CO2 Concentration and Flux in a California's Oak/Grass Savanna

The measurement of CO₂ concentration is important in estimating canopy carbon storage term associated with partitioning diurnal net ecosystem carbon exchange (NEE). In this study, the eddy covariance towers were installed over and below canopy in an oak/grass savanna in California. We measured ambient CO₂ concentration profile (at the height of 0.35, 1.8, 6.0, and 23.5 m) with LI-800 GasHound CO₂ Analyzer, coupling with the measurement of CO₂ concentration with LI-7500 Open Path CO₂ /H₂ O Gas Analyzer. Comparing with canopy carbon storage term estimated with the 4-layer profile data, carbon storage term was overestimated 31% if only the 2-point CO₂ concentration was applied. CO₂ concentration showed significant diurnal pattern, and the diurnal variation was changeable in seasons. Annual mean CO2 concentration was 402.4±16.3, 397.8±13.3, 393.4±10.7, and 388.8±9.0 ppm at the height 0.35, 1.8, 6.0, and 23.5 m, respectively. Daily mean and maximum of CO₂ concentration appeared to be exponentially related to the height (R2 = 0.99), while daily minimum of CO₂ was stable around 378 ppm at all height in a course of year. We also examined the influences of temperature, wind speed and direction, and soil moisture on diurnal patterns of CO₂ concentration, canopy storage term, and NEE. The results suggested that canopy storage term was a considerable source of uncertainty in partitioning NEE. Our study provided a basic understanding on the relationships between atmospheric CO₂ concentration and NEE in an individual site and will be useful in modeling regional CO₂ concentration and NEE.