Carbon dioxide fluxes from an Araucaria forest in southern Brazil

Since November 2009, eddy covariance measurements of carbon dioxide and energy fluxes have been taken over an Araucaria moist forest in Parana State, southern Brazil. This is a typical ecosystem of southern Brazil and northeastern Argentina. The tower is located at a 32-ha experimental station, where the prevailing species are Araucaria angustifolia, Ocotea Porosa and Ilex paraguaiensis. Average canopy height is 20 m. A total of 60 days of observations are presented. The carbon dioxide fluxes are analyzed, in terms of their daily cycle and of the variability among days. Special emphasis is given to the determination of the nocturnal fluxes. It is shown that, in a good portion of the nights, there is flux underestimation, associated to low turbulent intensities at the site. This problem is quantified and different methods to tackle it are analysed. They are based on the determination of the flux temporal scale, to maximize capturing low-frequency exchange, and on the analysis of the temporal evolution of the fluxes along a given night, to identify periods with potential horizontal loss of carbon by advection. Finally, the best estimates of nocturnal fluxes are proposed, and the total site exchange is estimated.