DU Researcher Explains How Tree Rings Forecast Weather Patterns

Every tree tells a story, and at the heart of each story is a journey of growth unfolding year after year. The narrative of a tree is etched in its rings, with each ring—unique in size, width and color—revealing a chapter, telling scientists about its age, its environment and even the changes in climate over time.  

Diego Pons Ganddini, a research assistant professor in the College of Natural Sciences and Mathematics, studies these stories. He recently received a National Science Foundation grant to explore how tree rings can predict variability in precipitation across the Neotropics—specifically Guatemala, Honduras, Colombia, Peru and Bolivia.  

What tree rings can tell us 

To understand the future, we must look to the past, Ganddini says.  

While it’s widely known that you can date a tree by counting its rings, there’s so much more stored inside a tree’s trunk. Oxygen isotopes in the tree rings provide clues about rainfall.  

In a pilot study, researchers measured the ratio of two oxygen isotopes, oxygen-18 and oxygen 16, in tree ring cellulose. They found that when there was more rain, the levels were lower. When there was less rain, the values were higher.  

“We can go back and assess the precipitation that is stored in the tree rings to understand the magnitude of the precipitation and also the source where the tree is gathering that type of water,” Gandini says.  

By studying these rings, scientists can reconstruct past patterns and better predict future changes. Understanding these patterns isn't just about the trees, it's also about where they grow.

Why the Neotropics? 

The Neotropics is one of the Earth’s eight biogeographic regions, covering most of South and Central America, the Caribbean and the southern part of North America. This area, Ganddini says, plays a significant role in the climate system due to the Intertropical Convergence Zone (ITCZ). 

The ITCZ is a band of clouds that encircle the globe near the equator, where the northeast and southeast trade winds converge. Its location varies seasonally as it follows the sun. The ITCZ’s position and width is important for scientists, as it plays a major role in influencing rainfall patterns. Right now, scientists are unsure how climate change will affect the ITCZ. Models suggest it is narrowing due to greenhouse gases, but they differ on whether its latitudinal position will change. A narrower ITCZ band leads to heavier rainfall and storm development within the band, while limiting precipitation in adjacent regions. 

Forecasting the future  

The climate system in tropical regions hasn’t been well-documented, and studying tree rings can help fill the gaps. Right now, Ganddini and other scientists are modeling the future behavior of the ITCZ. With more historical data, they can refine these predictions and provide policymakers with better insights to address the rapidly evolving climate crisis. 

“The climate system is chaotic by definition, but the more we try to understand and assess the system, the better we can forecast, the better we can predict. That translates into saving lives,” he says, such as being able to better prepare for climate disasters like floods and wildfires. 

For Ganddini, these stories signal the start of a new era of discoveries—one where humanity can course-correct one of its most pressing challenges.