A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Tower- and Biometry-based Measurements of Tropical Forest Carbon BalanceScott D. Miller, Michael L. Goulden, Mary C. MentonDepartment of Earth System Science, University of California IrvineHumberto R. da Rocha, Helber C. Freitas, Adelaine Michela Silva e Figueira, CleilimAlbert Dias de SousaDepartment of Atmospheric Sciences, University of Sao PauloCorresponding author:Scott D. MillerDepartment of Earth System ScienceUniversity of CaliforniaIrvine, CA 92697-3100sdmiller@uci.eduVoice 949 824-2314Fax 949 824-3256We used two independent approaches, micrometeorology and biometry, todetermine the Net Ecosystem Production (NEP) of a primary forest in Para, Brazil. Eddycovariance measurements of CO 2 exchange were made during 93% of the time from July2000 to July 2001 using both open and closed-path gas analyzers. The annual sum (NEP)was calculated using 12 different combinations of hardware and software to gauge thesensitivity to methodology. The annual sum calculated using the closed-path gas analyzerand 30-minute averaging was –4.0 T C ha -1 yr -1 , consistent with tower measurements inother Amazonian forests. We directly show that this result is not significantly affected byhardware configurations and flux-calculation methods. This tower-based result issignificantly different from three inventories of forest biomass at the site over a 16-yearperiod that indicate the forest has not been accumulating a large amount of carbon(0 ± 1.5 T C ha -1 yr -1 ). The tower-based measurements indicated nocturnal respirationwas underestimated during calm conditions. The annual sum changed by ~4 T C ha -1 yr -1 ,and became consistent with the biometric measurements, when the NEE during periodswith u * < 0.2 ms -1 were replaced with observations from more turbulent periods. Weconclude that this u * filter provides the best estimate of annual CO 2 exchange, resulting ina NEP of +0.1 T C ha -1 yr -1 , such that the forest neither gained nor lost large amounts ofcarbon during the study interval. The high Gross Primary Production and calm nightscharacteristic of tropical forest amplify the uncertainty in tower-based annual sumscompared to temperate and boreal forest sites, and hence we attach a confidence intervalto the tower-based NEP of –5.2 and +1.2 T C ha -1 yr -1 .Key words: biosphere-atmosphere exchange, tropical forest, CO 2 exchange, NetEcosystem Production, eddy covariance, LBA