Monitoring carbon assimilation in South Americaʹs tropical forests: Model specification and application to the Amazonian droughts of 2005 and 2010

Net primary production (NPP) is a key variable for monitoring and understanding the impacts of environmental change on ecosystems and for generating realistic global and regional carbon budgets. In this paper we present a regional algorithm (RATE) for automatically monitoring the rate of carbon fixation (as measured by NPP) of tropical forests in South America. The algorithm is based on a modification of the SITE ecosystem model and uses data obtained from the procedures for aggregation and correction of data from the MOD12Q1 and MOD15A2 products and meteorological data from the National Centers for Environmental Prediction (NCEP). A correction procedure for the MOD15A2 Leaf area index (LAI) and Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) used by the RATE algorithm produced satisfactory LAI and FAPAR estimates when compared against observed values. The algorithm was successfully validated in eight field sites from two types of tropical forests in South America (Amazon rainforest and the Atlantic forest), producing an average error of only 4.72%. When applied to Amazonia, RATE indicates that NPP showed little variation in the 2005 and 2010 drought years (NPP = 1.28 kg-C m− 2 year− 1) in comparison to non-drought years (NPP = 1.31 kg-C m− 2 year− 1). RATE also provides some limited evidence for small decreasing trends in Amazonia carbon assimilation during the period 2001–2010.