SAN FRANCISCO — As hotter and bigger fires blaze through the West and yet another year passes with a disastrous toll, America's wildlands are having a harder time bouncing back. Some spots, from the singed valleys of Wine Country to the steep slopes of the Sierra, may never look the same.

The fiercer fires are killing more of the vegetation needed to provide seeds for regrowth, and scientists are learning that even when new trees sprout, many are struggling with the warmer and more extreme weather wrought by climate change.

While wildfires have historically benefited the natural landscape, cycling soil nutrients and clearing space for new plants, for example, scientists say more burned areas are likely to end up devoid of trees and populated instead with weeds and grasses. This new vegetation is even likelier to burn, setting in place a vicious cycle of only more fire and less forest.

The latest research on forest regeneration, published last month in the journal Ecology Letters, found that as much as one third of the areas burned since 2000 saw no trees return, about double the treeless land that resulted after fires before 2000.

In 54 percent of the areas burned this century, the research suggests, too few trees grew back to ensure a full forest recovery.

The study looked at nearly 1,500 sites that burned between 1985 and 2015 across five mountain states.

"Regardless of whether you have a chaparral ecosystem burning in Southern California or a forest ecosystem in the Sierra, there's always a concern about whether they're going to bounce back from a wildfire," said the study's lead author, Camille Stevens-Rumann, an assistant professor of forest and rangeland stewardship at Colorado State University. "Many areas are not bouncing back as they have."

In the past, new trees would typically sprout within two to three years. The researchers expect forest regeneration rates to continue to slip in coming decades while "alternative shrubs or grassland states" take hold.

Last year's extraordinary fire season, which burned 9.6 million acres nationwide and 1.2 million acres in California, has set the stage for more such changes.

Beyond climate change, forest recovery is being hurt by more intense fires destroying trees and their seed source, said Kristen Shive, a forest ecology researcher at the University of California at Berkeley who was not involved in the new study.

These larger, hotter fires are driven by both the warming planet and decades of fire suppression, which has left a surplus of flammable vegetation. Before firefighting, the forests would naturally burn, reducing the fuel buildup.