Increasing Mortality of Large Trees Shifts Southwestern Amazon Forests Into a Source of CO2

The Southwestern Amazon is undergoing rapid changes due to deforestation, forest degradation, fires, and impacts from climatic changes. Yet, the responses of forests to these changes are unknown in this region. In particular, the processes controlling ecosystem Gross Primary Production (GPP, the atmospheric carbon dioxide fixed during photosynthesis per unit of time) are not well understood in this region due to limited in situ observations. Using inventory plot data (RAINFOR TAM-04 and 08, from 2006 to 2013) and eddy covariance flux measurements (from 2017 to 2018) over old-growth forests in the Tambopata region of the Southwestern Amazon, we found consistent patterns of GPP whose values ranged from 25 to 35 MgC/ha/yr. These values of GPP are within the estimates derived from MODIS GPP (25 MgC/ha/yr). In terms of net CO2 exchange, both tree inventory data and eddy covariance measurements also showed that in 2017 and 2018 the forests represented a net source of CO2 to the atmosphere (~5 MgC/ha/yr ). These emissions were likely related to an increase in mortality of big trees (20% of dead trees had DBH values ranging from 20 to 65 cm) as shown by the analysis of data from permanent forest plots. Mechanisms for this increase in tree mortality include, among others, the abundance of lianas, and extreme weather events such as prolonged droughts. Using the Community Earth System Model, Version 2 (CESM2) Single Column Integrated Surface Model (SCISM) prescribed with local ecosystem characteristics and monthly conditions, we assess the changes in ecosystem carbon fluxes. Southwestern Amazon is changing, and understanding the drivers of tree mortality and what triggers them are urgent.