The annual cycles of CO₂ and H₂O exchange over a northern mixed forest as observed from a very tall tower

Abstract

Abstract We present the annual patterns of net ecosystem‐atmosphere exchange (NEE) of CO 2 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 CO 2 mixing ratio measurements at 11, 30, 76, 122, 244 and 396 m is described. The annual cycle of CO 2 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 CO 2 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 CO 2 are less than 60 g C m −2 yr −1 . The seasonal pattern of NEE of CO 2 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 CO 2 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 CO 2 , with a lag in the spring when water vapor fluxes increased before CO 2 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 CO 2 mixing ratio in the ABL at the tower led the seasonal pattern of NEE of CO 2 in time: in spring, CO 2 mixing ratios began to decrease prior to the onset of daily net uptake of CO 2 by the forest, and in fall mixing ratios began to increase before the forest became a net source for CO 2 to the atmosphere. Transport as well as local NEE of CO 2 are shown to be important components of the ABL CO 2 budget at all times of the year.