Turbulent fluxes of carbon, water vapor, and temperature were continuously measured above an upland forest in north central Wisconsin during 1999 and 2000 using the eddy covariance method. Maple (Acer saccharum), basswood (Tilia americana), and green ash (Fraxinus pennsylvanica) species found in this forest also comprise a substantial portion of the landscape in the northern Great Lakes region and area, and it has been hypothesized that forests of this age (60-80 years) are responsible for net uptake of atmospheric CO 2 over North America. Mean CO 2, water vapor, and temperature profile measurements were used to improve flux estimates during periods of low turbulence, and were effective for friction velocities (u *) >0.3 m s -1. Unique observations at this site included nighttime and early morning venting anomalies that seemed to originate from a seemingly homogenous area within the forest. These elevated NEE measurements, some as high as 80 mol m -2 s -1, appeared in valid turbulent flux observations for hours at a time, and provided circumstantial evidence for preferential venting and/or existence of pooled CO 2 in low-lying areas. We observed that the forest was a moderate sink for atmospheric carbon, and cumulative NEE of CO 2 was estimated to be -334 g C m -2 year -1 during 2000. Sensitivity to low-turbulence flux corrections was very small (21 g C m -2 year -1), and discrepancies between annual estimates of NEE and NEP were similar to other sites. A normalized measure of ecosystem respiration, the free energy of activation, was presented and its seasonal variations were analyzed. Gross ecosystem production (GEP) was high (1165 g C m -2 year -1) and ecosystem respiration (ER) was low (817 g C m -2 year -1) in comparison to spatially integrated, landscape-scale observations from WLEF (914 and 1005 g C m -2 year -1, respectively), a 477 m tower located 22 km to the northeast [Glob. Change Biol. 9 (2003) 1278]. Forest transpiration was responsible for most of the water released to the atmosphere. Stomata closed under intense sunlight and high vapor pressure deficits (VPD > 1.5 kPa). Effect of stomotal closure on annual CO 2 uptake was minimal due to adequate soil moisture and moderate VPD during the growing season.
Bibliographical noteFunding Information:
This research was funded in part by the National Institute for Global Environmental Change through the US Department of Energy (DoE). Any opinions, findings, and conclusions or recommendations herein are those of the authors and do not necessarily reflect the view of DoE. The Atmospheric Chemistry Project of the Climate and Global Change Program of the National Oceanic and Atmospheric Administration supported P. Bakwin. The authors wish to thank Art Johnston, Aaron Berger, Dan Baumann, and all those who contributed to the construction and maintenance of this flux tower. We also extend our appreciation to Dana Carrington for data analysis, and to Tom Steele, Gary Kellner, and Karla Ortman at the Kemp Natural Resources Station, University of Wisconsin, who provided technical support and accommodations throughout this project.
- Carbon cycle
- Deciduous forest
- Eddy correlation
- Nocturnal boundary layer
- Stomatal closure