Trees are losing the battle against climate change

Forests have long been resilient, adapting to shifting climates over centuries. However, the pace of modern climate change presents an unprecedented challenge. Trees that have thrived in certain regions for generations now face rising temperatures, prolonged droughts, and increasing disturbances from wildfires and pests.

Scientists have expected tree species to migrate naturally, shifting toward cooler and wetter areas to survive. Yet, a recent study from Colorado State University reveals a different reality – tree populations in the interior U.S. West are shrinking, but they are not expanding into new, more favorable territories.

Climate change is killing trees

The study examined data from over 25,000 forest plots across the interior U.S. West.

The researchers found that trees are struggling to regenerate in the hottest parts of their ranges, which aligns with expectations. However, the more alarming discovery is that they are not establishing themselves in cooler, wetter locations either.

Study lead author Katie Nigro emphasized the significance of this pattern, stating that trees play a crucial role in providing clean water, clean air, and essential habitat for wildlife. She noted that without intervention, some species may struggle to maintain their presence in the landscape.

“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,” said Nigro.

Climate change outpaces tree adaptation

For decades, ecologists have assumed that as warming temperatures push trees out of certain areas, they will gradually expand into more suitable habitats. However, the study challenges this assumption by demonstrating that most species are not shifting as expected.

Shrinking ranges were observed in both undisturbed forests and those affected by wildfires, insect outbreaks, and disease. Researchers tested whether disturbances such as wildfires could clear the way for trees to establish themselves in cooler regions by removing competition.

The expectation was that, with fewer adult trees blocking sunlight and taking up resources, new seedlings could take root in more favorable microclimates. But the study found little evidence of such shifts occurring.

“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.”

Forests are shrinking faster than expected

The study, published in Nature Climate Change, provides a large-scale analysis of how trees are responding to changing conditions. The results highlight an overarching pattern – trees are disappearing from the hottest, driest parts of their range, yet failing to gain ground along the cooler, wetter margins.

Nigro pointed out that some tree species, particularly slow-growing subalpine varieties, may need more time to establish themselves in new areas.

However, she also warned that the rapid pace of climate change may be outstripping the ability of trees to adapt naturally. More localized studies will be necessary to determine which species can survive where and whether interventions will be needed.

“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.

Physical barriers prevent tree migration

Even when trees attempt to shift their ranges, they face numerous obstacles. Climate change is not only altering temperatures but also increasing wildfire frequency and intensity.

These disturbances can prevent regeneration by eliminating seed sources. Additionally, seeds trying to move upslope to cooler temperatures must overcome significant physical barriers.

“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.”

The study highlights how trees remain restricted to their current locations due to environmental and physical constraints.

Rocca noted that understanding where trees are naturally persisting versus where they need human intervention is key to ensuring future forest sustainability.

“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,” she said.

Tough trees survive climate shifts

While most tree species struggle to expand their ranges, a few have managed to regenerate successfully.

The researchers identified four species that continued to regenerate despite climate challenges, wildfires, and disease outbreaks. However, three of these species were relatively rare, making it difficult to assess their long-term adaptability.

One species, Gambel oak, proved particularly resilient due to its ability to resprout even after disturbances. Unlike other species that rely on seed dispersal for regeneration, Gambel oak can regrow from its root system, allowing it to survive in harsh environments.

Role of data in tracking forest change

The study used long-term data from the USDA Forest Service’s Forest Inventory and Analysis program, often referred to as the national “tree census.” This program continuously tracks tree growth and losses across the nation, providing invaluable insights into forest dynamics.

Study co-author Kristen Pelz highlighted the importance of the study’s findings.

“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.

By analyzing the leading and trailing edges of tree ranges, rather than just the overall average, the study offers a more detailed picture of how tree species are shifting.

This information is particularly useful for land managers trying to make informed decisions about future forest conservation strategies.

Assisted migration as a possible solution

Since trees are not expanding into better-suited environments on their own, the study suggests that human intervention may be necessary.

Assisted migration involves helping tree species move to more favorable climates by planting them in areas where they are more likely to thrive.

“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.

She noted that in some cases, assisted migration might involve using seeds from the same species but sourced from populations already adapted to warmer conditions.

A changing forest landscape

Nigro continues her work as a postdoctoral fellow with the Rocky Mountain Research Station, where she is studying which seeds from a single species have the highest survival potential in harsher climates.

Meanwhile, her Ph.D. adviser, co-author Miranda Redmond, is conducting related research at UC Berkeley, focusing on how tree species are adapting to climate change.

“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.

As climate change reshapes the natural world, forests of the future may look quite different from those of today. Some tree species may disappear from certain landscapes, while others may need human intervention to persist.

“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,” said Nigro.

The study makes it clear that trees alone may not be able to keep up with the rapid shifts in climate. Without intervention, some species could be lost, altering forest ecosystems in ways that could have lasting effects on wildlife, water resources, and air quality.

The challenge now is determining the best strategies for ensuring forests remain a vital part of the Western landscape for generations to come.

The study is published in the journal Nature Climate Change.

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