“Hotter Droughts” and the fate of Tropical Forests
Recent Amazonian droughts have drawn attention to the vulnerability of tropical forests to climate perturbations. Ground and satellite observations over a period of about 15 years show the legacy of the droughts in terms of impacts on the ecological structure and function and the carbon exchange of the forests. The amplification of hydrological cycle and the increase of surface temperature together tip off the balance between precipitation and potential evapotranspiration, causing hotter and drier conditions in Amazonia. These conditions cause hydraulic failure in trees, putting large and long-lived trees at the great risk of mortality. The persistence of low canopy water content observed by microwave scatterometers, increasing surface temperature detected by thermal sensors, along with a delay in the recharge of the hydrological system measured by GRACE all point to a 3-5-year legacy of droughts. The post-drought loss of carbon associated with tree mortality or damage of canopy trees detected by spaceborne LiDAR observations (2003-2008) show Amazon forests switching from a long-term carbon sink to a carbon source to the atmosphere. With more frequent droughts expected in future, the results from this study suggest that tropical forests may lose their role as a robust sink of atmospheric carbon, leading to significant positive climate feedbacks and exacerbating climate warming trends.