Climate change has long been expected to prompt tree species to migrate toward more favorable environments. However, after analyzing data from over 25,000 national forest inventory plots in the U.S. West-excluding coastal states-the researchers found that trees were failing to regenerate in the warmest parts of their range, as anticipated. More unexpectedly, most of the 15 tree species examined were also not spreading into cooler regions, suggesting that without human intervention, trees may not be able to relocate quickly enough to match climate shifts.
"Trees provide a lot of value to humans in terms of clean water, clean air, wildlife habitat and recreation," said lead researcher 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."
The study, published in Nature Climate Change, found that tree range contractions were evident across both disturbed and undisturbed areas. Researchers examined 30 years of wildfire, insect, and disease data to determine whether disturbances might facilitate tree migration by clearing competition and allowing seedlings to take root in preferred climate zones. However, their findings showed limited evidence of trees naturally expanding into cooler areas, even in regions impacted by wildfire.
"Just like us and every species, trees can only function within a certain climatic tolerance, and different species have different climatic tolerances," Nigro explained. "I thought we would find more shifts into cooler zones, especially in burned areas."
The results highlight an overarching failure of tree regeneration in the hottest and driest portions of their range, coupled with a lack of expansion into the more favorable conditions found at higher elevations or latitudes. Nigro noted that tree migration could be lagging behind climate change, particularly for slow-growing species in subalpine environments. More localized research is necessary to determine which species may be able to adapt in different regions.
The study underscores the potential necessity of human-assisted tree migration to counteract the pace of climate change.
"One of the potential issues is that we may get bigger and bigger mismatches between where trees are living and their ideal climate," Nigro noted.
Wildfires, insect infestations, and disease outbreaks further hinder natural regeneration by reducing seed sources, making it even harder for trees to colonize cooler regions. Even in cases where trees could theoretically migrate upslope, factors like seed dispersal limitations make such movement difficult.
"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."
Rocca emphasized the study's importance in identifying areas where forests are naturally persisting versus where human intervention may be required to maintain tree populations.
A few tree species demonstrated resilience. Of the four species that continued to regenerate despite climate stressors, three were relatively rare, making their responses harder to assess. The fourth, Gambel oak, is known for its drought and heat tolerance, allowing it to survive under harsher conditions.
The study relied on long-term data from the USDA Forest Service's Forest Inventory and Analysis program, often referred to as the national "tree census." Co-author Kristen Pelz, analysis team lead for the program, emphasized the significance of the research.
"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."
Unlike past studies that focused on average range shifts, this research examined both the warm and cool margins of tree ranges, providing more precise insights for land managers. If trees were naturally expanding into cooler zones, assisted migration would be less of a concern.
"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 explained. Assisted migration, she noted, could involve using seeds from the same species that are better adapted to higher temperatures.
Nigro, now an Oak Ridge Institute for Science and Education postdoctoral fellow at the Rocky Mountain Research Station, is investigating which seeds within a species have the best survival prospects under harsher conditions. Co-author Miranda Redmond, Nigro's Ph.D. adviser at CSU, is also expanding the research by studying tree adaptation strategies 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 concluded that human intervention, including tree planting, may be necessary to preserve forests in their current locations. However, some areas will inevitably transform, potentially leading to new ecosystems with different species.
"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."
Research Report:Trailing edge contractions common in interior western US trees under varying disturbances
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