Department of Earth System Science

We used eddy covariance to measure the net exchange of CO2between theatmosphere and an old-growth tropical forest in Para ́, Brazil from 1 July 2000 to 1 July2001. The mean air temperature and daily temperature range varied little year-round; therainy season lasted from late December to around July. Daytime CO2uptake under highirradiance averaged 16–19mmol·m22·s21. Light was the main controller of CO2exchange,accounting for 48% of the half-hour-to-half-hour variance. The rate of canopy photosyn-thesis at a given irradiance was lower in the afternoon than the morning. This photosyntheticinhibition was probably caused by high evaporative demand, high temperature, an intrinsiccircadian rhythm, or a combination of the three. Wood increment increased from Januaryto May, suggesting greater rates of carbon sequestration during the wet season. However,the daily net CO2exchange measured by eddy covariance revealed the opposite trend, withgreater carbon accumulation during the dry season. A reduction in respiration during thedry season was an important cause of this seasonal pattern. The surface litter was desiccatedin the dry season, and the seasonal pattern of respiration appears to be a direct result ofreduced forest floor decomposition during drought. In contrast, canopy photosynthesis wasnot directly reduced by the dry season, probably because deep rooting allows the forest toavoid drought stress