Natural ecosystems can be used to offset manmade CO2 and other greenhouse gas emissions that we add to the atmosphere, but the accounting needs to be right for carbon offset policies to be effective.
For years, it's been suspected that forest inventory and, more recently, biometeorological methods bias the carbon sequestration equation by potentially miscounting the quantity of ecosystem respiration in the carbon cycle.
A group of researchers from across Europe and the United States, including Lawrence Livermore National Laboratory (LLNL), have recently united to tackle this terrestrial carbon budget question by conducting experiments to reduce the uncertainties of whether forests serve annually as CO2 sinks or sources.
One method used across the world to quantify the terrestrial sinks and sources of carbon is the eddy covariance technique. This observational method uses two key pieces of instrumentation to quantify carbon dioxide fluxes between the atmosphere and biosphere: a sonic anemometer for the wind components and an infrared gas analyzer for carbon dioxide concentrations. The measurements are correlated in time and space to calculate a net CO2 flux, which once accumulated over time (e.g. annually), is used to determine the sink or source strength of an ecosystem.
The data are then used to determine whether a forest or cropland is a net sink of carbon from the atmosphere, and to what extent. This has ramifications for the policy of carbon offsets.
"However, applying the method in natural ecosystems is not as simple as simply installing instrumentation," said Sonia Wharton, the LLNL lead on the project. "The data must be carefully collected and analyzed to determine annual sink or source status."
Also, the assumption must be true that the instruments, which are placed above the forest or cropland, are measuring all sinks and sources of CO2 in the ecosystem, from photosynthesis (a sink) to autotrophic and heterotrophic respiration (a source).
"This is notoriously difficult to do in tall forest canopies, which also represent one of the globe's largest natural sinks of CO2," Wharton said.
To help clear up the uncertainties, three sites - two in Europe and one in the United States - have been heavily instrumented since early summer 2024 with eddy covariance towers in the subcanopy to study a phenomenon called forest canopy decoupling. During canopy decoupling, the air inside the canopy is not well-mixed and often CO2 from respired carbon pools is transported out of the canopy and not measured by the primary eddy covariance system, resulting in a potential bias toward a stronger carbon sink strength. To date, there is no standardized approach to address decoupling.
The new subcanopy towers are located in a U.S. old-growth conifer forest that is estimated to be more than 500 years old and stands approximately 50 meters (165 feet) tall. The site, called Wind River, is a biologically rich temperate rainforest in the western Cascades and carbon sequestration has been measured there since 1998. However, forest inventory records date back to the 1920s. This new research at Wind River is led by researchers from LLNL and Washington State University with collaborations from the University of California, Davis, Pacific Northwest National Laboratory, the University of Utah and the National Ecological Observatory Network (NEON). Wind River is an active research site operated by the U.S. Forest Service and is one of NEON's ecological field sites.
The European sites in the Czech Republic and Italy are led by the Integrated Carbon Observatory System.
Other LLNL researchers on the project are Bobby Arthur, Nipun Gunawardena and Matteo Puccioni. The research is funded by the DOE Wind Energy Technologies Office.
