There are 154 national forests in the United States, covering nearly 300,000 square miles of forests, woodlands, shrublands, wetlands, meadows and prairies. These lands are increasingly recognized as vital for supporting a broad diversity of plant and animal life; for water and nutrient cycling; and for the human communities that depend on forests and find cultural and spiritual significance within them. Forests could also be potential bulwarks against climate change. But, increasingly severe droughts and wildfires, invasive species, and large insect outbreaks — all intensified by climate change — are straining many national forests and surrounding lands.

A by a team of 40 experts outlines a new approach to forest stewardship that “braids together” Indigenous knowledge and Western science to conserve and restore more resilient forestlands. Published March 25, the report provides foundational material to inform future work on climate-smart adaptive management practices for land managers.

“Our forests are in grave danger in the face of climate change,” said , an associate dean of forestry at Oregon State University. “By braiding together Indigenous knowledge with Western science, we can view the problems with what is known as ‘Two-Eyed Seeing,’ to develop a path forward that makes our forests more resilient to the threats they are facing. That is what this report is working to accomplish.”

Eisenberg co-led the report team with , a fire ecologist in the School of Environmental and Forest Sciences at the 91̽��.

“Climate change is stressing these forests even as they are considered for their potential role in slowing rates of climate change,” said Prichard. “We want this report to provide not just guidance, but also hope — hope in the practical measures we can take now to promote resiliency and help forests thrive.”

Initiated by interest from the Forest Service on Indigenous knowledge and Western science, the report stems from direction to protect old and mature forests outlined in , signed by President Joe Biden in April 2022. These types of forests, some hundreds of years old, are often dominated by larger trees, with fewer seedlings and saplings. Some management practices over the past century have made many of these forests vulnerable to drought, fire, insects and other stressors, all of which will likely increase with climate change.

The executive order included guidance on strengthening relationships with tribal governments and emphasized the importance of Indigenous knowledge, a theme highlighted repeatedly in the new report. This knowledge includes the time-tested practices of Indigenous stewardship that for millennia shaped forest structure and species composition. Following European colonization, these practices were sharply curtailed by genocide, displacement, and forced assimilation of Indigenous peoples. Western scientists increasingly recognize that Indigenous stewardship practices built and maintained forests that were more resilient and ecologically diverse than today.

Many Indigenous cultures, for example, used a practice called intentional burning — also known as cultural burning — which decreased forest density, promoted healthy understory growth, and hosted a broad diversity of plant and animal life. These practices over time yielded “mosaics” of forests made up of diverse patches of trees varying in age, density, and overstory and understory composition. These “mosaic” forests are less prone to the types of large, severe wildfires that have burned swathes of North American forests this century, according to Prichard.

Other members of the core leadership team for the report are , a senior research ecologist with the Forest Service’s Pacific Northwest Research Station, and , a professor and director of the Center for the Future of Forests and Society at OSU.

“Two powerful ideas we heard from our Indigenous colleagues in developing this are those of reciprocity and the seven generations principle. Collectively, the writing team agrees that we can frame a more sustainable land ethic with these ideas,” said Hessburg. “These perspectives guided our recommendations, which suggest taking from the land and giving back in equal measure, and proactively stewarding these lands with seven generations in mind.”

come from tribal nations, universities, U.S. Forest Service research stations, consulting groups, Natural Resources Canada, Parks Canada, and Tall Timbers Research Station and Land Conservancy.

“Our report is deeper than changes in policy and management — it proposes a fundamental change in the worldview guiding our current practices,” said Nelson. “Our writing team’s cultural, geographic and disciplinary diversity allows for guidance on a shift in paradigms around how we approach forest stewardship in the face of climate change.”

The report may also inform Forest Service work on the proposed national forest land plan intended to steward and conserve old-growth forest conditions.

“We are very interested in understanding how Indigenous knowledge can be used in combination with western science to improve our management of all forest conditions including old growth,” said Forest Service Deputy Chief Chris French. “This report is a big step in improving our understanding of how to do that.”

The report is available for download , along with an interactive map highlighting more than 50 examples of forest adaptation strategies. It was funded by the U.S. Forest Service, the Resources Legacy Fund, the 444S Foundation, the Doris Duke Charitable Foundation, the Gordon and Betty Moore Foundation, and the Wilburforce Foundation.

For more information, contact Prichard at sprich@uw.edu, Hessburg at paul.hessburg@usda.gov, Eisenberg at Cristina.Eisenberg@oregonstate.edu, and the Forest Service press office at sm.fs.pressoffice@usda.gov.

The first volume of the fourth assessment, released in 2017, looked at the physical science underlying the report. , a research scientist at the 91̽��Joint Institute for the Study of Atmosphere and Ocean, was an author on chapter two, “,” that provides an overview and update of the first volume. The rest of the second volume, released Nov. 23, focuses on impacts, risks and adaptation across the United States.

, a 91̽��professor of both global health and environmental and occupational health sciences, was a lead author of the chapter on . This chapter looked at human health effects from exposure to heatwaves, floods, droughts and other extreme events; infectious diseases; changes in our food and water; and mental health and well-being. The chapter also assessed the health co-benefits of various mitigation policies that address climate change.

Previous versions of the climate assessments considered various impacts, such as from extreme weather events or for public health, separately, Ebi told . The new report, she said, includes regional chapters that consider the interconnected and often compounding risks within the Northwest and other regions.

Two researchers at the School of Environmental and Forest Sciences contributed to the new assessment’s chapter on . Professor was one of two coordinating lead authors, and research scientist was a technical contributor. The chapter looked at how extreme weather, including droughts, will make wildfires more frequent and intense nationally and in specific regions of the U.S. It also describes how climate change will affect other ecological disturbances, such as insects. The authors find that many options exist to reduce the largely negative effects of climate change, and list how federal agencies and other entities are already implementing adaptation measures across the United States.

The national assessment includes 10 chapters that focus on impacts, risks and adaptation in specific regions. The ‘s former deputy director, Joe Casola, was an author on the . (Amy Snover, director of the Climate Impacts Group, was a lead author in 2014 of the ). The new report emphasizes many of the same impacts on water, coasts, forests and agriculture in the Northwest. The Northwest region has warmed almost 2.0 degrees Fahrenheit since 1900, with a portion of the warming directly linked to human-caused climate change. The authors use 2015, a year characterized by record-breaking warm and dry conditions, to explore how climate change will be experienced in the Northwest region. This chapter, and the larger national assessment, emphasizes how climate change will disproportionately affect poor and disadvantaged people and Indigenous communities.

, an assistant professor of environmental and forest sciences and of civil and environmental engineering, contributed to the second , also released Nov. 23, for the first time in conjunction with the national climate assessment. This report each decade summarizes carbon-cycle science, or how increasing atmospheric carbon dioxide from burning fossil fuels moves through the Earth system across North America. Butman was the lead author of the chapter focused on and a contributing author to the second chapter on of rising atmospheric carbon dioxide. , an assistant professor at 91̽��Bothell, contributed to the carbon cycle report’s chapter on .

, a research scientist in the Polar Science Center at the 91̽��Applied Physics Laboratory, was a contributor to the Fourth National Climate Assessment’s regional chapter focused on . Former 91̽��research scientist , now a faculty member at the University of Connecticut, was an author on the climate assessment’s chapter on .

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For more information, contact Ebi at krisebi@uw.edu, Peterson at wild@uw.edu, Heidi Roop at the Climate Impacts Group at hroop@uw.edu and Butman at dbutman@uw.edu.

The book brings together years of conversation about what resource managers are seeing – and doing – on the ground. While Halofsky and Peterson wrote the introduction, other chapters were written by scientists and resource managers who are members of the , a group of 35 organizations that the two 91̽��environmental scientists co-lead.

Q: Where exactly are the Northern Rocky Mountains?

JH: We focused on the northern region of the U.S. Forest Service in the Rockies. Our area includes northern Idaho, all of Montana and North Dakota, a small portion of South Dakota, and all of the Greater Yellowstone Ecosystem (including part of Wyoming).

Q: How is this book different from other climate change reports?

JH: This book is a deeper dive into the Northern Rockies region, and federal forest lands and grasslands in particular. The book also focuses on the specific needs of the Forest Service and National Park Service, which manage millions of acres of land in this area. It puts it in the context of their management practices, and what they may be able to do about climate change.

Q: What new threats will this region face?

JH: Some of the biggest threats are lower snowpack, which affects a number of different areas. Obviously skiing and winter recreation are affected, but water supply is another big impact. Lower snowpack and less precipitation falling as snow means that the water runs off earlier in the season, and then you have lower streamflow in the summer. That affects water availability for cities and people, for agriculture, and for fish and other aquatic organisms. When we have lower streamflow and lower moisture levels, we also see more wildfires.

Q: What are people doing, or what could they do, to prepare for these changes?

JH: A lot of the “what do we do about it?” is promoting healthy ecosystems. The idea is that a healthier ecosystem will be better able to respond to these changes. We can make sure that the streams are healthy, and that impacts from roads, livestock grazing, and other stressors are minimized.

Restoring the functionality of streams and floodplains is very important. Reintroducing the beavers can also be helpful, because they build dams that slow the water flow, retain cool water in the mountain streams and augment summer flows.

Q: What about forest fires? Is there any way to prevent that risk from increasing?

JH: For forests, it’s about reducing existing stressors. Fire suppression has really affected forest conditions — the forests are denser than they were historically. Doing things like thinning treatments, where you reduce the density of the forest, can help reduce fire risk and help trees respond to drought because they’re not competing with other trees. Reducing high fuels on the forest floor, with prescribed fire or other methods, is also important.

These actions are not necessarily going to decrease the number of fires, but thinning and other treatments can reduce the severity of the fire. So the fire won’t burn as hot, or damage the soil as much.

Q: What about the forest fires this summer that caused evacuations and smoky conditions as far west as Seattle? Were those fires related to climate change?

JH: It is difficult to say how much of a role climate change has played in recent wildfire activity. However, climatic conditions are a major driver for how much area is burned. Over the past century, wildfires in the mountainous areas of the West have seen larger areas burned during periods of low precipitation, higher temperatures and drought conditions. Climate change will bring higher temperatures and more severe droughts in fire-prone regions of the U.S., and that is projected to lead to larger areas burned.

Q: What do you hope this book will achieve?

JH: We hope that it increases awareness of climate change and its effects on natural resources, and also can give people some hope that there are things that can be done on the ground, and things that are already being done, that can help reduce the negative effects of climate change.

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For more information, contact Halofsky at jhalo@uw.edu.

Major forest die-offs due to drought, heat and beetle infestations or deforestation could have consequences far beyond the local landscape.

Wiping out an entire forest can have significant effects on global climate patterns and alter vegetation on the other side of the world, according to a led by the 91̽�� and published Nov. 16 in PLOS ONE.

“When trees die in one place, it can be good or bad for plants elsewhere, because it causes changes in one place that can ricochet to shift climate in another place,” said lead author , a 91̽��postdoctoral researcher in atmospheric sciences. “The atmosphere provides the connection.”

Just as conditions in the tropical Pacific Ocean can have distant effects through what we now understand as El Niño, the loss of a forest could generate a signal heard around the world — including by other plants.

Forest loss is known to have a nearby cooling effect, because without trees the Earth’s surface is more reflective and absorbs less sunlight, and loss of vegetation also makes air drier. These local effects of deforestation are well known. But the new study shows major forest losses can alter global climate by shifting the path of large-scale atmospheric waves or altering precipitation paths. Less forest cover can also change how much sunlight is absorbed in the Northern versus the Southern hemispheres, which can shift tropical rain bands and other climate features.

“People have thought about how forest loss matters for an ecosystem, and maybe for local temperatures, but they haven’t thought about how that interacts with the global climate,” said co-author , a 91̽��assistant professor of atmospheric sciences and of biology. “We are only starting to think about these larger-scale implications.”

The new study focused on two areas that are now losing trees: western North America, which is suffering from drought, heat and beetle infestations that span from the southwestern U.S. to Alaska, and the Amazon rainforest, which has been subject to decades of intense human development. The researchers ran a climate model with a drastic forest-loss scenario to investigate the most extreme potential climate effects.

Results show that removing trees in western North America causes cooling in Siberia, which slows forest growth there. Tree loss in the western U.S. also makes air drier in the southeastern U.S., which harms forests in places like the Carolinas. But forests in South America actually benefit, because it becomes cooler and thus wetter south of the equator.

In the second test case, removing most of the Amazon rainforest also caused Siberia to become colder and more barren, but it had a slight positive impact on southeastern U.S. vegetation. Losing Amazon forest had a significant positive impact on the neighboring forests in eastern South America, mostly by increasing the precipitation there during the Southern Hemisphere summer.

The study shows that when it comes to forests, one plus one does not always equal two. Removing both forests had different impacts than the combined effects of removing the two separately, since the effects can either reinforce one another or cancel each other out.

“I think it’s really interesting that these effects happen through different mechanisms depending on where you look,” Swann said.

The model’s parameters for forest changes are still preliminary, so the exact mapping of cause and effect at each location is not set in stone. The researchers are conducting field studies to better characterize the temperature and humidity changes from altering different forest types. They also hope to pinpoint which locations are most sensitive to triggering such shifts, or to being affected by the changes.

“The broader idea is that we must understand and include the effects of forest loss when modeling global climate and trying to predict how climate will change in the future,” said Swann.

Swann’s previous research looked at how a hypothetical in the Northern Hemisphere to slow global warming could have the unintended effect of changing tropical rainfall. More recent has shown how European deforestation over the past thousands of years may have reduced rainfall over modern-day Africa.

“This study shows that local events like forest die-offs in one part of the globe influence climate and ecology in other, often distant locations,” said Tim Kratz, program director at the funding agency, the National Science Foundation. “Unraveling these far-reaching effects is critical to understanding how nature works at continental to global scales.”

The study was also funded by the U.S. Department of Energy. Co-authors are Juan Villegas at the University of Antioquia in Colombia; David Breshears, Darin Law and Scott Saleska at the University of Arizona; and Scott Stark at Michigan State University.

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For more information, contact Garcia at esgarcia@uw.edu and Swann at 206-616-0486 or aswann@uw.edu.

New research by German, Finnish and U.S. scientists elucidates the process by which gas wafting from coniferous trees creates particles that can reflect sunlight or promote cloud formation, both important climate feedbacks. The is published Feb. 27 in .

“In many forested regions, you can go and observe particles apparently form from thin air. They’re not emitted from anything, they just appear,” said , a 91̽�� associate professor of atmospheric sciences and second author on the paper.

The study shows the chemistry behind these particles’ formation, and estimates they may be the dominant source of aerosols over boreal forests. The has named aerosols generally one of the biggest unknowns for climate change.

Scientists have known for decades that gases from pine trees can form particles that grow from just 1 nanometer in size to 100 nanometers in about a day. These airborne solid or liquid particles can reflect sunlight, and at 100 nanometers they are large enough to condense water vapor and prompt cloud formation.

In the new paper, researchers took measurements in Finnish pine forests and then simulated the same particle formation in an air chamber at Germany’s . A new type of chemical mass spectrometry let researchers pick out 1 in a trillion molecules and follow their evolution.

Results showed that when a pine-scented molecule combines with ozone in the surrounding air, some of the resulting free radicals grab oxygen with unprecedented speed.

“The radical is so desperate to become a regular molecule again that it reacts with itself. The new oxygen breaks off a hydrogen from a neighboring carbon to keep for itself, and then more oxygen comes in to where the hydrogen was broken off,” Thornton said.

Current chemistry would predict that 3 to 5 oxygen molecules could be added per day during oxidation, Thornton said. But researchers observed the free radical adding 10 to 12 oxygen molecules in a single step. This new, bigger molecule wants to be in a solid or liquid state, rather than gas, and condenses onto small particles of just 3 nanometers. Researchers found so many of these molecules are produced that they can clump together and grow to a size big enough to influence climate.

“I think unravelling that chemistry is going to have some profound impacts on how we describe atmospheric chemistry generally,” Thornton said.

Lead author did the work as a postdoctoral researcher in Germany, working in the group of co-author . Ehn is now based at the University of Helsinki in Finland.

Boreal or pine forests give off the largest amount of these compounds, so the finding is especially relevant for the northern parts of North America, Europe and Russia. Other types of forests emit similar vapors, Thornton said, and he believes the rapid oxidation may apply to a broad range of atmospheric compounds.

“I think a lot of missing puzzle pieces in atmospheric chemistry will start to fall into place once we incorporate this understanding,” Thornton said.

Forests are thought to emit exponentially more of these scented compounds as temperatures rise. Understanding how those vapors react could help to predict how forested regions will respond to global warming, and what role they will play in the planet’s response.

In related work, Thornton’s group was part of last summer to study air chemistry over the Southeastern United States, where aerosols formed by reforested areas or from pollution could help explain why that region has not warmed as much as other places.

“It’s thought that as the Earth warms there will be more of these vapors emitted, and some fraction of them will be converted to particles which can potentially shade the Earth’s surface,” Thornton said. “How effective that is at temperature regulation is still very much an open question.”

The 33 co-authors also include Felipe Lopez-Hilfiker and Ben Lee, both at the UW, and researchers from the University of Copenhagen in Denmark, the Institute for Tropospheric Research in Germany, Aerodyne Research Inc. in Massachusetts, and Tampere University of Technology in Finland.

The research was funded by the European Research Council, Academy of Finland Center of Excellence, U.S. Department of Energy, and the Emil Aaltonen Foundation.

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For more information, contact Thornton at 206-543-4010 or joelt@uw.edu and Ehn at mikael.ehn@helsinki.fi.