Number of trees that die years after wildfire likely bigger than thought, research shows

A year after a wildfire, the U.S. Forest Service assesses the damage to figure out how many trees need to be planted to restore the forest. But research shows that much more time is needed to calculate tree mortality. (Oregon Department of Forestry/Flickr)

One year after a wildfire burns in a forest, the U.S. Forest Service assesses the damage.

What the agency finds at this one-year mark informs its post-fire restoration efforts, including how many trees foresters are required to plant to replace ones that died due to the fire. But according to a growing body of research, one year just isn’t enough time to determine how many trees have died following a wildfire.

Two recent studies, created independently of each other, that examined the effects of wildfires in Oregon, Washington as well as other parts of the West, found that trees scorched by fire can continue to die for as long as five years after a wildfire.

While there is as yet no scientific consensus about what these findings mean for post-fire restoration, the implication, according to some scientists, is that the ecological damage caused by wildfires has been underestimated and restoration efforts are not keeping up.

The amount of climate-warming carbon lost to the atmosphere when trees die likely has also been underestimated, according to one of the two studies, implying that the current carbon-absorbing power of some forests has been overestimated.

The phenomenon being investigated is called “post-fire delayed tree mortality.”

“What we’re finding is fire effects are not static, they’re dynamic, (tree) mortality continues to happen for years, hence the word delayed,” said Andrés Holz, associate professor at Portland State University, director of the university’s Global Environmental Change Lab and coauthor of one of the studies.

The research potentially has wide implications: Though scientists do not know how many acres have been affected by post-fire mortality, wildfires burn millions of acres of forest a year. Last year, 56,580 wildfires burned in the U.S., scorching nearly 2.7 million acres (over 4,200 square miles), and in 2022, nearly 68,990 fires burned more than 7.5 million acres (over 11,800 square miles), according to the National Interagency Fire Center based in Boise, Idaho. The center coordinates fire resources nationwide.

Previous field research has shown that trees continue to die for years after wildfires. Holz and his colleagues’ research is some of the first to use satellite imagery to study the phenomenon remotely.

Wildfires in Cascades

Holz’s study, published in April in the journal Ecosphere , examined the effects of delayed tree mortality following wildfires in the Cascades and the northern and southern Rocky Mountains. To do this, Holz and his coauthors compared images of forests burned by wildfires taken one-year post-fire to images of the same forests taken five-years post-fire.

The analysis revealed that many forest sections that had previously shown up as green pixels in satellite images taken one year after a fire were no longer green by year five, suggesting that trees had continued to die over the four-year period. This one-to-five-year comparison was done in areas burned by 30 wildfires.

The study’s lead author, Sebastian Busby, said the research was inspired by fieldwork he did in the Cascade Mountains as a Portland State University graduate student working with Holz. Busby is now a forest carbon analyst for the nonprofit Nature Conservancy.

While in the Cascades, Busby said he used GPS data and fire severity maps created one year after a fire to locate where a fire perimeter should have stopped only to find that the reach of the dead trees had surpassed the perimeter. That was also reflected in the comparisons of the satellite imagery.

By year five, delayed mortality led to a decline in tree cover at the examined fire perimeters from 5% to 25%, according to the study. Busby said it’s likely that trees that previously had been lightly burned but were still living one year after a fire had been weakened enough by the flames to later die from other causes, including drought and insect attacks.

The study also looked at “fire refugia,” or patches of trees in a wildfire area that           experience low-intensity fire or no fire and continue to survive.

“The way I always think about (fire refugia) is they’re islands within an ocean, and the ocean is a severely burned forest,” said Busby.

These living islands provide the seeds that forests use to naturally regenerate. Busby and Holz found that while some fire refugia remained unchanged by year five, others shrank in size or disappeared entirely.

“Trees continue to die over time and sometimes at high magnitudes,” said Busby. “So, if you actually look further out in time, there’s less and less seed source available.”

The problem, Busby said, is most forest management agencies stop assessing fire damage one year after a fire. This means they are likely overestimating the number of surviving trees and the quantity of seeds they provide. According to Holz, this suggests the U.S. Forest Service is likely underestimating the number of trees it needs to replant.

Researchers disagree

Matthew Reilly, a U.S. Forest Service research forester and lead author of the second paper, also used satellite imagery to study post-fire tree mortality. His study, published last year in the journal Fire Ecology , examined tree mortality following six fires in Oregon, Washington and California. The study also found that trees in a wildfire zone can die up to five years later, with islands of surviving trees shrinking in the years that followed. “The big thing that we found very consistently is that fire refugia are essentially being cut in half in terms of their area,” Reilly said. “They’re not only cut in half, but they are now more fragmented.”

The study found that post-fire tree mortality occurred on 14% of the total area examined in the study. This amounted to 7,725 hectares, or over 29 square miles.

The total area impacted by post-fire mortality in the Western U.S. remains unknown. However, Holz, Busby and Reilly said the methods they developed in their two studies could be used to determine that number in the future. They agree the number is likely to be very large.

Because delayed mortality is a known problem, but nobody knows how big that problem really is, Reilly refers to it as “sizing up the elephant in the room.”

But he disagreed that his agency and other land managers aren’t planting enough trees post fire though he said the findings could “help inform strategies” related to the agency’s restoration practices.

Reilly said his study also suggests that by underestimating tree death scientists have also overestimated the amount of carbon that’s stored by trees, which helps fight climate change.

“We know that they’re underestimating it, but we don’t quite know by how much yet,” he said.

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