Climate change in the American West may be crossing an ominous threshold, making parts of the region inhospitable for some native pine and fir forests to regrow after wildfires, new research suggests.

As temperatures rise, the hotter, drier air and drier soil conditions are increasingly unsuitable for young Douglas firs and ponderosa pines to take root and thrive in some of the region's low-elevation forests, scientists write in a study published Monday in the Proceedings of the National Academy of Sciences. Wildfires in these areas could lead to abrupt ecosystem changes, from forest to non-forest, that would otherwise take decades to centuries, the study says.

"Once a certain threshold was crossed, then the probability of tree establishment decreased rapidly," said Kimberley Davis, a researcher at the University of Montana and lead author of the study. "The climate conditions are just a lot less suitable for regeneration."

The two iconic species are important to both the region's forest ecology and its economy, particularly its timber industry.

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Davis and her colleagues looked at growth rings of nearly 3,000 young trees in 33 fire-damaged areas of California, Colorado, the Northern Rockies and the southwestern United States to see when the forests recovered after fires over the past 30 years. Analyzing climate data over the same period, they found certain thresholds involving summer humidity for ponderosa pine, surface temperature for Douglas fir, and soil moisture for both species, beyond which there was a sharp decline in forest regrowth.

The warmer, drier air isn't harming mature trees, but it is preventing future generations from growing, Davis said.

"There could be a lot of areas where there is currently forest but if we have a fire we might not see regeneration," she said.

Thresholds 'Show How Much Is at Stake'

Davis and her colleagues found that most of the sites they looked at had crossed the temperature and humidity threshold at some point in the last 20 years.

They targeted the driest and warmest sites in the region to see if climate change was already beginning to affect forests. The researchers now plan to assess the extent to which regeneration is affected in relatively cooler, wetter sites in the region.

Several factors influence a forest's regrowth after a wildfire, such as the severity of the fire, regional drought and how the trees produce seeds. The researchers noted that as the region sees fewer years with climate conditions suitable for seedlings to grow, the nature of the trees' seed production, with heavy crops of cones only every few years, will further limit new growth.

Last year, wildfires burned more than 8.7 million acres nationwide, 32 percent higher than the 10-year average according to an annual report by the National Interagency Coordination Center released last week. More than 1.8 million of those acres were in California, the highest in recorded state history, according to state fire officials.

The inability of forests to bounce back from such fires is cause for concern, said Joe Fargione, science director for the Nature Conservancy's North America region.

"The thresholds identified here show how much is at stake—losing forests because trees can't grow back—if we don't accelerate the switch to clean energy and invest more in natural climate solutions," Fargione said. "This study will help land managers identify forests at the greatest risk of not regenerating post-fire."

Fargione said such high-risk forests can be targeted for selective thinning to reduce the risk of forest fires and for restoration efforts to improve the success of forest regeneration.

Trees Play a Critical Climate Role

Protecting forests is important for slowing climate change because of their ability to pull carbon dioxide out of the atmosphere and store the carbon in their trunks, branches and roots.

A study Fargione and others published last fall found better forest, farm and land management practices offer natural climate solutions that could mitigate 21 percent of the United States' annual greenhouse gas emissions.

"It really highlights the fact that we need to begin a national and international conversation about how we can enhance the resiliency of our global forests," Anthony Swift, director of the Natural Resources Defense Council's Canada Project, said of the current study.

Swift said intensive logging, road building and cutting trees to make room for transmission lines weakens the ability of forests to deal with added stressors like climate change.

"We don't need to stop all activity in global forests, but it does raise the need to reconsider how and where we extract timber and engage in other industrial activities in forests," he said.

A report last month by NRDC looked at how clear cutting boreal forests in Canada by some U.S. toilet paper manufacturers imperils forests and could hasten climate change.The report called for alternatives, like using recycled paper, that don't lead to the fragmentation and clear cutting of intact forests. Leaving forests more intact could also help protect Douglas fir and ponderosa pine forests in the American West, Swift said.

"Whether we are looking at wildfires or industrial activities, concerns that in a changing climate regeneration is going to be more difficult to achieve is an issue that we all need to be looking at more carefully because of the roll our forests have to play for the global climate," he said.