Climate is changing fire patterns in the West in a way that could substantially alter the vegetation and ecology of Yellowstone National Park and the Greater Yellowstone Ecosystem by the middle of this century, according to new research from Colorado State University and the University of California, Merced.
A study published online in the Proceedings of the National Academy of Sciences shows that climate change could increase the frequency of large fires in the Greater Yellowstone Ecosystem to a point that sparks dramatic shifts in the forest vegetation from mature and old-growth forests to younger forests and woodlands.
“Large, severe fires are normal for this ecosystem and have occurred about every one to three centuries over the past 10,000 years,” said study co-author Bill Romme, professor emeritus at Colorado State University and a landscape ecologist who has worked in the Greater Yellowstone area for more than 30 years. “But if the climate continues changing as projected and the current relationship between climate and large fires holds true, the warming climate will drive far more frequent large fires in the future than have occurred historically.”
Wildfires in this ecosystem are primarily climate-driven and are primed by hotter, drier conditions, such as those predicted by numerous global climate models. Already fire ecologists have noticed increased fire frequency in the West, associated with temperature increases of less than two degrees Fahrenheit and early spring snowmelt in the mountains.
For the new study, the researchers analyzed wildfire and climate data in the northern Rocky Mountains from 1972 to 1999, then combined the observed relationships with the trends predicted by climate models to project how climate change will likely impact fires during the 21st century.
“What surprised us about our results was the speed and scale of the projected changes in fire in Greater Yellowstone,” said co-author Anthony Westerling, professor of environmental engineering and geography at the University of California, Merced. “We expected fire to increase with increased temperatures, but we did not expect it to increase so much or so quickly. We were also surprised by how consistent the changes were across different climate projections.”
The researchers found that large fires (greater than 500 acres in size) will likely occur almost every year within the Greater Yellowstone Ecosystem by 2050, with the fire rotation – the time span over which an area the size of the entire landscape burns – reduced from a historical range of 100 to 300 years to less than 30 years. Interestingly, the predicted new fire regime resembles patterns more typical of other landscapes that are warmer and drier than Yellowstone, such as the ponderosa pine forests of the southwest.
“More frequent fires will not be catastrophic to the area, but they will undoubtedly lead to major shifts in the vegetation,” said study co-author Monica Turner, professor of landscape ecology at the University of Wisconsin-Madison who has worked in Greater Yellowstone for more than 20 years.
For example, the iconic lodgepole pines that dominate much of the current landscape may not have time to recover between big fires and the forests could shift toward fast-growing aspen and fire-tolerant Douglas-fir, or even shrubs and grassland in some places.
Such changes would also affect the region’s wildlife, hydrology, carbon storage and aesthetics. Despite these changes, the ecosystem will not be destroyed; Yellowstone will continue to have natural vegetation, wildlife, geysers and the other things that we value in this special place, Romme said.
Westerling, an expert on climate-fire interactions, cautions that the models used in the study will not work once the increase in fires creates a fundamental change in the ecosystem. As the landscape changes, the relationships between climate and fire will change as well.
“The biggest challenge for us is to understand what can happen when the ecosystem is transformed,” he said. “Our projections also depend on the climate models we are using – for example, if projections for winter snow pack or summer rainfall were to change significantly, that would change our results.”
Turner adds that availability of fuels will eventually become the limiting factor for fires rather than climate, at which point all of the models break down and the future becomes more unpredictable.
“Our research after the immense 1988 fires revealed surprises and tremendous resilience in Yellowstone’s ecosystems, and Yellowstone is likely to surprise us again in the future,” Romme said. “It is an incredibly valuable natural laboratory for studying how ecosystems adapt to changing environmental conditions.”
The other study co-authors are Erica Smithwick at Pennsylvania State University and Mike Ryan of the U.S. Forest Service Rocky Mountain Research Station in Fort Collins.
The work was funded by the Joint Fire Science Program, U.S. Forest Service Southern Research Station, and the National Oceanic and Atmospheric Administration.