Measurements of CO₂ and energy fluxes over a mixed hardwood forest in the mid-western United States

Results from the first full year of measurements (1998-1999) of above canopy CO₂ and energy fluxes at the AmeriFlux site in the Morgan-Monroe State Forest, IN, USA, are presented. The site is located in an extensive secondary successional broadleaf forest in the maple-beech to oak-hickory transition zone. The minimum fetch is 4 km. Turbulent flux measurements are obtained by an eddy-covariance system at 46 m (1.8 times the canopy height) with a closed-path infrared gas analyzer. Peak vegetation area index (VAI) was determined as 4.7±0.5 and the mean albedo during the vegetative period was 0.15±0.02. The aerodynamic roughness length was estimated as 2.1±1.1 m. It showed little variation with wind direction or season. The seasonal variations of energy partitioning and of net CO₂ exchange are discussed in terms of the phenological development of the forest. To estimate the annual net ecosystem production (NEP) and carbon sequestration, eddy-covariance measurements during periods of poorly developed turbulence at night, and missing data were replaced by a simple parametric model based on measurements of soil temperature and photosynthetically active radiation (PAR). The night-time flux correction reduces the annual sequestration estimate by almost 50%. The corrected estimate of annual NEP for the 1998-1999 season is 2.4 t C ha^-1 per year ±10%. (C) 2000 Elsevier Science B.V.