Small anomalies in dry-season greenness and chlorophyll fluorescence for Amazon moist tropical forests during El Nino and La Nina

The Amazon Basin, a major driver of atmospheric CO2 fluxes, is composed of moist tropical forest (> 2000 mm mean annual precipitation), seasonally moist tropical forests (< 2000 mm mean annual precipitation), croplands, and pastures. It is debated whether there is a dry-season increase in photosynthesis for moist forest and a large reduction in photosynthesis of tropical South America was recently cited as a major driver of the historically high atmospheric CO2 growth rate during the 2015/2016 El Nino. To address this debate and to gain insight into changes in dry-season greenness, SIF, and photosynthesis during El Nino-Southern Oscillation (ENSO) events, here we investigate (1) dry-season changes in satellite-based greenness, solar-induced chlorophyll fluorescence (SIF), and photosynthesis during 2007-2017 and (2) anomalies of satellite-based dry-season greenness, SIF, and photosynthesis for two El Nino events (2009/2010 and 2015/2016) and two La Nina events (2007/2008 and 2010/2011). We hypothesize that satellite-based greenness, SIF, and photosynthesis of moist tropical forests should increase during the dry season, and find this to be the case using two MODIS BRDF-adjusted vegetation indices (EVI and NDVI), GOME-2 SIF data, and the Vegetation Photosynthesis Model (VPM). We also hypothesize that dry-season greenness, SIF, and photosynthesis should be anomalously high during the El Ninos, due to anomalously high photosynthetically active radiation (PAR) and a relatively normal preceding wet season, and anomalously low during the La Ninas because these dry seasons were preceded by anomalously low amounts of wet-season precipitation. For this hypothesis, we present results for moist tropical forest and at the basin scale to determine if and by how much their anomalies differ. We find dry-season greenness, SIF, and photosynthesis of moist tropical forest and at the basin scale were statistically significantly lower than normal during the La Ninas, significantly higher than normal during the 2009/2010 El Nino, and were mixed for the 2015/2016 El Nino. Although statistically significant, the magnitudes of the dry-season anomalies were not substantial. Our findings provide additional evidence that photosynthesis of moist tropical Amazon forest increases during the dry season and narrows the potential drivers of perturbations to the atmospheric CO2 growth rate during the last four ENSO events, as anomalies in dry-season greenness, SIF, and photosynthesis during these ENSO events were minute.

Automated summary

The study found that in the Amazon Basin, there is a seasonal increase in greenness, solar-induced chlorophyll fluorescence (SIF), and photosynthesis of moist tropical forests during the dry season, and anomalies were higher during El Nino events and lower during La Nina events. The magnitudes of these anomalies were statistically significant but not substantial, providing additional evidence that photosynthesis in the Amazon forest increases during the dry season and plays a role in perturbations to atmospheric CO2 growth rate during ENSO events.