The findings, published in Nature Climate Change, illustrate the complexities of managing reforestation efforts and old-growth forests under climate change, which is fueling more frequent and intense weather extremes.
“The majority of the world’s forests are fairly young,” said Justin Maxwell, a co-author of the study and an associate professor in the IU Bloomington College of Arts and Sciences’ Department of Geography. “That hasn’t been true until the last couple hundred years. With climate change bringing more frequent drought, it’s important to understand how young forests will respond.”
Because of their carbon capture and storage capabilities, trees have been touted as a natural solution to the climate crisis. Each year, global forests absorb a net 7.6 billion metric tonnes of carbon dioxide — about 1.5 times more carbon than the U.S. emits annually. That carbon-trapping ability slows during times of drought, however, as trees photosynthesize less — younger trees especially.
“Our study shows that young canopy trees slow their growth rate about 7% more than older trees,” said Tsun Fung Au, a postdoctoral fellow at the University of Michigan’s Institute for Global Change Biology who led the study while completing his Ph.D. at IU Bloomington. “That may not sound like much, but at a global scale that amounts to a lot of carbon remaining in the atmosphere.”
To conduct the study, researchers analyzed a global dataset of nearly 22,000 trees, grouping individual trees across species by age distribution. Tree rings, the concentric circles marking new annual growth, serve as a reliable record of tree age as well as past climatic conditions. Investigating the historical data captured by these specimens revealed a significant difference between the youngest and oldest trees’ response to past droughts. Under drought conditions, young canopy trees reduced their growth by 28%, while older canopy trees slowed growth by just 21%.
The differences between young and old didn’t stop there, however. While young canopy trees needed about a year to recover from drought conditions, older trees were more likely to require multiple years to return to pre-drought growth levels, a ripple effect with consequences for forests’ long-term carbon storage potential.
Furthermore, the effects of drought on young and old trees differed by biome. For example, young trees in temperate and alpine forests proved more sensitive to drought than old trees, while in deserts and tropical regions, the growth gap between young and old was much narrower.
“These differences in drought response across tree ages and biomes suggest that we need to prioritize the structure, composition and diversity of our forests in order to mitigate climate change,” Au said.
Additional authors on the paper include IU faculty members Scott Robeson, Kim Novick and Richard Phillips, IU graduate student Sacha Siani, Jinbao Li of the University of Hong Kong, Matthew Dannenberg of the University of Iowa, Teng Li of Guangzhou University, Zhenju Chen of Shenyang Agricultural University, and Jonathan Lenoir of the French National Centre for Scientific Research.