Masson pine (Pinus massoniana) is a tree species widely planted in central and south China. In the present pioneer study, we reported about our two years of carbon flux observations over an even-aged pure Masson pine stand. Light intensity could explain nearly half (47%) of the variance in daytime net ecosystem exchange (NEE). Daytime dark respiration was lower than nighttime NEE, suggesting a possible effect of light inhibition on respiration. The mean annual NEE was -557 g C m-2 yr-1, which indicated that this stand is a medium to large carbon sink. This NEE estimates were defensive because we checked the data with thorough quality controls and in consistency with previous independent estimates. An unexpected seasonal pattern of NEE was observed with a clear reduction around the transitional period between summer and autumn (around July). This NEE reduction is probably a consequence of water stress induced stomatal control, and not of a decrease in light intensity. The updated optimal stomatal theory did not provide the best description of stomatal control in relation to photosynthesis. Whether this is a new emergent property of the ecosystem scale needs further investigation.