3-PG Productivity Modeling of Regenerating Amazon Forests: Climate Sensitivity and Comparison with MODIS-Derived NPP

Abstract

Potential forest growth predicted by the Physiological Principles in Predicting Growth (3-PG) model was compared for forest and deforested areas in the Legal Amazon to assess potential differing regeneration associated with climate. Historical deforestation and regeneration have occurred in environmentally marginal areas that influence regional carbon sequestration estimates. Effects of El Niño?induced drought further reduce simulated production by decreasing soil water availability in areas with shallow soils and high transpiration potential. The model was calibrated through comparison of literature biomass and with satellite-based estimates. Net primary productivity (NPP) for mature Amazonian forests from the 3-PG model was positively correlated (r?2 = 0.77) with a Moderate Resolution Imaging Spectroradiometer (MODIS)-derived algorithm, though with some bias. Annual total NPP for the study area using a 1961?90 average climatology was 4.6 Pg C yr?1, which decreased to 4.2 Pg C yr?1 when simulated with climate from the severe 1997/98 El Niño event. From a regional analysis, results showed that biomass accumulation is almost entirely controlled by the availability of soil water. Also, areas currently forested in the eastern Amazon are more sensitive to extreme El Niño?induced drought than southern areas with the greatest deforestation extent.