Harvard Forest

Harvard Forest

Partitioning whole-ecosystem CO2 fluxes to understand mechanisms of carbon sequestration in a temperate forest

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Eddy flux tower at the Harvard Forest site for measuring net ecosystem exchange of CO2 via the eddy covariance technique. Photo credit: Rick Wehr.

Ecosystem-atmosphere carbon exchange is key both to the ecosystems and to the atmosphere. On the one hand, the atmosphere is the carbon source for the construction of plant biomass, and on the other, ecosystems influence atmospheric carbon dioxide (CO2) levels and thereby climate. Ecosystem models differ widely in their depictions of how forest carbon dynamics will interact with a changing climate, and that interaction is a large source of uncertainty in climate predictions. Accordingly, we have been investigating the mechanisms controlling carbon exchange at the Harvard Forest by integrating stable carbon isotope analyses with a suite of measurement approaches including eddy covariance, soil chambers, plot trenching, and minirhizotrons. The data are being integrated inu2014and used to refineu2014an ecosystem model called Ecosystem Demography 2 (ED2). This work is in collaboration with Boston University, Harvard University, and the Woods Hole Research Center.

Within the collaboration, our group has focused on measuring ecosystem-scale photosynthesis and respiration by eddy covariance. It is now over 20 years since the first long-term eddy covariance measurements ofu00a0netu00a0ecosystem-atmosphere carbon exchange began, and such measurements have become a matter of routine at overu00a0500 eddy covariance tower sites aroundu00a0the world (the first site was the Harvard Forest). However, it is not enough to know the net exchange. The net exchange is the balance ofu00a0two somewhat independent and somewhatu00a0coupled components: ecosystem-scale photosynthesis and respiration. In order to understand or predict NEE, one must understand these two components. Unfortunately, there has been no means to measure photosynthesis or daytime respiration directly at the ecosystem scale; instead, people have made guesses by u2018partitioningu2019 NEE measurements according to prescribed behaviors for photosynthesis and/or daytime respiration. What is desired instead is a method tou00a0discoveru00a0those behaviors, and so to test whether the prescriptions of the last 20 years have been correct.

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Aerodyne Quantum Cascade Laser Spectrometer for measuring 12CO2, 13CO2, and 18O12C16O in air, working away in the shack at the base of the eddy flux tower. Photo credit: Rick Wehr.

Isotopicu00a0flux partitioning (IFP) is such a method. It works byu00a0identifying the photosynthetic and respiratory componentsu00a0of NEE by theiru00a0distinct stable isotopic signatures. The ratio ofu00a013Cu00a0tou00a012C differs between photosynthesized and respired carbon becauseu00a0thereu00a0is a strong isotopic fractionation by photosynthesis that varies on timescalesu00a0shorter than the mean ageu00a0of the substrate for respiration. We have been using IFP at the Harvard Forest as an important part of our study of carbon exchange. Among other things, we have found that ecosystem respiration is especially sensitive to soil water content during the day, and that photosynthesis becomes more efficient through the summer, contrary to expectations. These results elucidate the controls on forest ecosystem productivity and carbon sequestration, with consequences for predicting the evolution of the coupled biosphere-climate system.

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