Vulnerability of Water and Carbon Cycling of Humid Tropical Forests to Coupled Land Use and Climate Stressors

Humid tropical forests (HTFs) host the largest reservoir of carbon, water, energy, and biodiversity on the Earth and are important for regulating the climate system.  Although these forests are historically adapted to some level of disturbance, they are now facing increasingly severe and persistent stresses from land use, climate change, impacting their function and health. Here, we quantify their vulnerability to these stresses by evaluating their ecological responses, particularly in cycling carbon and water across space and time. We show that globally, stress from climate change is emerging as widespread and important as land-use across the tropics.  Together, these stressors have slowed the recovery rate of forest aboveground biomass (AGB), gross primary productivity (GPP), and evapotranspiration since the early 1980s.  Temporal autocorrelation, as an indicator of this slow recovery, rises substantially for evapotranspiration and gross primary production when climate stress reaches a critical level.   These forest responses to stress vary regionally and have both declining and increasing trends, suggesting large-scale deviations from normal behavior.  Forests in the Amazon Basin exhibit extensive vulnerability to increasing moisture deficit and drying of atmosphere by reducing GPP and Evapotranspiration, while those in the Congo Basin appear more resilient, and even show increasing trends due to warming and potential shift in radiation regime.  We present a spatially explicit climate and land-use vulnerability index to systematically track the response of tropical forests to future stressors and provide early-warning signals of critical transitions.