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A VCU-led experiment will improve forecasts of how the forest carbon cycle responds to disturbances — a key question for climate change

Millions of trees die each year from invasive pests and pathogens, threatening the ability of forests to serve as a natural solution to climate change.
christopher gough holds up the stem girdle of a tree

Right: Christopher Gough, Ph.D., associate professor in the Department of Biology, led the experiment that involved 3,600 trees across 50 acres in Michigan.

A project led by researchers at Virginia Commonwealth University experimentally simulated an insect outbreak across nearly 50 acres of forest with a goal of understanding how different levels of disturbance — such as from invasive insects — affect the capacity of forests to sequester carbon.

The project, called the Forest Resilience Threshold Experiment, or FoRTE, aims to answer important questions about forests’ ability to maintain carbon uptake and storage amid increasing threats from pests and pathogens.

Forest disturbance is on the rise in North America, with invasive insects killing millions of trees each year. The ecological consequences include a loss in forests’ ability to scrub carbon dioxide from the atmosphere.

“When a tree dies, its leaves no longer pull carbon dioxide out of the atmosphere, eliminating the manufacture of raw carbon-containing materials needed to build new wood, roots, flowers and fruit,” said Christopher Gough, Ph.D., an associate professor in the Department of Biology in the College of Humanities and Sciences. “As a result, when many trees die — or disturbance severity is greater — the capacity of forests to sequester carbon may be compromised more than when only a few trees are killed by insect pests, pathogens and extreme weather.”

kayla mathes removes tree bark and sugar-transporting tissues immediately under it, using a process called stem girdling
Kayla Mathes, a doctoral candidate in the integrative life sciences program of VCU Life Sciences, removes tree bark and sugar-transporting tissues immediately under it, using a process called stem girdling. (Courtesy photo)

And when trees die and decompose, they release carbon dioxide back into the atmosphere.

“So, tree mortality has the double effect of reducing carbon uptake and increasing carbon emissions by fueling decomposition,” Gough said. “If carbon dioxide uptake by forests declines too much, or emissions increase too drastically, forests can become temporary emitters of carbon dioxide to the atmosphere.”

Yet forest structure and composition can affect how forests respond to different levels and kinds of disturbance. The amount of carbon sequestration changes in response to disturbance can vary depending on whether the trees are arranged evenly or variably, how many trees are present, and the species.

Gough and the research team conducted their experiment at the University of Michigan Biological Station, a 10,000-acre site where VCU has long partnered with other major research universities to conduct large-scale forest manipulations.

With assistance from professional sawyers, the researchers used chainsaws and pry bars to stem girdle — the removal of bark and the sugar-transporting tissues immediately under it — 3,600 trees, disrupting the flow of carbohydrates from leaves to roots, similar to disturbances caused by wood-boring insects.

They replicated the treatment in four different forest types and applied four levels of severity, ranging from zero to 85% tree mortality. The experiment included large and small trees to emulate disturbances concentrated in the upper and lower canopy.

The field experiment was coupled with computer modeling to evaluate what ecological information is needed to improve predictions of how disturbance affects carbon sequestration and, ultimately, the climate system, Gough said.

laura hickey uses a pry bar to stem girdle a tree
Laura Hickey, a VCU biology master's degree student, uses a pry bar to stem girdle a tree as part of the experiment. (Courtesy photo)

The project’s results are reported in two newly published papers. Additionally, the researchers’ data from the field was made available via an open-access website prior to the studies’ publication.

The first paper, “Structure and Parameter Uncertainty in Centennial Projections of Forest Community Structure and Carbon Cycling,” published in the journal Global Change Biology, is led by Alexey Shiklomanov, Ph.D., a research physical scientist with NASA who was previously with the U.S. Department of Energy’s Pacific Northwest National Laboratory, which is a collaborator on the project.

The paper shows that modeled responses to disturbance are highly sensitive to the kinds of ecological information used to drive simulations. For models to make accurate forecasts of how forest carbon sequestration responds to disturbance, the collection of certain kinds of ecological data should be prioritized by field researchers in support of improving model predictions.

“One of the objectives of this project is to project how our study forest will look in the future,” Shiklomanov said. “The best tool we have for making those kinds of projections is computer models that simulate how trees in a forest grow and compete with each other under different environmental conditions. But before making those predictions into the future, we wanted to see whether our model could reliably predict the forest of the present day starting from the post-harvest conditions of 100 years ago. We found that to do this successfully, we had to tune the model in very precise ways because tiny changes to model parameters resulted in completely different forests.”

students measuring the emissions of carbon dioxide from forest soils
Kayla Mathes, a VCU Ph.D. candidate in integrative life sciences, and Carly Rodriguez, a Western Colorado University undergrad and National Science Foundation Research Experiences for Undergraduates Fellow. Mathes is measuring the emissions of carbon dioxide from forest soils. (Courtesy photo)

The second paper, “Forest Structural Complexity and Biomass Predict First-Year Carbon Cycling Responses to Disturbance,” was published in the journal Ecosystems and was led by Gough, with co-authors including three VCU graduate students, as well as researchers with the University of Michigan and the Pacific Northwest National Laboratory. The paper shows that the amount and arrangement of plant biomass prior to disturbance affects initial changes in carbon uptake and emissions following the girdling experiment.

“These results are useful because they reveal that the ecological features of a forest prior to disturbance determine, in part, how much carbon sequestration is likely to change in response to rising disturbance severity,” Gough said. “Forests that contain a greater quantity of heterogeneously arranged vegetation exhibit greater stability, possibly buffering them from large immediate changes in carbon sequestration. These findings have implications for forest management, too, suggesting that the cultivation of biomass-rich, heterogeneous forest structures may buffer ecosystems against the effects of disturbance.”

Kayla Mathes, a doctoral candidate in the integrative life sciences program of VCU Life Sciences, is a collaborator on the project, focusing on how forest disturbances of different types and severities impact soil carbon cycling.

“Forests play a hugely important role in climate change mitigation through carbon sequestration, but climate change is also causing an increase in frequency and severity of forest disturbances at a global scale,” Mathes said. “This experiment is crucial for us to understand how the forest carbon cycle is responding to different types of disturbances to better forecast how changing forest disturbance regimes might impact the global carbon cycle.”

"Forests play a hugely important role in climate change mitigation through carbon sequestration, but climate change is also causing an increase in frequency and severity of forest disturbances at a global scale."

VCU doctoral candidate Lisa Haber studies how surviving trees respond to ecological disturbance at different severity levels in terms of how their leaves function. Among her roles in the project was to use equipment that measured the photosynthetic rate of leaves, as well as aspects of leaf form and chemistry, to sample the forest before, during and after the stem girdling.

“We need a better understanding of the ecological mechanisms supporting continued forest growth following disturbance so that we can more accurately forecast our climate future, and the size and strength of the forest carbon sink on planet Earth under global change,” she said.

Maxim Grigri, who received his master’s degree from VCU this year, also worked on the project. Grigri was responsible for assessing above-ground wood production following the experiment’s manipulated forest disturbance.

maxim grigri places a mark on a tree with a sharpie
Maxim Grigri, who received his master’s degree from VCU this year, measures a tree's diameter from a permanently affixed metal ruler. This allows the researchers to gauge how fast trees are growing and how much carbon they are investing in wood. (Courtesy photo)

“Trees produce tons of biomass every growing season, sequestering atmospheric [carbon dioxide] and storing it in their roots, stems, branches and leaves. While leaves fall every autumn, carbon in wood remains for potentially hundreds of years acting as a buffer to anthropogenic climate change. Unfortunately, invasive pests and pathogens threaten the ability of the forests to serve as a natural climate solution,” he said. “There are many grand and exciting ideas about how to solve the climate crisis, but I like to focus on the one all around us — trees.”

The project is funded by an $815,000 grant from the National Science Foundation.

In addition to Mathes, Haber and Grigri, the project included VCU collaborators Jeff Atkins, Ph.D., a postdoctoral scholar, and former undergraduate students Laura Hickey, Alexandra Barry, Autym Shafer and Sheryl Bradford.

Collaborators also include co-principal investigator Ben Bond-Lamberty, Ph.D., a research scientist with the Pacific Northwest National Laboratory who is leading the modeling focus of the project; earth scientists Stephanie Pennington and Kalyn R. Dorheim of Pacific Northwest National Laboratory; Jason Tallant of the University of Michigan; Liz Agee, Ph.D., of the Department of Energy’s Oak Ridge National Laboratory; and Robert Fahey, Ph.D., an assistant professor of forest ecology and management at the University of Connecticut.

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