MISSOULA – Warmer and drier climate conditions in western U.S. forests make it less likely trees can regenerate after wildfires, according to new research from the University of Montana published in the Proceedings of the National Academy of Sciences.
Importantly, the research also finds that ecologically based forest management can partially offset climate-driven declines in tree regeneration by limiting fire-caused tree death, but only if action is taken quickly.
According to the authors, the study provides timely information to optimize new state and federal initiatives to increase the pace of ecologically based forest management across millions of acres of Western forests.
Forests are adapted to different types of fire across the West, but hotter and drier conditions in recent decades have intensified the way fires burn, resulting in more trees being killed. All of this can result in fewer seeds available for forests to regenerate after wildfires. Even when seeds are available, a warming climate increasingly limits the chances that seedlings can establish and grow.
“Climate change increasingly limits tree establishment after wildfires because seedlings can be killed by hot temperatures and dry conditions,” said lead author Kim Davis, who completed the study at UM and now works as a research ecologist with the U.S. Forest Service Rocky Mountain Research Station at the Missoula Fire Sciences Laboratory.
The research examined how the severity of a fire – the number of trees it kills – in combination with the climate conditions after the fire affected the chances of tree seedlings regenerating to establish a new forest. It is the most extensive such study to date, assessing regeneration of eight major tree species after 334 wildfires across the West. Information was gathered from over 10,000 field plots and collected by more than 50 research teams.
Researchers found that warmer, drier conditions over the past four decades led to a decline in tree regeneration after wildfires, and this trend is expected to continue in the future. For example, from 1981 to 2000, 95% of the areas studied had climate conditions suitable for tree regeneration after wildfires, but this is projected to decrease to only three-quarters of the West by 2050 under future climate scenarios.
The most vulnerable forests were in drier regions in the Southwest and California, while forests in the wetter and cooler regions of the northern Rocky Mountains and Pacific Northwest are still expected to support conifer regeneration in the near future.
“The impacts of climate change and wildfires vary across the West,” said Philip Higuera, a study co-author and UM professor of fire ecology in the W.A. Franke College of Forestry. “And the large scope of this study allowed us to highlight where these changes are most concentrated and happening first.”
The study also found ecological forest management in vulnerable dry forests could offset climate-driven changes by reducing the number of trees killed in wildfires. Specifically, in almost half of the study region, regeneration after wildfires is projected to be likely only if future fires burn at lower severities because fewer trees that produce seeds needed for forest recovery are killed in these fires.
“We know from prior research that forest thinning and controlled burns in overgrown dry forests effectively reduces fire severity and subsequent tree death,” said study co-author Marcos Robles, lead scientist for The Nature Conservancy in Arizona. “Land managers can’t do much about drought and climate change in the short term, but they can reduce the area in which forests are vulnerable to severe wildfires by accelerating ecological based forest management.”
Prior research published by The Nature Conservancy demonstrates that ecological forest management in a large restoration initiative in Arizona would not only reduce wildfire-caused tree death, but also provide additional co-benefits. Those include significant reductions in drought-related tree death, while increasing carbon storage, stream flow and tree growth.
“But the clock is ticking,” Robles said. “It’s urgent that we implement these treatments in our forests now, lest we lose them altogether.”
Even in cooler high-elevation forests in the Rocky Mountains and Pacific Northwest that experienced large tree-killing wildfires in the past, the researchers also found that climate change makes it less likely for trees to regenerate after wildfire. In these forests, planting trees after wildfire may take on increased urgency, given a shortening window of opportunity for trees to establish in climate conditions that are warmer and drier than in the past.
The federal government allocated around $3 billion to fund ecological forest management and reforestation efforts across 50 million acres in the next 10 years throughout the West.
“Our paper provides much-needed information that can inform these efforts, particularly given the rapid pace of change our Western forests are experiencing,” Higuera said.
The study was published with 63 co-authors, including 12 current or former UM researchers: Higuera, Davis, Alina Cansler, Solomon Dobrowski, Andrew Larson, Jamie Peeler, Mark Kreider, Kyra Clark-Wolf, Caitlin Littlefield, Alan Tepley, Julia Berkey and Michael Schaedel.
The new study is online at: https://www.pnas.org/doi/full/10.1073/pnas.2208120120.
Note: Davis and co-authors published a companion article in the Conversation titled “The West’s iconic forests are increasingly struggling to recover from wildfires – altering how fires burn could turn that around.”
Contacts: Kim Davis, USDA Forest Service research ecologist, Rocky Mountain Research Station, firstname.lastname@example.org; Philip Higuera, UM professor of fire ecology, email@example.com.