Data from this project can be downloaded here.

As interest in rising atmospheric CO₂ has grown over the last decade, there has been a surge in research programs directed at quantifying the carbon dioxide exchanged between terrestrial ecosystems and the atmosphere. An international network (FLUXNET) is made up of more than two hundred flux tower sites in North and South America (AmeriFlux and Fluxnet Canada), Europe (CarboEurope), and elsewhere. These sites represent most of the biomes of the world’s vegetation. Tower-based eddy covariance measurements are used to assess the biosphere-atmosphere exchange of CO₂, H₂O, and energy at these locations.

Eddy covariance is an amazingly powerful technique which provides net CO₂ flux measurements (often called net ecosystem exchange or NEE), which are the combination of simultaneous production of CO₂ by respiration and uptake of CO₂ by photosynthesis (NEE = respiration - photosynthesis). Measurement of the stable isotope variants of CO₂ (¹²C¹⁶O¹⁶O, ¹³C¹⁶O¹⁶O, and ¹²C¹⁸O¹⁶O) allows additional examination of carbon cycle processes.

With funding from the Department of Energy's Terrestrial Carbon Processes program (through the joint NASA/DOE/USDA Carbon Cycle Science program), we are examining the dynamics of ecosystem atmosphere exchange of stable isotopes of CO₂ at a coniferous subalpine forest in Colorado (Niwot Ridge AmeriFlux Site). The location is part of the Niwot Ridge Long-Term Ecological Research site.  We are using state of the art instrumentation (tunable diode laser absorption spectrometry) that allows determination of carbon or oxygen isotope ratios of atmospheric CO₂ at several heights in the forest every few minutes.  This data set is allowing us to examine 1) the influence of environmental variables on the isotopic content of respiration, 2) photosynthetic discrimination against ¹³C by an entire forest, and 3) the isotopic disequilibrium between the photosynthetic and respiration fluxes.  These three topics are critical to understanding the carbon cycle at spatial scales from leaf to globe, and provide substantial insight into ecosystem carbon cycle processes.

The links below provide additional information about the project.

• measurements at three spatial scales (forest, tundra, aircraft)

• diurnal patterns of CO₂ and isotopes in forest air

• weekly patterns of CO₂ and isotopes

• seasonal patterns of CO₂ and isotopes

• seasonal patterns of CO₂ and isotopes in the forest at multiple heights

• continuous CO₂ data from the tundra site (collected by Britt Stephens)

• photos of the tunable diode laser system

• photos of the T-van tundra site

• photos of the equipment used to sample at the Carr aircraft site

• photos of the Niwot Ridge AmeriFlux tower (compiled by Sean Burns)

• public data availability from our project

• links to other research at Niwot Ridge

• Niwot Ridge tundra cam

Publications related to this project:

Zobitz JM, Keener JP, Schnyder H, Bowling DR (2006) Sensitivity analysis and quantification of uncertainty for isotopic mixing relationships in carbon cycle research, Agricultural and Forest Meteorology, 136:56-75. (PDF)

Bowling DR, Burns SP, Conway T, Monson R, White JWC (2005) Extensive observations of CO₂ carbon isotope content in and above a high-elevation subalpine forest, Global Biogeochemical Cycles, 19, GB3023, doi:10.1029/2004GB002394. (PDF)

Bowling DR, Sargent SD, Tanner BD, and Ehleringer JR (2003) Tunable diode laser absorption spectroscopy for stable isotope studies of ecosystem-atmosphere CO₂ exchange, Agricultural and Forest Meteorology, 118, 1-19 (PDF)