In order to understand the relationship between environmental change and terrestrial ecosystems, the Environmental Protection Administration is promoting a network of carbon dioxide flux monitoring station in Taiwan. The ChiLan flux monitoring station (CLM) has operated since May, 2005. Fluxes of carbon dioxide, water vapor, and sensible heat fluxes over the forest canopy were measured by the eddy covariance method with a 3‐D sonic anemometer and an open‐path analyzer mounted on top of a 24‐meter instrument tower over a natural regenerated stand consists mainly of the tree species of Taiwan cypress (Chamecyparis obtuse var. formosna). Due to the rain or fog water often block the window of open path gas analyzer, calculation of fluxes during foggy and rainy period has been difficult at the Chilan site. A system which combines open path and close path gas analyzers has been established since August, 2007. Synchronized analog signals from the both gas analyzers and the 3‐D sonic anemometer were recorded with a sampling rate of 10 Hz by a data logger. Raw data as well as 30‐minute statistics were then retrieved by a desktop computer and stored for later analysis. Located on a valley slope of 15°, the Chilan site is considered to be a difficult steep slope site for the eddy covariance technique. Considerable amount fluxes released by the soil respiration under the height of flux measurement instrument might be drained away by drainage/advection flows and escape from the eddy flux measurement. This uncertainty of flux estimations is an inherit problem of the horizontal homogeneity requirement of the eddy covariance technique. Nocturnal flux underestimation due to poor development of turbulence in the nighttime is another serious limitation of the eddy flux measurement. In this project, several gap‐filling techniques were used to quantify the annual carbon assimilation at CLM. Based on this two data gap filling and nocturnal flux correction schemes, the annual net ecosystem exchange (NEE) of the Taiwan cypress plantation at the Chilan site is estimated in between 2.3 ton C/ha/year to 10.0 ton C/ha/year.

carbon dioxide flux, eddy covariance, long term ecological research