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 cm(-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 +/- 19g 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.