Trees might need our help to survive climate change, CSU study finds

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."