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 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 of fluxes due to local advection are difficult to measure and the uncertainty of flux estimations is an inherit problem of the horizontal homogeneity requirement of the eddy covariance technique. The Chilan flux tower site, therefore not only suited to be a long term forest carbon flux measurement site but also can contribute significant scientific findings for heat, water, and carbon fluxes measurement of mountainous forest area. Flux measurement indicated that there is no clear seasonal pattern of net ecosystem exchange at the Chilan site comparing with similar forest tree species stand of temperate climate region. Monthly CO2 fluxes range from 200 – 300 g CO2/m2/month and sum up to an annual total CO2 sequestration of 32.5 metric ton/ha/year or 8.9 metric ton C/ha/year. Nighttime mean CO2 fluxes generally varied with soil temperature except for 2007/11 when the forest soil was near its saturation as the monthly rainfall amounted over 1000 mm in two consecutive months. Mean GPP estimated by subtracting the nighttime CO2 fluxes from the daytime CO2 fluxes were related the amount of diffuse PPFD suggesting that fog is the main control factor of CO2 flux. The CLM site is located at a valley slope of 15o and is considered to be a difficult site in terms of the requirement of the application of eddy covariance measurement of fluxes. Further study is still needed to clarify the problems inherent from the advection flow at steep slope forest site.

carbon dioxide flux, eddy covariance, long term ecological research