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A view of one of the research areas

Long-term experiment aims to ‘train’ trees, measure soil carbon

Interdisciplinary project combines multiple areas of natural resources expertise

Walk through a forest and you’re witnessing a quiet competition.

As trees grow, they compete with other plants for water and light. Sometimes this means one tree dominates the resources, but other times the plants find a balance that benefits each.

This mutually beneficial relationship is the idea behind a new experiment at the University of Georgia, where researchers are testing how one species of hardwood trees can help another grow straighter. Together, the trees may also shed light on another aspect of the experiment, which is measuring the soil’s ability to capture carbon.

It may take decades to see results, but it’s worth the time, said professor Daniel Markewitz of the UGA Warnell School of Forestry and Natural Resources, one of the collaborators on the project. Ideally, the project can help reinforce the importance of replanting hardwood trees as they are harvested across the Southeast. These days, they are often replaced with faster-growing pine trees.

“What we tend to do is cut out some hardwoods when we’re cutting out pines, and then we come back and plant pine. So, over time, we’re cutting out hardwoods,” said Markewitz. But a diversity of hardwoods and softwoods is beneficial for several reasons, including wildlife and soil health. He also noted a potential shortage of white oak looming—it’s used in barrels for aging wines and spirits, and the wood is in high demand.

Markewitz, along with colleagues David Clabo, Rebecca Abney and Dan Johnson of Warnell and Nina Wurzburger of the UGA Odum School of Ecology, have planted three large plots primarily with poplar and white oak trees at the university’s demonstration forest in Athens. Another stand of trees has been planted on state-managed lands in central Georgia.

The long-term project serves several research purposes across the sites. For starters, Markewitz and Clabo aim to measure how well the faster-growing poplars crowd the oaks, forcing them to grow straighter in the constrained space. After several decades, when the poplars are harvested, the oaks will continue to grow unencumbered—ideally, adding girth to their now-straight trunks. At the B.F. Grant property in central Georgia, Clabo is managing a variety of hardwoods and softwoods for a similar purpose. The trees have been selected and paired based on their need for sunlight, growth rates, drought tolerance and canopy structure. 

If researchers can show that the faster-growing trees can “train” the slower growing hardwoods, the method can give southern landowners a potential income stream where they might struggle to market pine wood products. The process could yield hardwoods that are straighter with higher-quality wood. Although it may be decades before results are known—oak trees may grow for 60 years or longer before they are harvested. 

It’s a time-intensive project that requires coordination across generations of faculty members. “Nobody is going to do this on their own,” added Markewitz, “so, this is why it’s important for universities like UGA to do this kind of work.”

It’s also not something a typical landowner could take on, but the project could help shape how small landowners grow trees in the future.

“There’s other benefits to it than just wood quality,” added Clabo, an assistant professor of silviculture outreach. “It creates more stand structure variability and offers a variety of food sources for wildlife, and there’s research that shows diversity of canopy structure in other mixed forest types may make those stands more resilient to windstorms. And on the economics side, if you’re a landowner in an area with both hardwood and pine markets, you can potentially manage for both hardwoods and pine in the same stand.”

He added that depending on how the trees are established, the management of pine-hardwood forests can be a lower cost per acre for landowners with fewer acres.

A second aspect of the research goes underground to analyze the soil. The hardwood trees were planted on land that had supported pine trees. As the tree species changes, says Abney, so will the network of root-based fungi that work in the soil to support the trees’ health. The relationship between roots and fungi in the soil is called mycorrhizae, which can also affect how much carbon is stored in the soil.

As the seedlings grow, researchers will test soil around the roots of the poplars, the oaks and the two species planted together.

“This area was all pine, which is associated with one variety of mycorrhizae,” said Abney, an assistant professor at Warnell who studies soil health. “Where we’ve replanted with hardwoods, hopefully we’ll see a shift over time.”

Some aspects of these fungal changes will be easy for Abney to identify, she said. Often, the type of tree, especially ones that drop their leaves in the fall, help define soil properties. But while the properties of mycorrhizae have been well studied, the effects when a forest changes are less known.

“This is what we’re trying to figure out—we know what an existing forest looks like. How can we change that?” she added. “I’m interested in what the carbon is attached to and other molecules in the soil.”

Even as forest carbon markets continue to evolve, even less is known about carbon captured in soil. Globally, said Abney, it’s estimated there is five to six times as much carbon stored in the soil than on land and in the atmosphere. This is why it’s worth considering methods to measure carbon captured by root systems. The research can also inform forest management techniques that can help landowners capture more carbon in the future.

“For example, take burn intervals—what if you burned every seven years? Or two years? Could you sequester more carbon in the soil by changing your intervals, and could a landowner get paid for burning?” she asked. As carbon markets develop, quantifying these differences will be important. “But also, we have to consider tree productivity—that’s where Dr. Clabo comes in.”

As the trees mature, Clabo will note how they develop. It may take a decade to begin to see changes, while also gauging factors such as drought tolerance, shade, slope of the land and the architecture of the crown. Eventually, he’ll be looking for stem quality characteristics like knots or seams in the trunk.

There has been some research done in mixed-species plantings, but not with commercial trees such as pine, he said. “My major professor in graduate school turned me on to the idea; he did studies in different regions that looked at species with different crown architecture and growth strategies and found some interesting relationships after years of the stands developing,” said Clabo.

It’s playing a long game, but just as Markewitz joined a team shepherding tree stand research begun more than 30 years ago, he sees this research as planting the seeds for his future colleagues.

In 30 or 40 years, changes in technology or markets may open new opportunities for measuring below-ground carbon. If that’s the case, the research team will be ready, with a project that serves landowners both above the ground and below it. 

“Now you’re starting to see experiments like this popping up, but we’re not going to see answers for 10 to 20 years,” said Markewitz. “This mixing of applied research on tree growth and fundamental research on soil carbon is a shining example of the interdisciplinary forest science at UGA.”


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