Climate warming can slow how tree roots feed soil microbes

A plant's roots do much of their work out of sight, pulling in water and nutrients and holding the plant in place. That makes it hard to see how plants respond when conditions underground change, and how a warming climate affects them. New Dartmouth-led studies look at the strategies plants use below the surface to adapt and survive.

"Changes in climate profoundly affect trees and other plant life, with very real impacts on almost every ecosystem—including subterranean ecosystems," says Thomas Muratore, first author of a new study published in New Phytologist and a postdoctoral researcher in the lab of Shersingh Joseph Tumber-Dávila, an assistant professor in the Department of Environmental Studies. "Research into belowground plant systems is critical to helping us make informed decisions in fields ranging from conservation to agriculture."

Muratore's study focused on "exudation," which occurs when plants release soluble carbon compounds from the tips of their roots. This process effectively feeds microbes in the surrounding soil, making them more active. Those microbes then break down the soil, which makes more nutrients available for the roots to absorb.

Scientists are beginning to understand the basics of exudation. How climate change might affect it is still an open question, with consequences for both plants and soil.

"The study I led examines how two common tree species respond to higher temperatures and to higher amounts of nitrogen in the soil—both of which can be byproducts of climate change," says Muratore. "The results were surprising."

Muratore and his collaborators thought warming and nitrogen addition would both increase all kinds of plant activity, so exudation would increase across the board. But they were wrong.

The researchers found that the size and shape of the root systems didn't change at all, and that higher temperatures actually drove down exudation. "Increased nitrogen levels increased exudation, regardless of temperature," Muratore says. "But we're unlikely to see nitrogen levels go up everywhere. So exudation will likely decrease in places where temperatures go up but nitrogen levels don't—and that means the dynamic interaction between plants and soil in those places will decline."

Digging wider

The study Muratore led focused on one specific root function. Two other recent papers take a much broader view. Both grew out of Tumber-Dávila's work with sUnderfoot, an international working group based at the German Center for Integrative Biodiversity Research.

Tumber-Dávila, who also is an E.E. Just Faculty Fellow, is co-author of a paper published in Scientific Data describing a new database of more than 10,000 plant species, containing every measurable trait they could think of associated with these subterranean strategies. Using this new global data set, he led a concept paper in New Phytologist that calls on researchers to study those traits together rather than in isolation.

"We wanted to look at all the things plants are doing underground," says Tumber-Dávila. "That can include things like exudation, but it also includes behaviors like cloning—where underground stems become genetic copies of the original plant. Plants engage in multiple underground strategies to help them survive and thrive, and our goal was to examine those strategies collectively in order to better understand what makes plants resilient.

"We made that database publicly available and are calling for the research community to look at these underground traits and strategies holistically," Tumber-Dávila says. "People who study roots need to talk with the people who study clonality, and so on. These things don't exist in a vacuum, and we need to consider the extent to which they may be interconnected and affect one another."

Part of the problem is geographic, Muratore says. Some underground traits are more common in certain parts of the world, and researchers tend to study them where they're common. Most of what scientists know about root systems comes from work in the U.S. and Europe.

"We need to do more to better understand plant behaviors in the Global South, particularly given the biodiversity and density of plant life involved," Muratore says. "We can't have a thorough understanding of plant systems and strategies if we aren't looking at plant systems and strategies globally."

Closer to home, Muratore is also bringing his focus on roots and soil to a Tumber-Dávila Lab study of hiking trails at the Oak Hill Outdoor Center in Hanover. The team's fieldwork asks how to design outdoor recreation spaces that hold up to heavier use and the warmer, wetter winters New England is seeing. One part of the study looks at how tree roots along trail edges respond, and what that reveals about a trail's ecological footprint. Results from the first season already suggest the roots may be responding in unexpected ways, Muratore says.

"One of the particularly novel aspects of our work is that we are approaching trail ecology from the bottom up, focusing on tree roots and soils," he says. "We know relatively little about tree roots relative to aboveground tree components, largely because of the challenges associated with studying them. In the context of trails, we still lack a basic understanding of how roots and soils interact and respond to recreational disturbance."

"Our recent papers underscore how little we know about these fundamental plant functions and strategies," says Tumber-Dávila. "What traits should we be measuring? How do they relate to each other? Why are plants doing these things? These are big questions. But, hopefully, we're also shedding light on how to answer them."