Stockton Maxwell finds environmental secrets in tree rings

by Chad Osbourne

Radford, VA (06/04/2022) — The value of trees is immeasurable.

They provide oxygen, improve air quality, conserve water, preserve soil, support wildlife and are often thought to positively affect our mental and social well-being. The list of benefits goes on and on. Trees can be a tremendous educational resource, too, teaching us about a region's ecological and environmental history. But to find that information, you have to look inside, at a tree's rings.

It's not easy to get there, but that's what Stockton Maxwell, Ph.D., does.

As a dendrochronologist, Maxwell focuses much of his research on vegetation dynamics and fire disturbance history using tree-ring analysis. He's cored trees for research all across the United States, from Virginia to Maine to California, and exotic locations around the world, helping with efforts in Peru, Brazil and Zambia.

Maxwell is a teacher, too, willing to talk with anyone, of any age or stature, about the science that captured his imagination nearly 20 years ago.

"I can show a tree sample to my son's fourth-grade class and ask them to spot the differences in the rings and what causes the difference between the big rings and small rings," Maxwell explained. "So, I'm teaching small kids a really tangible science to get them excited about the environment.

"It also works for big kids, too," he said with a smile, referring to the students he teaches at Radford University, where he is an associate professor of geospatial science and directs the Tree Ring Laboratory in Reed Hall, just a few steps away from his office.

Most of the students he teaches major in geospatial science or a related field. But sometimes, students from other majors enroll in a dendrochronology class because, as one political science major claimed, "I just thought it was interesting."

"Yeah, it is interesting," Maxwell said enthusiastically.

Maxwell first fell in love with tree ring science while working on his master's degree in forestry in the mid-2000s. During the process, he attended the North American Dendroecological Field Week workshop in Idaho. A few years later, colleagues asked Maxwell to be an instructor at the workshop. He eagerly agreed and now regularly teaches and organizes the workshop's tree-ring analysis sessions.

Maxwell's summers are usually consumed with teaching and research. In July 2021, he taught a tree-ring analysis workshop at Yellowstone National Park, and in October, he traveled to Zambia to teach and help launch new projects there. In December, Maxwell worked with a small group of conservationists in Brazil to study Brazil nut trees "and what they can teach us about the past climate in the Peruvian Amazon," he explained.

That's not all. Maxwell and a colleague help organize training for researchers in Congo, Zambia, South Africa, Namibia and Ghana.

"We had a couple of dozen professors, graduate students and researchers come in for two weeks of training so we could show them how we do it in the United States," Maxwell explained. "If you look at a map of where all of the tree-ring analysis projects have occurred, the entire continent of Africa is almost empty. We are helping them launch a network where they can build on this science."

Maxwell hopes to soon give his Radford University students similar opportunities and access to this important research across the world.

"It is a great way to teach students of any age and interest to better understand their environment," he said while further explaining, in a brief Q&A, the importance of his research and teachings.

What can be learned from tree rings?

"How long you got?" Maxwell asks jokingly and with a laugh.

"Some of my research focuses on learning about past environments in terms of what has happened in the ecosystem. I have projects right now looking at wildfire history. For example, I just published a paper in 2021 on wildfire history in West Virginia, in the northern part of the state.

I also have a project in the greater Yellowstone ecosystem, and there we have a record of fire history for over 1,500 years. We have really long-living trees there that have been recording for centuries. One of the things that we found out west was that not only were fires really frequent, but they often occurred during the really dry years. And that's not a really novel thing because the fuels get really dry, and that's when the fire can get going.

We've made these long-term connections between fire and climate. If we want to put good fire - prescribed fire - back on the landscape, to help maintain a healthy and resilient ecosystem, we can now, from our research findings, tell resource managers, here's how often you should probably burn the forest and maintain a healthy ecosystem because this is what happened in the past before humans disrupted the fire cycle.

In the photo is a sample from the species Brachystegia boehmii growing in the Miombo Woodlands of Zambia. Maxwell collected the sample as part of a workshop he taught in Zambia in October and November 2021. The thin white lines on the sample represent the boundary of the annual growth ring, Maxwell said.

How can tree rings be used to reconstruct past climates?

The Intergovernmental Panel on Climate Change just released its new reports this year and over the past year, and it basically says the climate is warming, and we have to deal with it now.

We know the climate is warming right now, but we have to put it in the context of something. In the past, maybe in the hundreds or thousands of years ago, what were the temperatures? Well, we didn't have thermometers back then.

Trees are a good substitute or a proxy for not having a thermometer or a rain gauge. We use the trees to help us understand past climates, and the relationship is fairly straightforward. In the eastern U.S., a big tree ring means we had a wet year, and a narrow ring means you had a dry year. Maybe in high mountain environments, a small ring is a really cold year, and wide rings are a really warm year. We can use that pattern of narrow and wide rings to help us interpret the past climate.

Large worldwide reports can say that the Earth is warming, and we know it's true because we have reconstructions of climate, precipitation and temperature for the past 2,000 years or longer that we can find in these trees. That's why tree rings are really good to give us this long-term perspective on climate and environments."

Are we experiencing more wildfires than usual in the Appalachian region?

"Some of my colleagues did a study in the 1990s [in the Brush Mountain area near Blacksburg, Virginia], and it showed that, historically, before humans started building neighborhoods there, fires used to burn on average of five to 10 years - maybe up to 15 or 20 years in some cases. Low-level, low severity fires would burn through there. These same ecosystems extend down into Tennessee and North Carolina, where you have steep slopes and pine trees. Fire was historically common; we know that from the tree-ring data.

We can learn lots from the tree-ring research to help inform resource managers throughout the Appalachian Mountains. We live in a pretty rural and forested area, and it would be naive to think that a big forest fire is not going to happen to us here. We can prepare for it now and be fire-aware."

What can Radford University students learn from conducting research in the Tree Ring Lab?

"I teach them from start to finish how to build a project. We go out and core trees at Selu Conservancy. Once we have the samples, the first step is to prepare the samples, sand them down and make them nice and polished. Then, we scan the samples at a high resolution on a flatbed scanner, so we can zoom in and mark the boundaries between the rings and measure them. I teach students how to date the samples, and at the end of the semester, they can present their work at one of the campus research forums to show and explain their projects.

The Tree Ring Lab space gives us a really great opportunity to work with students. I think everybody should have some exposure to these types of research and scholarship opportunities."

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