Soil Moisture Thresholds Explain a Shift from Light-limited to Water-limited Sap Velocity in the Central Amazon during the 2015-16 El Nino Drought

The 2015-16 El Niño drought provided a unique opportunity to examine whether transpiration in Central Amazon forests is constrained by water under severe lack of rainfall. We examined the controls of net radiation and volumetric soil water content on sap velocity in the central Amazon, and used partial correlation analysis and multiple linear regression to identify the most likely drivers of sap velocity variability. We further identified critical thresholds of soil moisture limitation on sap velocity. Our analysis showed a marked decline in soil moisture by 31.6% during the drought (0.26 m3/m3) compared to the wet season (0.38 m3/m3). In conjunction, sap velocity and its variability experienced dramatic drops of 56.8% and 87.8% respectively, during the drought. Sap velocity was largely limited by net radiation during wet and dry seasons, however, shifted to be limited only by soil moisture during the drought. Soil moisture thresholds on sap velocity were identified to be 0.31 m3/m3 ~ 0.33 m3/m3 (-36 ~ -61 KPa in soil matric potential). We also used The Functionally Assembled Terrestrial Ecosystem Simulator (FATES) to simulate the changes in canopy transpiration under different drought conditions, which were generated by reducing the rainfall by 10 - 50%. We found a strong correlation between canopy transpiration and soil moisture only when soil moisture is below 0.3 m3/m3, which matches the observational results well. Our results suggest strong soil moisture limitation on rainforest transpiration in the central Amazon during extreme drought conditions, and such limitation will likely become more frequent under future climate, due to increased surface temperatures combined with an increased frequency, intensity, duration and extent of extreme drought events.