The Amazon basin represents more than 50 percent of tropical rainforest area, about half of total terrestrial biomass (120 Pg of carbon out of 247 Pg carbon globally). Modeling analyses suggest that the Amazon basin may experience a drying trend and/or intensification of the hydrological cycle in a warming world. The two mega-droughts in the last decade (2005 and 2010) have been used as evidence of drying effects. These droughts were observed to have persistent legacy effects on the ecosystem canopy water content and structure from remote-sensing data. However, the causes and variability of these legacy effects are not well understood. In addition, there exist wide debates on the consequences for carbon fluxes during the droughts and the subsequent recovery period. With increasing evidence of more frequent droughts in Amazonia, the question of how and to what extent the droughts can impact the function and the carbon storage and fluxes of the forests remains unresolved and challenging.
Anomalies of Ku-Band Scatterometer data (associated with canopy water content) averaged over southwest Amazon during the 2005 drought and the post-drought recovery period. There is a clear and persistent legacy effect. Figure adapted from Saatchi et al. (2013).
This project combines satellite observations of water storage, canopy water content, forest structure and photosynthetic activity, and in-situ measurements of carbon dynamics and fluxes with the Ecosystem Demography 2 (ED2) terrestrial biosphere model with a novel mechanistic plant hydrodynamic module, to investigate the responses of Amazon forests to recent droughts and the sensitivity or vulnerability of the Amazon to future water stress.
Novel plant hydrodynamic module within ED2 model. The new module can account for the variations of plant water flow, canopy water status, and plant responses to water stress in a mechanistic way. For details see Xu et al. (2016).
In this project, site-level census and fluxes data will first be combined with forest structure data inferred from LiDAR observations to initialize, optimize, and validate the ED2 model. After successful validation, model predictions of canopy water content will be compared with Ku-band scatterometer data as well as microwave-band vegetation optical depth, both of which have been associated linearly with canopy water content. The optimized model can help to interpret the variations of the diurnal, seasonal, and inter-annual variations of the observed changes in canopy water contents. In addition, the ED2 model can help to relate changes in canopy water contents to ecosystem responses to droughts such as variations of transpiration, carbon fluxes, forest structure and above-ground biomasses. Finally, regional simulations of the optimized model can help to assess the vulnerability and recovery trajectories of Amazon forests under different drought scenarios.
This project will have several expected deliverables. First, the project can provide assessment of the magnitude of the drought effects and their legacy on carbon and water dynamics in Amazon forests from both empirical analysis and theoretical model predictions. Second, the project can help to quantify the sensitivity/vulnerability/resilience of the Amazon forests to potentially more frequent extreme water stress in the future, constrained by ground and remote-sensing observations as well as current ecophysiological knowledge of tropical forest dynamics. Third, the project can improve the ED2 model particularly in terms of plant water stress, which belongs to one of the largest uncertainties in current terrestrial ecosystem models.