Future Drought-Induced Tree Mortality Risk in Amazon Rainforest

The future evolution of the Amazon rainforest remains uncertain not only due to uncertain climate projections, but also owing to the intricate balance between tree growth and mortality. Many Earth System Models inadequately represent forest demography processes, especially drought-induced tree mortality. In this study, we used ORCHIDEE-CAN-NHA, a land surface model featuring a mechanistic hydraulic architecture, a tree mortality sub-model linked to a critical loss of stem conductance and a forest demography module for simulating regrowth. The model was forced by bias-corrected climate forcing data from the ISIMIP-2 program, considering two scenarios and four different climate models to project biomass changes in the Amazon rainforest until 2100. These climate models display diverse patterns of climate change across the Amazon region. The simulation conducted with the HadGEM climate model reveals the most significant drying trend, suggesting that the Guiana Shield and East-central Amazon are approaching a tipping point. These two regions are projected to transition from carbon sinks to carbon sources by the mid-21st century, with the Brazilian Shield following suit around 2060. This transition is attributed to heightened drought-induced carbon loss in the future. This study sheds light on uncertainties in the future carbon sink in the Amazon forests, through a well-calibrated model that incorporates tree mortality triggered by hydraulic damage and the subsequent recovery of drought-affected forests through demographic processes. Whether the Amazon rainforest will remain as net carbon sink or not has long been of great concern as the drought events are predicted to become more frequent and more intense in the future and such extreme events highly threaten the forest net carbon uptake capacity. Here we use a process-based model embedding drought-induced tree mortality scheme that can perform well regarding past drought events over Amazon basin, to predict the future drought-induced tree mortality risk and the evolution of net biomass carbon sink. The climate models present consistent warming but different wetting/drying patterns, although most of them consistently predict a drier trend in northeastern Amazon. Simulations forced by one climate model showed a carbon sink turning to a carbon source in more than half of Amazon rainforest since the middle of the 21st century. This work can inform the forest area with high tree mortality risk in the future, which calls for more concerns on mitigation policies. We used a land surface model forced by ISIMIP2 climate data to simulate the future drought-induced tree mortality risk in Amazon rainforest While climate models differ in projections of wetting/drying patterns, many of them suggest a drying trend in the northeastern Amazon Simulations forced by HadGEM model indicate the Guiana Shield and East-central Amazon will transition from carbon sink to source from 2050s