The Annual Cycles Of CO2 And H2O Exchange Over A Northern Mixed Forest As Observed From A Very Tall Tower

We present the annual patterns of net ecosystem-atmosphere exchange (NEE) of CO₂ and H2O observed from a 447 m tall tower sited within a mixed forest in northern Wisconsin, USA. The methodology for determining NEE from eddy-covariance flux measurements at 30, 122 and 396 m above the ground, and from CO₂ mixing ratio measurements at 11, 30, 76, 122, 244 and 396 m is described. The annual cycle of CO₂ mixing ratio in the atmospheric boundary layer (ABL) is also discussed, and the influences of local NEE and large-scale advection are estimated. During 1997 gross ecosystem productivity (947−18 g C m⁻² yr⁻¹), approximately balanced total ecosystem respiration (963±19 g C m⁻² yr⁻¹), and NEE of CO₂ was close to zero (16±19 g C m⁻² yr⁻¹ emitted into the atmosphere). The error bars represent the standard error of the cumulative daily NEE values. Systematic errors are also assessed. The identified systematic uncertainties in NEE of CO₂ are less than 60 g C m⁻² yr⁻¹. The seasonal pattern of NEE of CO₂ was highly correlated with leaf-out and leaf-fall, and soil thaw and freeze, and was similar to purely deciduous forest sites. The mean daily NEE of CO₂ during the growing season (June through August) was −1.3 g C m⁻² day⁻¹, smaller than has been reported for other deciduous forest sites. NEE of water vapor largely followed the seasonal pattern of NEE of CO₂, with a lag in the spring when water vapor fluxes increased before CO₂ uptake. In general, the Bowen ratios were high during the dormant seasons and low during the growing season. Evapotranspiration normalized by potential evapotranspiration showed the opposite pattern. The seasonal course of the CO₂ mixing ratio in the ABL at the tower led the seasonal pattern of NEE of CO₂ in time: in spring, CO₂ mixing ratios began to decrease prior to the onset of daily net uptake of CO₂ by the forest, and in fall mixing ratios began to increase before the forest became a net source for CO₂ to the atmosphere. Transport as well as local NEE of CO₂ are shown to be important components of the ABL CO₂ budget at all times of the year.