The US is struggling to replant forests destroyed by increasingly intense wildfires, and many areas are unlikely to recover on their own.
Researchers are studying which species are likely to survive - and where - as climate change makes it difficult or impossible for many forests to regrow. But they say the U.S. also lacks sufficient seed collection, seedling production and workers trained to replant trees on the scale needed to offset mounting losses.
The Forest Service says the biggest roadblock is the years-long task of completing environmental and cultural assessments and preparing severely burned land for replanting.
Here's what you need to know:
Climate impact
Larger and more intense fires, fueled by climate change, destroy seed trees that normally allow regeneration, or leave fire scars too large for trees to naturally bridge the gap.
The climate has changed so much that trees can often no longer regrow. Even when seedlings gain a foothold, they are often killed by drought and repeated fires.
Hot fires in particular can harden the ground and leave bare slopes susceptible to being washed away in rainstorms and polluting waterways. Researchers say some once-forested areas in the southwest and west may never recover and instead turn to grassland or shrubland.
Reforestation Gap
Nineteen of the 20 largest wildfires ever recorded in the contiguous U.S. have occurred in western states since 2000, according to Sean Parks, a Forest Service research ecologist. Then the region fell into a prolonged megadrought.
The US was once able to reliably replant burned forests. But now the gap between areas in need of replanting and the ability to do so has grown to at least 1.5 million hectares - and that could triple by 2050, says Solomon Z. Dobrowski, a forest management expert at the University of Montana.
Researchers say forests will be more likely to grow back, regardless of fire intensity, because of hotter and drier weather.
Targeted tree planting
Researchers are trying to figure out which seedling species survive and where. In general, survival is poorer at lower elevations, where it is warmer, drier and more open. So replanting the same trees in the same areas is likely to fail.
Scientists are replanting at higher altitudes and are also investigating whether seedlings survive better when planted in clusters or near trees that can provide shade and promote water absorption. Some researchers even wonder whether other species should replace trees wiped out by fire.
University of New Mexico forest ecologist Matthew Hurteau said the 2011 Los Conchas fire decimated a huge swath of ponderosa pine forest and most replanting efforts have failed.
So he planted seedlings of different species at different heights and on slopes facing different directions, and then monitored soil moisture, temperature and humidity. A resulting computer model can predict with about 63% accuracy the probability that a seedling will survive in a given spot, and will be used for planting this fall.
"Let's not do the old plant-and-pray method," Hurteau said. "Let's plant where we know their chances of survival are quite high."
Forest Service rules generally require planting the same species at the same height as before a fire, but the agency will have to be "flexible in the future," said Jason Sieg, acting supervisor of the Arapaho and Roosevelt National Forests & Pawnee National Grassland.
For now, that may mean replanting at different heights or collecting seeds from a different location. Ultimately, researchers say this would require planting species not native to an area - an option that many have opposed.
"I've seen people go from 'Absolutely, we can't move trees' to 'Maybe let's at least try it and do some experiments to see if this will work,'" says Camille Stevens-Rumann. , interim director at the Colorado Forest Restoration Institute.
"We have to get creative if we want trees in our landscapes," she said. "We're in a place of such drastic climate change that we're not talking about whether or not some of these places will be a different kind of forest, but whether they will be forests at all."
Restoration challenges
Hurteau, a researcher at the University of New Mexico, said ecologists and the state realized there wouldn't be enough seedlings to reforest millions of acres burned by wildfires.
So several New Mexico universities and the state's forestry department started the New Mexico Reforestation Center to build a nursery that could produce 5 million seedlings per year for state, tribal and private lands. This year the first seedlings will be planted.
The number of Forest Service nurseries - once funded by deposits from timber sales - dropped from 14 to six in the 1990s as timber harvests declined and habitat protections were introduced, according to a Forest Service report on the history of the nurseries.
Most Western seedling production is private and takes place in Oregon, California and Washington, says Solomon Dobrowski, a forest management expert at the University of Montana.
In places like New Mexico and the Rocky Mountains, "we don't really have a base of facilities to support widespread reforestation," Dobrowski said. "We ask ourselves, 'What's going to fill the gap?'"
The Forest Service is modernizing farms and looking for ways to expand internal capacity and partner with private industry, states and groups like the New Mexico Reforestation Center. But officials say the biggest challenge is that the number of intense wildfires is outpacing the ability to prepare sites for replanting.
Experts say more seed collection and skilled workers are needed to make even modest progress in closing the reforestation gap. And they say public and private collaboration is essential.
Collecting seeds is expensive and labor intensive. It takes a typical western conifer a few years to develop cones before contractors harvest them. Growing, planting and monitoring seedlings amid frequent droughts adds uncertainty, time and money.
Experts say there will be areas where trees never return, but it's critical that the U.S. does as much as possible in a thoughtful way.
"Trees last hundreds of years, so we have to think about what is good when we plant trees today," Hurteau says. "Are we placing the right species and densities in the landscape, given what the next 100, 200 and 300 years will look like?"
___ AP data reporter Mary Katherine Wildeman contributed from Hartford, Connecticut. ___
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