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1. Tower-based eddy covariance and measurements of the vertical CO2 concentration gradient within the canopy were used to quantify the seasonal variations in the net ecosystem CO2 exchange (NEE) of a 28-30 m tall transitional tropical forest (cerradao). The study was conducted near the city of Sinop, Mato Grosso, Brazil (11 degrees 24.75\' S; 55 degrees 19.50\' W), which is located in the ecotone of two major regional ecosystem types of South America (tropical rainforest and savanna). 2. The NEE during the dry season (August-September) was in balance, but during the transition period between the dry and wet seasons (October-November) the cerradao stand became a net source of 50-150 mmol m(-2) day(-1) CO2 to the atmosphere. Measurements during the wet season (February, April) indicate that the forest was a net sink of between -55 and -102 mmol m(- 2) day(-1). 3. The NEE of the transitional tropical forest was more similar to that of tropical rainforest during the wet season, but during the dry season the NEE of the transitional forest was more similar to that reported for tropical savanna. The data suggest that seasonal variations in rainfall have important implications for the seasonal pattern of NEE in cerradao

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