Observations of Subcanopy Flow and the Carbon Budget in Two Amazon Rain Forest: Santarém and Manaus Lba-Eco Sites

Under LBA-ECO NASA project, measurements of horizontal and vertical fluxes and gradients of CO2 and wind, were made in two Amazon tropical rain forest sites, the Tapajós National Forest Reserve (FLONA-Tapajós - 54o58'W, 2o51'S) and Cueiras Reserve (Manaus - 60o12'W, 2o36'S). Two observational campaigns in 2003 and 2004 were conducted in Santarem Site to describe subcanopy flows, clarify their relationship to winds above the forest, and estimate how they may transport CO2 horizontally. Also in Manaus Site were made observations during october 2005 to sptember 2006, with same instrumentations system. It is now recognized that subcanopy transport of respired CO2 is missed by budgets that rely only on single point Eddy Covariance measurements, with the error being most important under nocturnal calm conditions. We tested the hypothesis that horizontal mean transport, not previously measured in tropical forests, may account for the missing CO2 in such conditions. A subcanopy network of wind and CO2 sensors was installed. Significant horizontal transport of CO2 was observed in the lowest 10 m of the canopy. Results indicate that CO2 advection accounted for 73% and 71%, respectively of the carbon budget for all calm nights evaluated during dry and wet periods. We found that horizontal advection is likely important to the canopy CO2 budget even for conditions with the above-canopy friction velocity higher than commonly used thresholds. On the Manaus LBA-SITE very different pattern have been found, with reversal wind flow in the lee side of hill observed up sloping during nighttime and inverted during daytime. The subcanopy flow and CO2 horizontal and vertical gradients over slope in complex terrain it was very different than Santarem Site. The diel cycles of the subcanopy flow and CO2 gradients were associated with a "gully Flow" or channel flow over the micro basin at Manaus Site. This channel flow just above the canopy has important role in the subcanopy flow and CO2 gradient strength.