Changing Amazon biomass and the role of atmospheric CO2 concentration, climate, and land use

• Published in: Global biogeochemical cycles 2016-01, Vol.30 (1), p.18-39
• Main Authors: Almeida Castanho, Andrea D., Galbraith, David, Zhang, Ke, Coe, Michael T., Costa, Marcos H., Moorcroft, Paul
• Format: Article
• Language: English
• Subjects: Amazon Basin; Atmospheric chemistry; Biogeochemistry; Biomass; Biosphere; carbon cycle; Carbon dioxide; Climate change; CO2 fertilization; Demography; Drought; dynamic vegetation model; Extreme drought; Forest biomass; Greenhouse gases; Heterogeneity; Land use; land use change; Mortality; Old growth; Tropical forests; Vegetation
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1002/2015GB005135

The Amazon tropical evergreen forest is an important component of the global carbon budget. Its forest floristic composition, structure, and function are sensitive to changes in climate, atmospheric composition, and land use. In this study biomass and productivity simulated by three dynamic global vegetation models (Integrated Biosphere Simulator, Ecosystem Demography Biosphere Model, and Joint UK Land Environment Simulator) for the period 1970–2008 are compared with observations from forest plots (Rede Amazónica de Inventarios Forestales). The spatial variability in biomass and productivity simulated by the DGVMs is low in comparison to the field observations in part because of poor representation of the heterogeneity of vegetation traits within the models. We find that over the last four decades the CO2 fertilization effect dominates a long‐term increase in simulated biomass in undisturbed Amazonian forests, while land use change in the south and southeastern Amazonia dominates a reduction in Amazon aboveground biomass, of similar magnitude to the CO2 biomass gain. Climate extremes exert a strong effect on the observed biomass on short time scales, but the models are incapable of reproducing the observed impacts of extreme drought on forest biomass. We find that future improvements in the accuracy of DGVM predictions will require improved representation of four key elements: (1) spatially variable plant traits, (2) soil and nutrients mediated processes, (3) extreme event mortality, and (4) sensitivity to climatic variability. Finally, continued long‐term observations and ecosystem‐scale experiments (e.g. Free‐Air CO2 Enrichment experiments) are essential for a better understanding of the changing dynamics of tropical forests. Key Points CO2 fertilization is a major contributor to the increase in simulated biomass of old growth forests in the last 40 years Land use change reduces the simulated Amazon biomass comparable in magnitude to the biomass increase from CO2 fertilization Better representation of mortality from extreme climate events is required in DGVMs