Examination of the seasonal variation and its mechanism of canopy carbon and water fluxes of deciduous forests in tropical region is important for understanding global carbon and water cycling at present and in the future.

We monitored seasonal variation of canopy carbon and water fluxes with eddy covariance method at Mae-Moh teak plantation in northern Thailand, which is deciduous in the dry season. Occasional measurements of maximum rate of electron transport (J_max), nitrogen content, and leaf mass per area (LMA) was also conducted to consider the effect of leaf photosynthetic capacity on canopy carbon flux, in addition to the continuous monitoring of leaf area index (LAI).

Net ecosystem exchange (NEE) and evapotranspiration show clear seasonal variation, but NEE started to decline gradually before the decline of evapotranspiration and LAI. J_max of the upper canopy leaves shows similar value in the beginning and the mid- of the period of the maximum LAI, though NEE was smaller in the latter case. These results suggest that the early stage of the NEE decline was not caused by the decline of LAI nor leaf photosynthetic capacity. Further analysis is planned to examine the effects of atmospheric drought and stomatal control to the canopy carbon and water fluxes.