What will forests look like at the end of the century? A new study led by UC Boulder suggests that a large number of forests in the western United States will no longer exist. The researchers found that when forests burn across the Southern Rocky Mountains, many will never recover, but will instead transition into grasslands and shrublands.
“We project that post-fire recovery will be less likely in the future, with large percentages of the Southern Rocky Mountains becoming unsuitable for two important tree species – ponderosa pine and Douglas fir,” said lead author Kyle Rodman.
In California and other western states, the wildfire season is arriving earlier and lasting longer every year. This new trend is primarily driven by climate change, which means that as global warming intensifies, so will the duration of the fire season.
Rising temperatures in the spring and summer, reduced snowpack, and earlier spring snowmelt combine to create longer and more intense dry seasons that make forests more susceptible to severe burns.
Previous studies from UC Boulder have examined the sites of individual fires. These forests recovered very slowly, if at all. In the plots surveyed, 80 percent still had no new trees 15 years after the fire.
The research team, which included experts from the U.S. Forest Service, Northern Arizona University, Colorado State University and the University of North Carolina Wilmington, set out to expand on the earlier work. The goal of the current investigation was to project the future by looking at the past.
The study was focused on 22 burned areas, including 710 square miles from southern Wyoming through central and western Colorado to northern New Mexico. Some of these regions that had burned as long ago as 1988.
The team looked at ponderosa pine and Douglas fir forests, which make up about half of the forested area in the region. “For those of us who live along Colorado’s Front Range, these are the trees that we see, live and recreate near on a daily basis,” said Rodman.
The researchers used satellite images and field measurements to reconstruct what the forests looked like prior to the fire. Next, by counting juvenile trees and looking at tree rings, they assessed how well the forests were recovering.
Forests at higher elevations with lower temperatures and more precipitation were found to be in the best shape. Those with more surviving trees nearby were also more likely to rebound.
On the other hand, lower-elevation forests proved to be much less resilient. When the scientists compared the study sites to regions that burned in the 19th and early 20th centuries, the areas that burned more recently failed to bounce back.
“This study and others clearly show that the resilience of our forests to fire has declined significantly under warmer, drier conditions,” said study co-author Professor Tom Veblen.
Computer modeling revealed that by 2051, with moderate greenhouse gas emissions, less than 18 percent of Douglas fir and ponderosa pine forests will recover if burned. Under a higher emissions scenario, that number drops to 6 percent for Douglas fir and 3.5 percent for pine forests.
Meanwhile, said Professor Veblen, the number and intensity of wildfires will continue its steady rise. The number of acres burned annually across the country has already doubled since the 1990s.
“The big takeaway here is that we can expect to have an increase in fire continue for the foreseeable future, and, at the same time, we are going to see much of our land convert from forest to non-forest,” said Professor Veblen.
Rodman hopes the findings will help land managers better plan where to invest their resources, or not, after a fire. For example, it would be more beneficial to plant seedlings in regions that are likely to recover. Rodman also hopes the projections will people one more reason to care about climate change.
“This was a hard study to write and can be a bit depressing to read, but there are some positive takeaways,” said Rodman. “If we can get a handle on some of these trends and reduce our greenhouse gas emissions, the outcomes may not look so dire. The future is not written in stone.”
The study is published in the journal Global Ecology and Biogeography.