Trees might need our help to survive climate change, CSU study finds
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Nine years after the 2012 High Park Fire burned a stand of lodgepole pine that had been severely affected by the mountain pine beetle, lodgepole and aspen saplings were rebounding on this slope in the Arapaho and Roosevelt National Forests. A CSU-led study has found that forests are not regenerating fast enough to keep pace with climate change, wildfire, insects and disease. Photo by Katie Nigro, September 2021
view moreCredit: Katie Nigro
A new Colorado State University study of the interior U.S. West has found that tree ranges are generally contracting in response to climate change but not expanding into cooler, wetter climates – suggesting that forests are not regenerating fast enough to keep pace with climate change, wildfire, insects and disease.
As the climate becomes too warm for trees in certain places, tree ranges have been expected to shift toward more ideal conditions. The study analyzed national forest inventory data for more than 25,000 plots in the U.S. West, excluding coastal states, and found that trees were not regenerating in the hottest portions of their ranges – an expected outcome.
More surprising to the researchers was that most of the 15 common tree species studied were not gaining any ground in areas where conditions were more favorable, indicating that most tree species likely will not be able to move to more accommodating climates without assistance.
“Trees provide a lot of value to humans in terms of clean water, clean air, wildlife habitat and recreation,” said lead author Katie Nigro, who conducted the study as a CSU graduate student. "If forest managers want to keep certain trees on the landscape, our study shows where they can still exist or where they might need help.”
Shrinking ranges were prevalent across undisturbed areas as well as those impacted by wildfire, insects and disease. Using 30 years of disturbance data, the researchers tested the idea that disturbances – particularly wildfire – might catalyze tree movement into cooler, wetter areas by killing adult trees and eliminating competition for seedlings to establish in their preferred climate zone.
"Just like us and every species, trees can only function within a certain climatic tolerance, and different species have different climatic tolerances,” Nigro said. “I thought we would find more shifts into cooler zones, especially in burned areas.”
Results of the study, published in Nature Climate Change, give a broad overview of the predominant pattern – an overall failure to regenerate in the hottest, driest portions of a tree’s range, but also failure to expand along the range’s cooler, wetter border. Nigro cautioned that it’s possible not enough time has passed to see new tree establishment in cooler, wetter areas, especially for slow-growing subalpine species. She added that more, local studies are needed to determine which species will survive where.
The paper makes the case for human-assisted tree migration because rapid warming from climate change is likely to outpace regeneration.
"One of the potential issues is that we may get bigger and bigger mismatches between where trees are living and their ideal climate,” Nigro said.
Trees seeking cooler temps face uphill battle
Increasing wildfire, insect and disease disturbances due to climate change also can prevent regeneration by removing seed sources, and seeds literally have an uphill battle in trying to gain ground upslope, where conditions are cooler.
“There's a lot of things that prevent a seed from moving uphill, including gravity,” said co-author Monique Rocca, an associate professor of ecosystem science and sustainability. “A lot of conditions need to be in place for a tree to be able to move to cooler, wetter sites.”
She continued, “This study digs into some of the details of where trees are staying on the landscape on their own versus where we may need to intervene if our goal is to keep Western landscapes covered in trees.”
A few species fared better than others. Of the four species that continued to regenerate in the areas they already occupied regardless of climate change, wildfire, and insect and disease outbreaks, three of them are rarer on the landscape, so it is harder to accurately gauge their response, and one, Gambel oak, is a resilient, heat- and drought-tolerant resprouting species.
The study used long-term field data from the USDA Forest Service’s Forest Inventory and Analysis program, sometimes referred to as the national “tree census.” Study plots on forested areas across the nation are surveyed continuously to track individual tree growth or losses through harvest, disease or death. Kristen Pelz, analysis team lead with the inventory and analysis program, co-authored the study.
“Dr. Nigro harnessed the power of our field-collected data to show how forests are changing across the interior West – not theoretically, but today,” Pelz said. “Her work is important because it considers how things like fire and native insects interact with climate, which is essential where natural disturbances have been a primary driver of forest dynamics for millennia.”
Rather than looking at changes in the average tree range, as past studies have done, this study went a step further and examined the cold and warm margins of species’ ranges – the leading and trailing edges – which specifies how tree ranges are shifting in more detail and provides actionable insights for forest managers. If trees were expanding into cooler areas on their own, assisted migration wouldn’t be as important.
“This research can help land managers and foresters decide whether to hang on to trees in the hottest portions of their ranges for as long as possible or to transition to a more heat- and drought-tolerant system,” Nigro said, adding that sometimes assisted migration can be done with seeds of the same species that are adapted to a warmer environment.
In her current research as an Oak Ridge Institute for Science and Education postdoctoral fellow with the Rocky Mountain Research Station, Nigro is trying to identify which seeds from a single species might have the best odds of surviving under harsher climate conditions. Co-author Miranda Redmond, who was Nigro’s Ph.D. adviser at CSU, is also following up on this research by studying tree species adaptations at UC Berkeley.
“These efforts are becoming increasingly critical due to the rapid pace and scale of tree die-offs from wildfires, drought and other climate-driven disturbances, coupled with tree regeneration failures observed in many areas,” Redmond said.
Nigro added, “Planting likely will be required to keep trees on the landscape where they are most valued, and we may need to accept new ecosystems in areas that are inevitably going to change. Our future forests might look different and contain different trees than they do today."
Arapaho and Roosevelt National Forests show patchy tree regeneration in this photo from 2022, 10 years after the High Park Fire. A new study found that tree ranges are shrinking in response to climate change. Photo by Katie Nigro
Credit
Katie Nigro
Journal
Nature Climate Change
Article Title
Trailing edge contractions common in interior western US trees under varying disturbances
Nominations open at end of February for new Champion Trees across U.S.
The National Champion Tree Program to take nominations for next register through August
University of Tennessee Institute of Agriculture
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The National Champion Tree Program will begin taking nominations for new Champion Trees on February 28. Crowned champions, like Frémont’s Cottonwood National Champion in Arizona (pictured), will appear in the 2025-2026 register.
view moreCredit: Photo credit: Brian Kelley, Gathering Growth Foundation, with permission from American Forests.
The National Champion Tree Program (NCTP) will take nominations for new Champion Trees on its website starting February 28. The list of eligible tree species for the 2025-2026 register includes more than 1,200 species of trees native and naturalized to the U.S., a steep increase from the 900 species eligible for the 2024 register. It is available online in the Register of Champion Trees. Nominations for potential Champions will stay open through August 2025.
“Each year, people find ‘new’ Champions all over the country,” Jaq Payne, NCTP director, said. “It could be the tree in your backyard, the tree in front of your church, or the tree in one of your local parks or state forests.” For the first time in the program’s 84-year history, an additional list of “culturally important non-native” eligible species will be included to represent common, widely recognized urban species previously not found on the register.
Champion Trees are identified based on a point system including the trunk circumference, height and average crown spread. After members of the public nominate trees, the NCTP will work with state coordinators to verify the submissions and their measurements. Verified trees will be added to the program’s data management system. National Champion Trees are crowned once every two years and must be re-verified every 10 years.
The program put out its first register since 2021 on January 18, and people can access it on the program’s website. There you can find Champion Trees for different species, see the trees’ measurements and read the cultural importance of the trees, if known. The program is still collecting the trees’ histories and would appreciate any help from community members.
The register started as a short list of 77 big trees in the April 1941 edition of American Forests magazine. By 2021, it had grown to 562 Champion Trees across the country. The NCTP moved from American Forests to the University of Tennessee School of Natural Resources in 2023. American Forests is providing $200,000 through April 2025 to support the program’s move to UT.
The National Champion Tree Program’s mission is to protect, preserve and keep record of the largest trees in the United States through public education and engagement.
The UT School of Natural Resources focuses on a mastery learning approach, emphasizing practical, hands-on experiences. The school’s faculty, staff and students advance the science and sustainable management of our natural resources through various programs of the UT Institute of Agriculture (UTIA).
UTIA is comprised of the Herbert College of Agriculture, UT College of Veterinary Medicine, UT AgResearch and UT Extension. Through its land-grant mission of research, teaching and extension, the University of Tennessee Institute of Agriculture touches lives and provides Real. Life. Solutions. to Tennesseans and beyond. utia.tennessee.edu.
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