Severe wildfires across the American West may already be altering the future of forests there.

The big picture: Fires are catalyzing changes in forests already under pressure from climate change. Scientists are trying to determine how forests are responding and whether they can be rebuilt to withstand future fires and combat climate change.

Driving the news: California has now seen the highest number of acres — about 2.5 million — burned in a single season on modern record. And the season still has several months to go.

• More problematic, though, is that recent fires are increasingly severe, completely burning larger numbers of trees, and expanding rapidly, says climate scientist Daniel Swain of the University of California, Los Angeles.
• The Bear Fire in Northern California grew by an estimated 230,000 acres in just 24 hours this week.
• Strong offshore winds arrived on Tuesday morning, further increasing the risk of rapid fire spread.
• "Things could get worse before they get better, especially in California, Oregon and Washington. The rainy season in California is at least a couple months out, and offshore wind season is just beginning and is early and it could last through November," Swain says.

What's happening: A potent mix of climate change and dense forests — the latter partially the product of nearly a century of fire suppression — is resulting in an abundance of drier vegetation that can fuel fires.

• The window for fires is increasing as the fire season lengthens, starting earlier in the spring with premature snowmelt and stretching longer into the fall due to a delayed rainy season.
• And within that window, the frequency of autumn days in California with extreme fire weather more than doubled since the early 1980s, according to a recent analysis by Swain.
• More dense, dry fuel means more severe fires with trees — and their seeds — burning in their entirety.
• And more land is burning and viable seeds from areas that don’t burn can’t be dispersed far enough to recolonize large areas.

One result: Fewer forests are naturally reestablishing after fires.

• A recent study of forests in the Southern Rocky Mountains projects this trend will continue, even under an unlikely scenario where carbon emissions are significantly curbed during the next 20 years.

"We need to talk moreabout how to reforest these areas that burn in megafires," says forest ecologist Julia Burton of SUNY College of Environmental Science and Forestry.

• Reforestation efforts should include planting lower tree densities but also pattern-planting seedlings in widely spaced clumps so there is less continuity of fuels to carry fires, she says, adding that the composition of species replanted is key as well.
• "You could make climate change worse if you plant the wrong forest in the wrong place. You could make fires worse if you plant the wrong forest in the wrong place," Swain says.

But climate change itself is affecting what types of trees can be established and where after fires.

• Trees can't migrate to cooler, moister elevations fast enough to keep pace with changes in climate.
• "In the Western U.S. where there is significant drying, the potential land area where you could support trees is dwindling because of climate change," says Matthew Hurteau, who studies the effects of climate change and fire on forests at the University of New Mexico.
• "The range of climatic conditions a mature tree can tolerate is much wider than a seedling or juvenile," he says. "If wildfire comes through and kills off mature trees, you can’t get the same species to establish because it is too dry and warm."

What to watch: Replanting trees is expensive — about $500–$1,000 an acre (the Camp Fire of 2018 would cost about $75 million to reforest). It takes several years for the seedlings to be grown in nurseries and lots of labor for them to be planted.

• DroneSeed, which works with the Nature Conservancy, timber companies, small private landowners and others, is trying to scale such efforts by dropping seeds housed in vessels from large drones, which allows them to plant large areas faster.

The bottom line: "When we’re going to plant trees, our goal and question should be what is the right tree and the right place for the year 2100," Hurteau says. "And the answer may be no tree."