The use of eddy covariance to infer the net carbon dioxide uptake of Brazilian rain forest

1 Eddy covariance measurements of CO2 flux, based on four and six week campaigns in Rondonia, Brazil, have been used in conjunction with a model to scale up data to a whole year, and thus estimate the carbon balance of the tropical forest ecosystem, and the changes in carbon balance expected from small interannual variations in climatological conditions. 2 One possible source of error in this estimation arises from the difficulty in measuring fluxes under stably stratified meteorological conditions, such as occur frequently at night. Flux may be 'lost' because of low velocity advection, caused by nocturnal radiative cooling at sites on raised ground. Such effects may be detected by plotting the net ecosystem flux of CO2, F-eco, as a function of wind speed. If flux is 'lost' then F-eco is expected to decline with wind speed. In the present data set, this did not occur, and F-eco was similar to the nocturnal awe estimated independently from chamber measurements. 3 The model suggests that in 1992/3, the Gross Primary Productivity (GPP) was 203.3 mol C m(-2) y(-1) and ecosystem respiration was 194.8 mol C m(-2) y(-1), giving an ecosystem carbon balance of 8.5 mol C m(-2) y(-1), equivalent to a sink of 1.0 ton C ha(-1) y(-1). However, the sign and magnitude of this figure is very sensitive to temperature, because of the strong influence of temperature on respiration. 4 The model also suggests that the effect of temperature on the net carbon balance is strongly dependent on the partial pressure of CO2.