Seasonality in CO2 and H2O flux at an eastern Amazonian rain forest

Previous studies of CO2 fluxes in Amazonia have suggested seasonal variation in net ecosystem exchange. We find little evidence of this seasonality at a new site in eastern Amazonia, despite the expectation that this site would be particularly sensitive to seasonal fluctuation of rainfall. The average rate of peak net ecosystem exchange was −19 ± 0.9 (1 S.E.) μmol CO2 m−2 s−1. Canopy conductance, evaporation, and vapour pressure deficit were all increased during the dry season, consistent with an increase in bulk temperature and solar radiation. The lack of a dry season decrease in photosynthesis was thought to be due to the observed increase in leaf area following dry season flushing. This was accompanied by an increase in solar radiation, and we suggest that the effect of “dryness” was merely to preclude optimality of photosynthetic response to this increase in radiation. The gross primary productivity of this site was estimated to be 36 t C ha−1 yr−1. This is similar to that reported for other Amazon forest stands. The year may have been a particularly productive one due to the lack of an El Niño event.