Micrometeorological estimates of carbon exchange in an old-growth forest

Almost continuous micro- and bio- meteorological measurements of atmosphere-ecosystem carbon dioxide exchange indicate that old-growth forests assimilate carbon at relatively high rates, contrary to expectations. In a 400-600 year old coniferous temperate rain forest in Southern Washington, the annual sequestration of carbon was 1.2-3.6 Mg ha-1 yr-1 between July 1,1998 and July 30, 1999. These values are similar to those of younger AmeriFlux forests at the same latitude, in the 1997-1998 season. There was no sustained period of non-growth throughout the year.

The measurements were made with micrometeorological sensors mounted on and near the support tower of an 83 m tall construction crane (the world's largest canopy crane) at the Wind River Canopy Crane Research Facility, run by the U.S. Forest Service and the University of Washington. This facility, since 1995, has greatly facilitated holistic process-oriented ecosystem research of carbon flux. Eddy-covariance estimates of vertical water vapor and CO2 fluxes were made using sonic anemometers and a fast-response infrared gas analyzer (IRGA) which measured the velocity vector, sonic temperature, and the concentrations of H2O and CO2 at 10 times per second (10 Hz). Other mean advection terms in the Net Ecosystem Exchange conservation equation were also estimated, based on two intensive advection experiments with mean profiles of carbon dioxide and water vapor concentrations measured at several horizontal and vertical locations (described in another abstract).

These measurements are related to diurnal and seasonal variations, air temperature, radiation loads, and other meteorological and soil variables. As expected, temperature and radiation are dominant variables although other variables are also important.