Comparative measurements of carbon dioxide fluxes from two nearby towers in a central Amazonian rainforest: The Manaus LBA site

Abstract

[1] Forests around Manaus have staged the oldest and the longest forest-atmosphere CO 2 exchange studies made anywhere in the Amazon. Since July 1999 the exchange of CO 2 , water, and energy, as well as weather variables, have been measured almost continuously over two forests, 11 km apart, in the Cuieiras reserve near Manaus, Brazil. This paper presents the sites and climatology of the region based upon the new data sets. The landscape consists of plateaus dissected by often waterlogged valleys, and the two sites differ in terms of the relative areas of those two landscape components represented in the tower footprints. The radiation and wind climate was similar to both towers. Generally, both the long-wave and short-wave radiation input was less in the wet than in the dry season. The energy balance closure was imperfect (on average 80%) in both towers, with little variation in energy partitioning between the wet and dry seasons; likely a result of anomalously high rainfall in the 1999 dry season. Fluxes of CO 2 also showed little seasonal variation except for a slightly shorter daytime uptake duration and somewhat lower respiratory fluxes in the dry season. The net effect is one of lower daily net ecosystem exchange (NEE) in the dry season. The tower, which has less waterlogged valley areas in its footprint, measured a higher overall CO 2 uptake rate. We found that on first sight, NEE is underestimated during calm nights, as was observed in many other tower sites before. However, a closer inspection of the diurnal variation of CO 2 storage fluxes and NEE suggests that at least part of the nighttime deficits is recovered from either lateral influx of CO 2 from valleys or outgassing of soil storage. Therefore there is a high uncertainty in the magnitude of nocturnal NEE, and consequently preliminary estimates of annual carbon uptake reflecting this range from 1 to 8 T ha À1 y À1 , with an even higher upper range for the less waterlogged area. The high uptake rates are clearly unsustainable and call for further investigations into the integral carbon balance of Amazon landscapes.

Sources or sinks? The responses of tropical forests to current and future climate and atmospheric composition

“…80% of nights (Loescher et al 2003) and 92% of night hours ), and the resulting need to filter out data from the many periods when these conditions make the eddy covariance technique inoperable. As shown by Araú jo et al (2002) and Miller et al (2004) for eastern and central Amazonian forests, the same eddy covariance dataset for a given forest indicates NEE values that differ by more than 4 Mg C ha Ϫ1 yr Ϫ1 , depending on the treatment of the data from periods of still air. Recent studies combining biometric and eddy covariance approaches now point to the forests being a net CO 2 source or, at most, a quite small sink (Anonymous 2003;Miller et al 2004).…”

“…Malhi & Grace 2000;Andreae et al 2002). The published tropical forest eddy covariance analyses so far have been based on 17 days to 10 months of data from four Amazonian rainforest sites (Fan et al 1990;Grace et al 1995;Malhi et al 1998;Andreae et al 2002;Araú jo et al 2002;Carswell et al 2002) and on 3 years of measurements at a tropical wet forest site in Costa Rica (Loescher et al 2003). Using widely accepted practices to estimate net forest CO 2 exchange from the data, all these studies produced estimates of significant net CO 2 uptake by these tropical rainforests, all of which were assumed to be old growth.…”

Global data set of biogenic VOC emissions calculated by the MEGAN model over the last 30 years

“…The two measurement systems were located on the 52 m high walk-up tower (K34). The measurements are representative to 2-3 km 2 area around the tower (Araújo et al, 2002).…”

Influence of drainage status on soil and water chemistry, litter decomposition and soil respiration in central Amazonian forests on sandy soils

“…Annual rainfall, measured at the Ducke rain forest reserve near Manaus (1966Manaus ( -1992, amounts to 2442 mm, with a standard deviation of 306 mm. The rather weak dry season is between June and November (Hodnett et al, 1997;Araújo et al, 2002;Waterloo et al, 2006).…”

“…The Reserve belongs to the Instituto Nacional de Pesquisa da Amazonia (INPA) and is about 70 km north of Manaus along the Manaus -Boa Vista highway. The Reserva do Cuieiras study site has extensively been described by Araújo et al, (2002Araújo et al, ( , 2010 10 soils on slopes and plateaus are Oxisols and Ultisols with a significant clay fraction and cover 57% of the area (Chauvel et al, 1987;Rennó et al, 2008). The valley soils cover the remaining 43% of the area and consist of strongly leached quartz sands (Spodosols) with a low capacity to retain water or nutrients (Chauvel et al, 1987;Brinkmann, 1985;Waterloo et al, 2006).…”