A photo of a ponderosa pine forest in Arizona
Tree ring data combined with forest inventories showed that ponderosa pine forests in Arizona will absorb less carbon as climate warms. Credit: Richard N. Horne/Wikimedia Commons, CC BY-SA 4.0

Forests around the world pull carbon out of the atmosphere and are crucial in the global fight to stem climate change. But figuring out how much carbon forests are storing as the planet heats up is tricky. For instance, many countries don’t have a direct, systematic, and timely method for measuring how factors like drought or intense periods of rainfall might influence a forest’s carbon uptake.

But now a team of international researchers has published a study that puts forth a solution for acquiring these data: gather tree ring cores from live trees in national forests. “Tree rings really have this remarkable annual resolution data,” said Margaret Evans, a dendrochronologist at the University of Arizona’s Laboratory of Tree-Ring Research who coled the study with Justin DeRose, a forest ecologist at Utah State University. “You get this decade or even century scale of the entire life span of the tree and its response to interannual climate variability and conditions.”

The easiest way to collect these data, Evans and DeRose noted in the study, is to include tree ring sampling in existing national forest inventory programs. The inclusion would require minimal additional investment because the cost of revisiting inventory plots is already built into the programs’ budgets. And at least in North America, the foundation for such a network already exists in the form of legacy collections, totaling at least 405,092 cores from across Canada, Mexico, and the United States.

“Forests are always growing. They’re dynamic. Trees grow and die; disturbances are a natural part of the system. Then you lay this increasing temperature trend on top of that, and it really complicates things.”

National forest inventories like the U.S. Forest Service’s Forest Inventory and Analysis (FIA) Program can give a broad idea of how much carbon forests absorb. But, Evans said, because such inventories are conducted only every 5–10 years, they miss many of the nuances that are becoming more important as the planet heats up. “If you only have measurements every 10 years,” she explained, “then you can’t figure out whether a particular heat wave negatively affected the forest ecosystem functioning and its carbon emissions and removals.”

“Forests are always growing. They’re dynamic,” DeRose added. “Trees grow and die; disturbances are a natural part of the system. Then you lay this increasing temperature trend on top of that, and it really complicates things. And it certainly complicates our understanding of what happens in these systems.”

Kristina Anderson-Teixeira, a forest ecologist with the Smithsonian Conservation Biology Institute and the Smithsonian Tropical Research Institute who was not involved in the research, praised the study and spoke highly of the authors’ idea to combine the two data sources (tree rings and FIA) to better understand forest climate dynamics. “There is so much potential to do more work combining tree ring research with forest ecology,” she said. “This would be a really, really good thing to do.”

Anderson-Teixeira pointed out that the framework would not work for tropical forests, however. Unlike the trees in temperate forests, which have annual periods of growth and dormancy (reflected in the alternating light and dark bands in their core), tropical trees grow year-round and do not make such rings. “So [combining dendrochronology with forest ecology is] a partial solution—it’s not going to completely answer the question” about carbon uptake in forests, Anderson-Teixeira said. “But getting this information for one continent is huge. It’s important.”

From Squishy to Solid Data

DeRose and Evans said that because tree ring data are a direct method for measuring a tree’s carbon intake, having this information would greatly improve the ability of countries to report greenhouse gas emissions and removals as required by the Paris Agreement and other international treaties. Right now, they said, scientists have to infer how much carbon sequestration is happening in forests by looking at things like fossil fuel combustion, measurements of carbon in the atmosphere, and measurements of ocean acidity levels. “It’s squishy,” said Evans.

Tree core data would also sharpen models that scientists use to try to predict forest ecosystem behavior under climate warming because researchers could compare their predictions to the annual observations in the tree rings and make adjustments. “You can look at it and say, ‘Okay, where did my model go wrong? What hints does it give in terms of how my model could be improved?’” said Evans. “That’s how science marches forward.”

A recent study that Evans coauthored with Kelly Heilman, a postdoctoral research associate at the University of Arizona’s Laboratory of Tree-Ring Research, shows how integrating the two data sources can help with forest management. In the paper, they and their colleagues combined tree ring data with FIA data on Arizona’s ponderosa pines and were able to infer the size of the trees each year and see how they responded to climate variables such as rainfall and temperature.

Their study predicted a 56%–91% decline in individual tree growth under future climate scenarios. It also showed that denser ponderosa forests fare worse when its hotter and drier—which has implications for forester managers, who could mitigate some climate stress on the forests by thinning them. “If you have both an overly dense forest and climate warming happening at the same time, that’s a double whammy. But if you thin the forests, you can remove one source of stress,” said Evans.

—Nancy Averett (@nancyaverett), Science Writer

Citation: Averett, N. (2022), Tree carbon data that ring true, Eos, 103, https://doi.org/10.1029/2022EO220138. Published on 15 March 2022.
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