Estimating Forest Productivity Over Regional and Multi-Decadal Time Scales Using Carbon Isotopes in Tree Rings

Terrestrial ecosystems are a key component of the global carbon cycle and a better understanding of their role in carbon cycle dynamics is needed to project the future carbon cycle. Current ecosystem carbon cycle models give highly variable projections for the response of the terrestrial carbon cycle to changing climate. Constraining ecosystem carbon-budget projections is an important issue given the interest in managing forests to increase the strength of the carbon sink to mitigate anthropogenic carbon emissions. Spatially explicit data on forest ecosystem productivity and net carbon exchange between the forest and atmosphere that span multi-decadal to centennial time periods, are needed to constrain model parameters and to evaluate model performance. But, such records are sparse. Current observational and remotely sensed data are limited to the last few decades, and tree-ring width records have shown relatively poor correlation with contemporary net carbon flux measurements. In this research project, the investigators postulate that carbon isotopes in tree-rings represent a proxy of forest ecosystem productivity and net carbon exchange between forests and the atmosphere. This hypothesis is based on carbon isotope theory and carbon isotope measurements from tree rings located in the footprint of an eddy covariance carbon dioxide flux tower. This project will test two related hypotheses. The first is that carbon isotopes in tree rings provide a good estimate of forest productivity and net ecosystem carbon exchange. This will be determined by comparing isotope measurements from trees with measurements from nearby long-running eddy-covariance flux towers. The second hypothesis will investigate whether the temporal variability of the carbon isotope record in the tree ring records representing forest productivity and ecosystem exchange are coherent between two flux towers separated by several hundred kilometers. This will be determined by sampling tree rings at nine sites between two flux towers and analyzing the spatial coherence of the tree-ring isotope record and several other climate and forest productivity variables across the sites. If the hypotheses are supported, a network of ecosystem-specific records of forest ecosystem productivity and net carbon exchange could be established to parameterize the next generation of earth system and carbon cycle models. The science education impact of this project will extend beyond the university by developing learning modules for use in secondary schools that service underrepresented groups using an established NSF GK-12 project at Penn State.

This project will determine if carbon isotopes in tree rings record the exchange of carbon dioxide between forests and the atmosphere. Contemporary forest-atmosphere exchange measurements are only available for approximately twenty years, making it difficult to test models used to project the carbon budget of forests and other ecosystems into the future and to determine their likely contribution to future climate change. This project will demonstrate the potential of using carbon isotopes in tree rings to estimate forest productivity and net carbon storage by forests over time periods of a century or more at local to regional scales. More accurate estimates of regional century-scale forest productivity and net carbon storage will inform climate and carbon management-related policy decisions regarding carbon sequestration efforts to mitigate anthropogenic carbon emissions.