And it isn't over: land managers are still going to school on the Yellowstone experience. "The lessons of '88 in the park are like classic Led Zeppelin," said Mr. Renkin, a park ecologist. "They'll be around forever."