Forest Fires in Yellowstone: the Science of Burning and Regrowth

By Carrie Perkins, University of Maryland College Park

This article, and others in the series "Parks in Science History", was written by a graduate student at the University of Maryland. The articles highlight the roles that national parks have played in the history of science and, therefore, the world's intellectual heritage. More articles and videos will be produced in the future.

Yellowstone National Park

NPS Photo / Mike Lewelling

It was the fall of 1988. Dr. Monica Turner, a 29-year-old staff scientist at Oak Ridge National Laboratory, flew out to Yellowstone National Park to start an experiment in forest ecology. She got her first glimpse of the Park since it had been ravaged by huge fires, the likes of which no one had ever seen.

The fires had burned for months across large sections of the park. In fact, they were collectively the largest wildland fire ever known within the park boundary. Based on the scale of the burns, Dr. Turner had expected to see large swaths of uniformly burned forest, but instead the burned land below her looked like a mosaic, an intricate mixture of severely burned, lightly burned, and unburned areas.

Her curiosity was piqued. Why were the fires so big this year? Did weather conditions make them worse than usual? Did they burn down only the older trees? How far are the most severely burned areas from areas untouched by the fire? When I look at the landscape from far away, why do I see a mosaic pattern?

These are just a few of the questions Monica asked in the months that followed. Asking ecological questions on such a large landscape scale was new at the time. Only two years earlier, Monica had helped organize the first landscape ecology meeting in the U.S., during an era when traditional ecologists were still wary of studying patterns and processes across thousands of square kilometers. Though she had no idea at the time, she would become a leader in the field of landscape ecology, spending more than a quarter century studying burn patterns in Yellowstone and paving the way for future landscape ecologists.

Fire opens lodgepole pine cones so seeds can be released.

NPS Photo / Jennifer Jerrett

Understanding the effect of different types of fire on landscape patterns is important for predicting how fast the landscape will recover, and what that recovery will look like. If burned and unburned areas of the same type of forest are right next to each other, it is easier for plants to return to the burned places. However, if burned and unburned patches are far away from one another, it is harder for seeds to travel to and rejuvenate the burned areas. As the fires burned throughout 1988, people were worried that burned areas would be too far from unburned areas to have any chance at recovery.

Before 1988, Yellowstone - a 9,000 km² expanse of forests that is 80% lodgepole pine - was no stranger to fire. As in many parts of the Rocky Mountains, fires burn every year in the park and are a big influence on the landscape’s animals and plants. Until the early 1970’s, Yellowstone’s park managers extinguished all fires in the park because fire suppression was believed to protect both humans and wildlife. But they began to realize that these fires are a natural part of the landscape and shouldn’t be suppressed. Managers launched a natural fire program in 1972, and so fires caused by lightning were no longer put out. For more than a decade, fires were small, went out by themselves, and mostly just burned old trees.

You might wonder, then, why everyone was shocked by the 1988 fires. At first, 1988 seemed like any other dry year. But then weather conditions became very unusual, with unprecedented drought and wind. As the summer went on, the fires got bigger and bigger. By the end of September, they had burned more than 3,212 km² of forest containing trees of all ages. The 1988 fires in Yellowstone burned more square kilometers than anyone had ever seen.

To explore burn patterns across the landscape of Yellowstone, Monica and her colleagues visited each area of the park in the summers of 1989 and 1990 to find out how severely the forests had been burned. Each location was given a burn classification:

• The most severe burn classification was “Crown Fire,” and was given to places where the tree canopies were consumed by fire, meaning that all or most of the seeds stored in the canopies were destroyed.
• Places with fires that killed the trees but did not destroy the tree canopies were called “Severe Surface Burns”.
• “Light Surface Burn” was the classification given to places where the trees survived but stems were scorched.
• “Unburned” was given to places that were hardly burned at all.

While many areas of the park received one of these burn classifications, it would have been impossible to visit every single patch of trees in Yellowstone - that would take many years! So, Monica and her team used Landsat satellite images and their field data to predict, based on what they knew about the locations they visited, the burn categories in other areas. They mapped these across the central core of the park.

This map of fires from 1988 uses colors only to help you see fire boundaries. Colors do not indicate anything else.

NPS

When people looked at the maps made by Monica and her team using satellite data, they were pleasantly surprised. Many people had feared the fires would create big expanses of lifeless, barren land far away from unburned areas. To everyone’s relief, however, the maps showed a mosaic landscape pattern, where uniquely shaped patches of severely burned areas, lightly burned areas, and unburned areas were neighbors. The most severely burned areas were very close to lightly burned and unburned areas. This meant seeds from unburned forests could make their way to barren areas and in the years to come grow up into adult trees to replace those that had been burned.

Why did the landscape become a mosaic? The reason has to do with the most common type of fire that burned in Yellowstone in 1988. Of the three burn categories, crown fires were responsible for most of the area burned. Unlike other fires, very large crown fires are unaffected by landscape features (roads, rivers, mountains, and valleys) or tree age. Only wind can control these large crown fires. As wind speed and direction change, crown fires sweeping across the landscape burn certain places more severely while they skip over nearby areas. After a big crown fire, you will find places that were hardly touched by fire right next to areas that were highly affected. Looking down at Yellowstone from an airplane in the years following the 1988 fires, the mixture of severely burned, medium burned, and lightly burned places looked like a mosaic.

Mosaic fire pattern in Yellowstone National Park.

NPS Photo / Jim Peaco

Monica’s study of the 1988 fires, published in The Journal of Vegetation Science in 1994, is considered a classic study in ecology because few people had studied ecosystems on such a large scale - thousands of square kilometers - before. As in the early days of any new field, pioneers of landscape ecology including Monica endured an uphill battle to gain acceptance and recognition for their work. Monica’s early research at Yellowstone proved this new way of thinking was important. By using new tools such as satellite imagery and landscape modelling, she discovered unexpected landscape patterns which otherwise would have remained a mystery.

Yellowstone’s forests were very resilient, and so in the years following the 1988 fires burned areas filled up once more with young lodgepole pine trees. But will Yellowstone’s forests recover from large fires in the future? Monica worries that the answer might be “No”. Extreme weather events -- think of the drought and high winds that fed the 1988 fires -- are increasingly common. As the western U.S. becomes hotter and drier, there have been more and more big fires. Will young forests have enough time to recover between increasingly frequent burns? Or will the landscape look different in the future than it has in the past? Time, along with more large-scale studies and models, will tell.

For further reading, check out her 1994 article.

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References

Frequently Asked Questions: 1988 Fires - Yellowstone National Park (U.S. National Park Service). https://www.nps.gov/yell/learn/nature/1988firefaqs.htm. Accessed 13 Aug 2018b

Monica G. Turner – The Ecological Society of America’s History and Records. https://esa.org/history/turner-m-g/. Accessed 13 Aug 2018a

Photo Galleries - Yellowstone National Park (U.S. National Park Service). https://www.nps.gov/yell/learn/photosmultimedia/photogallery.htm. Accessed 6 Jun 2018

Turner MG, Hargrove WW, Gardner RH, Romme WH (1994) Effects of fire on landscape heterogeneity in Yellowstone National Park, Wyoming. J Veg Sci 5:731–742 . doi: 10.2307/3235886

Wildland Fire: Fire Interpretation: History of National Park Service Fire Policy | U.S. National Park Service. https://www.nps.gov/fire/wildland-fire/learning-center/history/history-of-NPS-fire-policy.cfm. Accessed 12 Aug 2018

Zagorski N (2007) Profile of Monica G. Turner. Proc Natl Acad Sci U S A 104:4779–81 . doi: 10.1073/pnas.0701264104