There are warning signs that some forests in the western U.S. may have a hard time recovering from the large and intense wildfires that have become more common as the climate warms.

After studying 14 burned areas across 10 national forests in California, scientists from UC Davis and the U.S. Forest Service said recent fires have killed so many mature, seed-producing trees across such large areas that the forests can't re-seed themselves. And because of increasingly warm temperatures, burned areas are quickly overgrown by shrubs, which can prevent trees from taking root.

"With high-severity fires, the seed source drops off," said study co-author Kevin Lynch, a forest researcher at UC Davis. "We aren't seeing the conditions that are likely to promote natural regeneration."

Historically, severe fires were uncommon in the forests covered by the study, largely made up of yellow pines and mixed conifers, but extended drought and heatwaves have exacerbated fire conditions across the West. The changing climate is also seen as a factor in recent wildfires in the Southeast, which is also mired in drought.

For the study, published Wednesday in the journal Ecosphere, the researchers surveyed 1,500 plots in burned areas at different elevations in the Sierra Nevadas, Klamath Mountains, and North Coast regions. There was no natural conifer regeneration at all in 43 percent of the plots, they reported.

"[O]ur data support growing concern that the well-documented trend toward larger and more severe fires is a major threat to conifer forest sustainability in our study region," the authors wrote. They said the study results could apply to mixed conifer forests across the West.

Welch said the study was aimed at helping forest managers decide where to apply limited funding to replant forests that aren't regrowing on their own.

"There aren't enough of the right kind of trees growing back, the sugar pines and the ponderosa pines," he said, describing the native species that are ecologically and commercially valuable.

Firs and cedars dominated in the study plots where there was regeneration, but those trees are much less resistant to future fires. Decades of logging of old fire-resistant trees and fire suppression shifted the composition of the forests, making them more dense. Add in the drying and warming climate, and it's a recipe for intense fires.

"Pretty much everyone agrees that's the climate change signal," he said.

That meshes with findings in other recent studies, said Ellis Margolis and Collin Haffey, U.S. Geological Survey forest scientists in New Mexico who were not involved in the new study.

"The story from recent fires is that, due to a warming and drying climate, combined with increased fuels from a century of fire exclusion, high-severity burn patches in dry conifer forests have been increasing in size," they said in an email response. "Importantly, some of these large, severe burn patches have no surviving trees within them."

Some areas are being scorched repeatedly because wildfires are also becoming more frequent. Those re-burns have killed off even more of the mature seed trees.

"The loss of seed sources and increasing moisture stress after fires, both related to climate change, does not bode well for the future of large areas of dry conifer forests," they said.

A wide-ranging 2015 study by other federal researchers suggested that entire forests could succumb to mega-disturbances like fire and insect outbreaks, and that climate change is already driving the transition of forests to shrub and grasslands in drier parts of the U.S. like the Southwest.

The new study offers some clues about short-term forest response, but regeneration after big fires can be a centuries-long process, according to Park Williams, a forest researcher with Columbia University's Lamont-Doherty Earth Observatory.

"Prior to the modern era, fires in this region seemed to typically be low-intensity," he said. "In the more intense fires that we've been seeing recently, the patches killed by the fire are tending to be far larger and it could take a very long time for the native tree species to repopulate these areas. With climate getting warmer in the coming centuries, it seems more likely that many large burned forest areas in the Southwest U.S. will be recolonized by shrub species that can reproduce quickly and tolerate heat and drought."

Alistair Jump, a forest ecologist in the U.K. who has studied forests on three continents, said recent forest die-offs around the world should be seen as part of a global forest crisis. The massive changes aren't just a symptom of climate change—they could drive changes in the global carbon cycle that would speed the buildup of heat-trapping pollution.

"Mortality events might be perceived as local. Even on a vast scale they are easy to dismiss as the problem of another state or country," he said. "However, the drivers of such events are substantially global (changing climate and its interaction with pests and disease) and many of their impacts are global. Consequently, we need a coordinated global approach to address the problem.

"There's a band of very dedicated people working on this issue across the globe but with staggeringly little recognition of the seriousness of the problem from most of our political leaders. In short, we risk a very significant exacerbation of our environmental problems at a global scale if we continue to overlook large scale mortality across the world's forests."