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%.