Our tower-based CO2 measurements examine how regional and ecosystem level processes in a mid-latitude forest contribute to global carbon cycling. Specifically, we endeavor to understand quantitatively how and why forested ecosystems take up or release carbon, on time scales from hours to decades, and to elucidate responses to climate changes and management interventions.
The tower was installed 1989 and the resulting eddy-flux measurements constitute the longest running record of the net-ecosystem carbon exchange in a North American Forest. The resulting long-term record of Net Ecosystem Exchange (NEE) has shown the effects of climate anomalies on carbon fluxes for seasonal and annual time scales. For example, reduced soil frost allows greater respiration in the winter leading to lower C sequestration. Cumulative gross photosynthesis depends on when the canopy emerges in the spring. Warmer springtime temperatures lead to greater uptake of C. These earlier results are reported in the 1996 Science paper, "Exchange of Carbon Dioxide by a Deciduous Forest: Response to Interannual Climate Variability" (abstract). The longer term trend of net uptake (2 tC/ha/yr on average) however is likely driven by succession of red oak to dominance over earlier pioneer species, enhanced by recent trends towards warmer spring temperatures and associated earlier canopy development. In short, the major factors that regulate carbon sequestration appear to be recovery from prior land use, length of the growing season, snow cover, cloudiness, and drought.
In 1993, ground-based ecological measurements were undertaken to compliment the tower-based measurements of CO2 exchange. High resolution dendrometer measurements, soil respiration chambers, vegetation surveys, leaf area index (LAI) measurements, leaf nitrogen analysis, and litter collection have all been implemented in 40 plots in the tower ‘footprint’ which spreads from the northwest to the southwest of the tower. Defining individual rates for these major biological processes will allow assessment of individual contributions from gross photosynthesis, C storage (by wood increment), and autotrophic and heterotrophic respiration to the net carbon flux.
As the NEE record is extended and augmented by supporting ecological measurements, we can further identify longer-term effects of climate perturbations on carbon fluxes and further define the relationship between stand history and carbon sequestration. Climatic anomalies in one season or year may have a longer term effect on the sequestration of carbon in subsequent seasons or years.
The flux and ecological measurements are coordinated with studies at other sites through the AmeriFlux network. By examining the relationships between carbon fluxes and the driving physical and biological variables across a range of sites we are enhancing understanding of the processes that control NEE. Data from both the tower-based and the ecological measurements are available through our data exchange.
For more information about the eddy flux tower data, contact Shawn Urbanski or Bill Munger. For more information about companion ecological measurements, contact David Bryant, Elizabeth Hammond Pyle or Lucy Hutyra.