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Study: Snow cover critically important for revegetation following wildfires

by Adam Duvernay, Eugene (Ore.) Register-Guard |

How well forests recover from wildfire is linked to the amount of snow that falls over burned areas, a worrying discovery with snowpack in the Pacific Northwest declining.

That is the finding of a new study from researchers at Oregon State University and the University of Nevada recently published in the Journal of Geophysical Research: Biogeosciences. As climate change increases how much winter precipitation falls as rain instead of snow, forests may take longer to recover from increasingly frequent wildfires. 

“Many of the forested, mountain landscapes in the Northwest are changing at an unprecedented rate, largely because the regional climate regime is changing,” OSU College of Forestry researcher Kevin Bladon said in a news release. “Forest responses to climate change are also being driven by shifts in the precipitation that influence soil water storage and groundwater.”

More than 80 percent of wildfires in the western United States burned within a seasonal snow zone between 2000 to 2012, a time period overlapping with the years the researchers studied. The researchers conducted their before-and-after vegetation analyses for two dozen high-severity wildfires in four distinct subregions of the Columbia River Basin.

The 260,000-square-mile Columbia River Basin is the largest watershed in the Pacific Northwest. Nearly 900 wildfires since 2010 burned through that watershed, which provides a water source for seven states and is home to about 700 species of wildlife.

“There are many short- and long-term effects from the forest fires that are hitting the West more frequently and more severely than they ever have, including erosion, debris flows and water quality issues, all of which can be bad for aquatic ecosystems and downstream community water supply,” Bladon said.

Bladon said the persistence of wildfire impacts is mitigated by forest revegetation after a fire, but what spurs growth in burned areas is complex and has not been well-studied.

“Climate change projections and changing wildfire regimes have added to concerns about postfire regeneration," Bladon said.

The study showed summer precipitation is the most important variable to post-fire revegetation across the regions they examined, but snow cover frequency, pre-fire forest composition and elevation all played important roles in reviving areas burned by fire.  

Wildfires are becoming more frequent as snowpack declines and snow stops falling sooner in the year, trends that mean forests will likely experience drought conditions more frequently than historically, which will affect revegetation, the researchers said.

“Snow matters to regrowing vegetation following fire, and with double impacts of declining snowpacks and increasing wildfires it is critical that we understand how these changes are affecting Pacific Northwest forests,” University of Nevada professor of geography Anne Nolin, who led the NASA-funded study that began while she was a researcher at OSU, said in the release.

Where snowpacks have declined, there likely will be ecosystem transitions that look like a shift from forest to non-forest and from evergreen to deciduous vegetation, she said.

Though much is made of returning forests to the same state they were in before a fire, Bladon said it may be better to consider if the current trends will make them more resilient to the future climate instead of trying to restore them.

“That’s at the heart of the challenge of reconciling a changing climate’s ecological forces with postfire forest management goals — the goals are often oriented toward re-establishing forests as they existed before the fire,” Bladon said in the release. “But with shifting climate trends in the region, that might not be the most adaptive path forward for forested landscapes.”