Drought and radiation explain swings in Amazon rainforest greenness during the 2015–2016 drought

Abstract. The 2015/16 Amazon drought was characterized by below-average regional precipitation for an entire year, which distinguishes it from the dry-season only droughts in 2005 and 2010. Studies of vegetation indices (VI) derived from optical remote sensing over the Amazonian forests indicated three stages in canopy response during the 2015/16 drought, with below-average greenness during the onset and end of the drought, and above-average greenness during the intervening months. So far, a satisfactory explanation for this broad temporal pattern, and spatial variation within the Amazonian forests of this broad response, has not been found. Better understanding of rainforest behaviors during this unusually long drought should help predict their response to future droughts. We hypothesized that below-average greenness could be explained by water deficit and heat stress occurring beyond the tolerance thresholds of rainforest. To test our hypothesis, we used monthly observations of terrestrial water storage (TWS), land surface temperature (LST) and vapor pressure deficit (VPD) for January 2003–December 2016. First, for each 1° grid cell, we determined the ‘normal’ range of monthly TWS, LST and VPD during non-drought years (i.e. 2003–2016, excluding 2005, 2010, 2015 and 2016), and identified the extreme values of ‘normal’ range, i.e. minimum TWS, maximum LST and maximum VPD. We considered the normal hydrological and thermal ranges to have been exceeded when (1) two or three of these variables were simultaneously beyond their extreme values, or (2) only one variable was beyond the extreme value, but the other two were significantly (p<0.05) different from the average for non-drought years. Using these criteria, regions experiencing hydrological and thermal conditions beyond the ‘normal’ range during different stages of the 2015/16 event were delineated. The results showed a gradual southward shift of these regions: from the north-eastern Amazon in August–October 2015, to the north-central part in November 2015–February 2016 and finally to the southern Amazon in July 2016. The majority of forests within the delimited regions experienced below-average greenness. Conversely, outside of these regions, greenness responded positively to radiation anomalies, as is expected under normal conditions. The opposing influences of drought and radiation anomalies together explained more than 70 % of the observed spatiotemporal patterns in greenness. These results suggest that our exceeding ‘normal’ ranges based approach, combining water storage, temperature and atmospheric moisture demand drivers, can reasonably identify the most likely drought-affected regions at monthly to seasonal time scales. Using observation-based hydrological and thermal condition thresholds can help with interpreting the response of Amazon rainforest to future drought events.