Long-term tall tower carbon dioxide flux monitoring over an area of mixed vegetation

A methodology is described to determine the carbon budget of an extended region with mixed vegetation. The eddy covariance measuring system is operated at 82 m elevation on a tall radio/television transmitter tower above an area covered by agricultural fields and forest patches. The methodology ensuring the high quality of the calculated fluxes is described. Net ecosystem exchange (NEE) of CO₂ is determined as the sum of the eddy flux at 82 m and the rate of change of CO₂ storage below the measurement level. The gap filling technique used to patch the missing measurements is also presented. In contrast to several long-term NEE measurements low turbulent conditions does not seem to bias the annual sums of NEE at the site. Maximum daytime NEE reached -1.4 to -1.5 mg CO₂ m^-2 s^-1 (negative value indicates uptake) during the most active phase of the vegetation, while mean night-time respiration was around 0.1-0.3 mg CO₂ m^-2 s^-1. During the dormant season typical net exchange was around 1 g C m^-2 day^-1, while the vegetation adsorbed around 2-4 g C m^-2 day^-1 in the growing season. The overall dependence of the landscape-wide NEE on the environmental factors is described with non-linear regression functions. It is demonstrated that tall tower NEE measurements may provide repeatable, consistent estimates of the landscape-wide carbon exchange. The results may contribute to a better constraint on the "bottom-up" flux estimates. During the period of 1997-2004 (year 2000 is missing) the region mostly behaved as a weak net CO₂ sink on annual scale. Year-round NEE was in the range of -107 ± 48 and 69 ± 37 g C m^-2 year^-1. Climate data are presented to explain the interannual variability of NEE, gross primary production (GPP) and total ecosystem respiration (R_eco).