Estudo da advecção horizontal de CO 2 em florestas na Amazônia e sua influência no balanço de carbono

Abstract

Horizontal and vertical CO 2 fluxes and gradients were obtained in an Amazon tropical rain forest, the Tapajós National Forest Reserve (FLONA-Tapajós - 54 o 58‟W, 2 o 51‟S). Two observational campaigns in 2003 and 2004 were conducted to describe subcanopy flows, clarify their relationship to winds above the forest, and estimate how they may transport CO 2 horizontally. It is now recognized that subcanopy transport of respired CO 2 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 CO 2 in such conditions. A subcanopy network of wind and CO 2 sensors was installed. Significant horizontal transport of CO 2 was observed in the lowest 10m of the canopy. Results indicate that CO 2 advection accounted for 73% and 71%, respectively of the carbon budget deficit (difference between total ecosystem respiration and respective eddy flux tower measured) for all calm nights evaluated during dry and wet periods. We found that horizontal advection was significant to the canopy CO 2 budget even for conditions with the above-canopy friction velocity higher than commonly used thresholds (u * correction). On the moderate complex terrain cover by dense tropical Amazon rainforest (Reserva Biológica do Cuieiras – ZF2 - 02◦36′17.1′′S, 60◦12′24.5′′W) subcanopy horizontal and vertical gradients of the air temperature, CO 2 concentration and wind field were measured for dry and wet periods in 2006. We tested the hypothesis that horizontal drainage flow over this study area is significant and it can affect the interpretation of the high carbon uptake reported by previous works. A similar experimental design to the one by Tota et al. (2008) was used with subcanopy network of wind, air temperature and CO 2 sensors above and below the forest canopy. It was observed a persistent and systematic subcanopy nighttime upsloping (positive buoyancy) and daytime downsloping (negative buoyancy) flow pattern on the moderate slope (~12%) area. Above canopy (38 m) on the slope area was also observed a downward motion indicating vertical convergence and correspondent horizontal divergence into the valley area direction. It was observed that the micro-circulations above canopy were driven mainly by the balancing pressure and buoyancy forces and that in subcanopy was driven similar physical mechanisms. The results also indicated that the horizontal and vertical scalar gradients (e.g. CO 2 ) were modulated by these micro-circulations above and below canopy, suggesting that advection estimates using the previous experimental approach is not appropriate due to the tri- dimensional nature of the vertical and horizontal transport locally.