Climate Change Is Hurting Regrowth In Forests, UM Researchers Say

Mar 20, 2019

Scientists at the University of Montana have found that climate change is already reducing the ability of some forests in the western U.S. to bounce back after wildfire. Their findings are confirming a long-suspected change.

For the past three years, UM post-doc Kimberly Davis has looked at how ponderosa pine and Douglas fir forests regenerate after fire, and she’s made an eye-opening discovery.

Some forests just won’t be coming back.

Davis says she found that once conditions get hot and dry enough, there’s a dramatic reduction in those trees ability to regenerate.

“So we call these thresholds, where all of a sudden, you cross a certain temperature, and then you see this big decline,” says Davis.

Climate change in the West is crossing a perilous threshold, Davis says, and some of Montana’s iconic pine and fir forests might not be able to regenerate if they get hit by wildfire.

In her lab in Missoula, Davis shows me some of the work researchers do that’s led to their conclusions.

“So if you look in there you’ll be able to see all the cells, so all those little circles,” Davis says as she squints through a microscope at a tiny tree trunk the size of a fingernail.

“They can be really little because they’re young or they can be really little just because they're supressed from competition and shade,” she explains.

For her study, Davis and a small team painstakingly counted the growth rings of 3,000 young trees and analyzed the climate conditions from the exact date each tree started growing.

She determined precisely when and under what climate conditions trees were able to regenerate at 90 burn sites across six states including Montana, California and Colorado.

When Davis looked at recent climate records from those same locations, she realized that a lot of the low-elevation forests she was studying were already crossing that climate threshold.

“What that means is that the climate at many of the sites that we studied is not really suitable for these trees to regenerate anymore,” says Davis.

One of those sites is in the Bitterroot Valley, where low-elevation forest butts up against grassland. Davis says when a fire passes through there in the future, she expects the burned forest edge will become more grassland. The seedling-friendly conditions that would allow pine and fir trees to regenerate are just getting too rare.

Ecologists have been predicting this kind of climate-linked tree regeneration failure for years, says Forest Ecologist Andrew Larson. He wasn’t involved in Davis’ research project, but says Davis is the first person to go beyond just hypotheticals.

“Nobody else has managed to find this sort of evidence yet," says Larson. "So it's right on the cutting edge of ecosystem change across the West.”

Larson says Davis’ work reveals that changes in forest regeneration that scientists had been predicting would happen by 2040 or 2050 are already happening.

“So 25 to 30 years ahead of the schedule that I was expecting. So to me, that's a surprise," says Larson. "That's saying that climate change impacts are not something that's coming in the future. They're here now, all across the West.”

It’s not all bad news, though.

Knowing where forests won’t be coming back means forest managers can focus their restoration efforts in other places where they’ll have a greater chance of success.

That can be a big help to forest managers who don’t often have the budgets for widespread restoration and don’t want their efforts to go to waste.

“It's a scarce resource that we need to make careful decisions about where we're going to spend those person-hours and dollars,” Larson says.

Larson says that even without new seedlings taking root in these hot, dry, low-elevation forests, there are still plenty of big adult trees with lots of life left in them, and forest managers can use strategies like fuel reduction and prescribed fires to make sure these remaining trees aren’t taken out by a severe blaze.

Still, Larson concedes that for many residents in the West, Davis’s findings are going to be a hard pill to swallow.

“Wow, in my lifetime, this place that has been forest -- in my lifetime, my parents’, my grandparents’ -- that’s gonna change to a grassland or some other non-forest community.”

Davis’s study was published March 11 in the Proceedings of the National Academy of Sciences