Changes in the carbon cycle of Amazon ecosystems during the 2010 drought

Satellite remote sensing was combined with the NASA-CASA (Carnegie Ames Stanford Approach) carbon cycle simulation model to evaluate the impact of the 2010 drought (July through September) throughout tropical South America. Results indicated that net primary production in Amazon forest areas declined by an average of 7% in 2010 compared to 2008. This represented a loss of vegetation CO2 uptake and potential Amazon rainforest growth of nearly 0.5 Pg C in 2010. The largest overall decline in ecosystem carbon gains by land cover type was predicted for closed broadleaf forest areas of the Amazon river basin, including a large fraction of regularly flooded forest areas. Model results support the hypothesis that soil and dead wood carbon decomposition fluxes of CO2 to the atmosphere were elevated during the drought period of 2010 in periodically flooded forest areas, compared to those for forests outside the main river floodplains.


1) CASA Carbon Flux Algorithms
As documented in Potter et al. (1993 and2009a), monthly NPP flux, defined as net fixation of CO 2 by vegetation, is computed in CASA on the basis of light-use efficiency. Maximum conversion efficiencies of photosynthetically active radiation (PAR) to NPP carbon gains can be approximated as being nearly constant across all natural ecosystems (Nemani and Running, 1989;Sellers et al., 1994;Goetz and Prince, 1998;Running and Nemani, 1998). For this study, we used MODIS collection 5 of the Enhanced Vegetation Index (EVI; Huete, et al., 2002 and2006) as an improvement in the model inputs for PAR interception.
Monthly production of plant biomass is estimated as a product of time-varying surface solar irradiance, Sr, and EVI from the MODIS sensor, plus a constant maximum light utilization efficiency term (e max ) that is modified by time-varying stress scalar terms for temperature (T) and moisture (W) effects (Equation 1).
The e max term is set uniformly at 0.55 g C MJ -1 PAR, a value that derives from calibration of predicted annual NPP to previous field estimates (Potter et al., 2003). This model calibration has been validated globally by comparing predicted annual NPP to more than 1900 field measurements of NPP (see section that follows). Climate drivers for the CASA model were from the National Center for Environmental Prediction (NCEP/DOE II) reanalysis data set (Kistler et al., 2001;Zhao and Running, 2010), and land cover settings were aggregated from the MODIS global 1-km product (Zhao and Running, 2010), by determining the majority 8-km resolution land cover class from the underlying 1km land cover pixels. This aggregation procedure provided the greatest assurance of high-quality, cloud-free VI inputs to the carbon cycle model.
The T stress scalar is computed with reference to derivation of optimal temperatures (Topt) for plant production. The Topt setting will vary by latitude and longitude, ranging from near 0 o C in the Arctic to the middle thirties in low latitude deserts. The W stress scalar is estimated from monthly water deficits, based on a comparison of moisture supply (precipitation and stored soil water) to potential evapotranspiration (PET) demand.
Evapotranspiration is connected to water content in the soil profile layers. The soil model design includes three-layer (M1-M3) heat and moisture content computations: surface organic matter, topsoil (0.3 m), and subsoil to rooting depth (1 to 10 m). Maximum rooting depth for cropland and grassland cover types was set at 1 m, whereas non-tropical forest was set at 2 m and tropical forest was set at 10 m (Nepstad et al., 1994;Poulter et al., 2009). These layers can differ in soil texture, moisture holding capacity, and carbon-nitrogen dynamics. Water balance in the soil is modeled as the difference between precipitation or volumetric percolation inputs, monthly estimates of PET, and the drainage output for each layer. Inputs from rainfall can recharge the soil layers to field capacity. Excess water percolates through to lower layers and may eventually leave the system as seepage and runoff.
Net ecosystem production (NEP) can be computed as NPP minus soil microbial respiration (

2) CASA Model Validation
Interannual NPP fluxes from the CASA model have been reported previously (Behrenfeld et al., 2001) and validated against multi-year estimates of NPP from field stations and tree rings (Malmström et al., 1997). Net ecosystem fluxes of carbon from CASA have been validated against atmospheric inverse model estimates over two decades (Potter et al., 2003). For this latest application, a comparison of observed NPP (n = 1927) from field based measurements to predicted annual values from the CASA model was made to provide validation of terrestrial NPP predictions across all ecosystem types.
Observed NPP values were compiled for the Ecosystem Model-Data Intercomparison (EMDI) activity by the Global Primary Productivity Data Initiative (GPPDI) working groups of the International Geosphere Biosphere Program Data and Information System (IGBP-DIS; Olson et al., 1997). Monthly MODIS EVI inputs resulted in a highly significant correlation (R 2 = 0.91) and a close 1:1 match of observed to CASA predicted NPP values, with the year 2001 selected as an example (Fig. S1).
In this comparison to observed NPP, the CASA model was also tested for sensitivity to the vegetation index monthly time series as well by driving the NPP algorithm separately with either MODIS-EVI or MODIS-FPAR monthly inputs, holding climate inputs constant. A lower level of saturation in the low-to-medium range of plant production estimated from CASA modeling with EVI inputs compared to FPAR inputs was discovered by comparison of the two scatter plots over the range of annual NPP from 100-300 g C m -2 yr -1 (Fig. S1). Not only did EVI result in less overall scatter in the predicted versus observed plot (i.e., R 2 = 0.81 using MODIS FPAR inputs), the match to observed NPP in the high global range (of greater than 1000 g C m -2 yr -1 ) was markedly more consistent with EVI compared to MODIS FPAR inputs.
By way of additional model validation in South American ecosystems, comparison of CASA seasonal NPP against measured monthly NEP fluxes from the Large-scale Biosphere-Atmosphere Experiment in Amazonia (LBA) showed close agreement at the Tapajos (Pará) forest experimental site (Potter et al., 2009a). At the ZF2 Manaus forest site, Chambers et al. (2004) directly measured respiration rates from live leaf, live wood, and forest soil surfaces to derive an indirect NPP flux estimate of 900 g C m -2 yr -1 . Annual NPP from CASA for this general area (2.5° S lat, 60° W lon) around Manaus varied between 782 -871 g C m -2 yr -1 from 2000 and 2004. These CASA model predictions are further validated by the estimated NPP range of 864 ± 96 g C m -2 yr -1 from a review of 29 carbon study sites in humid tropical forest areas (Luyssaert et al., 2007).

3) Flooded Forest Area Examples of Drought Impacts
Three