Diel and seasonal patterns of tropical forest CO₂ exchange

We used eddy covariance to measure the net exchange of CO2 between the atmosphere and an old-growth tropical forest in Pará, Brazil from 1 July 2000 to 1 July 2001. The mean air temperature and daily temperature range varied little year-round; the rainy season lasted from late December to around July. Daytime CO2 uptake under high irradiance averaged 16–19 μmol·m−2·s−1. Light was the main controller of CO2 exchange, accounting for 48% of the half-hour-to-half-hour variance. The rate of canopy photosynthesis at a given irradiance was lower in the afternoon than the morning. This photosynthetic inhibition was probably caused by high evaporative demand, high temperature, an intrinsic circadian rhythm, or a combination of the three. Wood increment increased from January to May, suggesting greater rates of carbon sequestration during the wet season. However, the daily net CO2 exchange measured by eddy covariance revealed the opposite trend, with greater carbon accumulation during the dry season. A reduction in respiration during the dry season was an important cause of this seasonal pattern. The surface litter was desiccated in the dry season, and the seasonal pattern of respiration appears to be a direct result of reduced forest floor decomposition during drought. In contrast, canopy photosynthesis was not directly reduced by the dry season, probably because deep rooting allows the forest to avoid drought stress