With wildfires devastating mountain ecosystems across the western United States, their successful forest revegetation recovery hinges on, among other factors, an adequate lasting snowpack, according to research by the University of ϱ, Reno and Oregon State University.
"Our study illustrated that summer precipitation, snow cover and elevation were all important drivers of revegetation success," said Anne Nolin, a hydrologist and geography professor at the University of ϱ, Reno and formerly at Oregon State University. "In particular, we found that snow cover was a critical explanatory variable for revegetation in the Oregon and Washington Cascades. This could help inform revegetation management practices following severe wildfires."
Climate change has already increased the fraction of winter precipitation that falls as rain rather than snow, reduced the spring snow water equivalent – a metric for how much water snow contains – and caused snowmelt to begin earlier in the spring than it used to, Nolin explained. Pacific Northwest snowpacks have seen the greatest declines of any seasonal snow region in the West.
The research, led by Nolin, examined the 260,000-square-mile Columbia River Basin in the Pacific Northwest. She teamed with co-author Andrew Wilson, a graduate research assistant in OSU’s College of Earth, Ocean, and Atmospheric Science, and co-author Kevin Bladon of OSU's College of Forestry for the study.
The NASA-supported study featured before-and-after vegetation analyses for two dozen high-severity wildfires. The fires occurred over a 10-year period among the four distinct subregions of the Columbia River Basin. There are many short- and long-term effects from these fires, including erosion, debris flows and water quality issues, which can affect the health of aquatic ecosystems and downstream community water supply, highlighting the importance of understanding post-fire forest rehabilitation.
In their paper published in the Journal of Geophysical Research – Biogeosciences, ”Assessing the Role of Snow Cover for Post-Wildfire Revegetation Across the Pacific Northwest," the show that given the trends of increasing wildfire activity, lower snowpacks, and earlier snow disappearance dates across the Pacific Northwest, forests will likely experience more frequent drought conditions, which will negatively impact the success of post‐wildfire vegetation recovery with a number of impacts to the ecosystem.
Post-fire management
"This knowledge may be used to facilitate adaptive post-fire management policies and decisions to ensure long-term forest health," Nolin, who is also director of the University of ϱ, Reno's Graduate Program of Hydrological Sciences, said. "For example, depending on the sub-region and species composition, reseeding efforts following low snow winters might employ more drought tolerant species or, replanting could be delayed one to two years until snowmelt and soil moisture conditions are more favorable for seedling propagation.
"However, climate change projections and shifting wildfire regimes have increased concerns about post-fire regeneration and, as such it is imperative that we broaden our understanding of the role of snowpacks in post-wildfire forest regeneration. The snowpacks’ role in aiding revegetation will become increasingly important across the West. And 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."
Wildfires continue to burn more area each year across many regions of the planet, including the Pacific Northwest. The Pacific Northwest’s largest watershed, the Columbia River Basin contains a variety of fire-prone landscapes that have seen almost 900 fires since 2010, serves as critical habitat for more than 700 species and is a water source for seven states.
“As wildfire activity continues to increase and intensify in the Northwest, understanding what shapes revegetation on severely burned forested landscapes is vital for guiding management decisions,” co-author Bladon said.
After the occurrence of a wildfire, revegetation over the burned area is critical to maintain or re-establish ecosystem functions from forests such as biodiversity, erosion control, water purification and habitat provision.
“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," Nolin said. "Positive relationships between snow cover and summer precipitation with post-fire greening suggest that active post-fire revegetation efforts will help facilitate recovery, especially during years when severe wildfires are followed by early snowmelt years or below average summer precipitation."
Summer precipitation
In the study, summer precipitation consistently appeared as the most important variable driving post-fire revegetation across all four subregions. Snow cover frequency, along with elevation, were shown to be secondary but significantly influential explanatory variables for revegetation in the Oregon and Washington Cascades.
More than 80% of wildfires in the western United States from 2000 to 2012 burned within a seasonal snow zone, a time period that overlaps with the years studied by the scientists.
“As wildfire activity continues to increase and intensify in the Northwest, understanding what shapes revegetation on severely burned forested landscapes is vital for guiding management decisions,” Bladon said. “But variables such as snow cover frequency, pre-fire forest composition, and elevation, were also shown to be significantly influential for revegetation in the Oregon and Washington Cascades.”
Wildfire season length in the western U.S. overall has increased by roughly 25 days in recent decades, including a massive increase in the Northwest from the mid-1970s, when it was 23 days, to 116 days in the early 2000s. That’s attributable mainly to warmer temperatures and drier conditions in the spring and summer.
“Snow cover has a strong influence on postfire vegetation greening, but the influence varied depending on subregion and dominant prefire conifer species, with the biggest impacts at low to moderate elevations in the Washington Cascades, the Oregon Cascades and western Montana Rockies,” Nolin said. “And with current climate change projections, snowpacks’ role in aiding revegetation will become increasingly important across the West.”
Bladon suggests fire can be looked at as an opportunity for forests to reassemble into ecosystems better suited to survive warmer winters, longer fire seasons and more drought stress.
“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. “But with shifting climate trends in the region, that might not be the most adaptive path forward for forested landscapes.”
