Temporal patterns of net CO₂exchange for a tropical semideciduous forest of the southern Amazon Basin

Abstract

The carbon cycling of tropical ecosystems has received considerable attention over the last 1–2 decades; however, interactions between climate variation and tropical forest net ecosystem CO2 exchange (NEE) are still uncertain. To reduce this uncertainty, and assess the biophysical controls on NEE, we used the eddy covariance method over a 3 year period (2005–2008) to measure the CO2 flux and energy balance for a 25–28 m tall, mature tropical semideciduous forest located near Sinop Mato Grosso, Brazil. The study period encompassed warm‐dry, cool‐wet, and cool‐dry climate conditions, and based on previous research, we hypothesized that the net CO2 accumulation of the semideciduous forest would be lower during periods of drought. Using time series of the enhanced vegetation index (EVI), a NEE‐light‐use model, and path analysis, we found that the estimated quantum yield (a′, μmol CO2μmol photons−1) was directly affected by temporal variations in the EVI, precipitation, and photosynthetically active radiation (PAR), while the optimal rate of gross primary production (FGPP,opt, μmol m−2 s−1) was directly affected by the EVI and PAR. However, indirect effects of precipitation on the a′ and FGPP,opt were stronger than direct effects because variations in precipitation also lead to variations in the EVI and the atmospheric vapor pressure deficit (VPD). Daytime ecosystem respiration (FRE,day, μmol m−2 s−1) was directly affected by temporal variations in temperature and VPD and indirect effects of other variables were of lesser importance. Net ecosystem CO2 uptake was often higher in the dry season than the wet season, not because of a dry season “green‐up” but because rates of ecosystem respiration declined relatively more than rates of canopy photosynthesis. Over interannual timescales, average daily NEE increased over the 3 year study period and was highest in 2007–2008, which was also the driest year in terms of rainfall. However, 2007–2008 was also the coolest year during the 3 year study period, and the low temperature appeared to compensate for the low rainfall. Overall, our data suggest that the NEE of tropical semideciduous forests is sensitive to temporal variations in surface water availability but that indirect effects of other variables, such as temperature and VPD, are important in controlling CO2 gain and loss. Such interactions will be important for the future NEE under warmer and drier conditions that are anticipated with anthropogenic climate change.

The effect of atmospheric aerosol particles and clouds on net ecosystem exchange in the Amazon

“…Figure 4 shows the diurnal cycle of NEE during the wet and dry season at both sites. The diurnal cycle of NEE is typical for tropical forests, with the magnitudes and peak hours of carbon absorption consistent with previous observations in other areas of the Brazilian Amazon forest (de Araújo et al, 2010;Hutyra et al, 2008;Vourlitis et al, 2011). Net ecosystem exchange is negative during daytime when photosynthesis is larger than respiration.…”

Modelling Gross Primary Production of Tropical Forest by Remote Sensing

“…Air samples were drawn at 1, 4, 12, 20, and 28 m above ground level using a diaphragm pump and solenoid switching system, and the vertical CO2 concentration profile was measured using a closed-path CO2 analyzer (LI-820, LI-COR, Inc., Lincoln, NE, USA). The gradient measurement system was operational for 30% of all observations, and during system failure canopy CO2 storage was quantified from the CO2 concentration measurements made at the top of the tower, which did not differ from those derived from the gradient measurements (VOURLITIS et al, 2011).…”

“…Net ecosystem exchange (NEE) and energy balance were measured using the eddy covariance method (VOURLITIS et al, 2011). Eddy covariance sensors were mounted on a walk-up tower at a height of 42 m above ground level.…”

“…Canopy CO2 storage was determined by quantifying the rate of change of the CO2 concentration of the air column between the ground surface and the eddy covariance sensors (VOURLITIS et al, 2011). Air samples were drawn at 1, 4, 12, 20, and 28 m above ground level using a diaphragm pump and solenoid switching system, and the vertical CO2 concentration profile was measured using a closed-path CO2 analyzer (LI-820, LI-COR, Inc., Lincoln, NE, USA).…”

“…Tropical rainforests sequester more carbon than any other biome or land cover class (AHLSTRÖM et al, 2015). However, rainforests have also been changed due to deforestation and human activity, which is altering the carbon cycle and microclimate of the region (VOURLITIS et al, 2011;BIUDES et al, 2014a). These changes modify the rate of carbon converted to biomass through photosynthesis, which on a canopy scale can be estimated as gross primary production (GPP) (XIAO et al, 2004).…”