Climatic and biotic factors influencing regional declines and recovery of tropical forest biomass from the 2015/16 El Nino

The 2015/16 El Nino brought severe drought and record-breaking temperatures in the tropics. Here, using satellite-based L-band microwave vegetation optical depth, we mapped changes of above-ground biomass (AGB) during the drought and in subsequent years up to 2019. Over more than 60% of drought-affected intact forests, AGB reduced during the drought, except in the wettest part of the central Amazon, where it declined 1 y later. By the end of 2019, only 40% of AGB reduced intact forests had fully recovered to the predrought level. Using random-forest models, we found that the magnitude of AGB losses during the drought was mainly associated with regionally distinct patterns of soil water deficits and soil clay content. For the AGB recovery, we found strong influences of AGB losses during the drought and of gamma. gamma is a parameter related to canopy structure and is defined as the ratio of two relative height (RH) metrics of Geoscience Laser Altimeter System (GLAS) waveform data-RH25 (25% energy return height) and RH100 (100% energy return height; i.e., top canopy height). A high gamma may reflect forests with a tall understory, thick and closed canopy, and/or without degradation. Such forests with a high gamma (gamma >= 0.3) appear to have a stronger capacity to recover than low-gamma ones. Our results highlight the importance of forest structure when predicting the consequences of future drought stress in the tropics.