UNC-Chapel Hill study uncovers global rules shaping the treeline under climate change

A new study from researchers at the University of North Carolina at Chapel Hill has revealed the key factors that determine where trees can grow at the highest elevations across the globe. By compiling the most comprehensive dataset of alpine treelines to date, more than 2,000 records worldwide, the research team uncovered a dual control system that explains why treelines form where they do, and which tree species dominate them. 

The study shows that cold temperatures act as a universal ceiling that limits where all tree species can survive. Once temperatures dip more than 35% below a species’ thermal comfort zone, it can no longer establish as a “tree” beyond that point. However, water availability acts as a filter, determining which species are able to grow at those upper limits. 

“This study reveals how heat and moisture shape the treeline, offering new insights into alpine ecosystems under climate change,” said Yuyang Xie, postdoctoral researcher in the department of biology at UNC-Chapel Hill and first author of the study. 

To better forecast how treelines will shift as the climate warms, the researchers developed a new tool called the Relative Distance to Optimum (RDO) index. Unlike past models, the RDO index accounts for species-specific differences and measures how far plants are living from their optimal environmental conditions. This breakthrough will allow scientists to more accurately predict treeline shifts worldwide. 

“Identifying the environmental limits of tree species helps accurately predict how alpine ecosystems will respond to warming,” said Xiao Feng, senior author and assistant professor in the department of biology at UNC-Chapel Hill. “These findings are crucial for guiding conservation in mountain regions under climate change.” 

The research not only advances fundamental understanding of treeline formation but also has direct applications for conservation. By predicting where treelines will move in the future, scientists and policymakers can better prepare for changes in mountain biodiversity and protect vulnerable habitats. 

“This study greatly advances our understanding of the complex drivers of treeline formation at the global scale,” Xie added. “It gives us the tools to anticipate how alpine ecosystems will shift in response to climate change.” 

The study is available online in the journal PNAS at: https://doi.org/10.1073/pnas.2504685122