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

Abstract

The 2015/16 El Niño 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 γ . γ 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 γ may reflect forests with a tall understory, thick and closed canopy, and/or without degradation. Such forests with a high γ ( γ ≥ 0.3) appear to have a stronger capacity to recover than low- γ ones. Our results highlight the importance of forest structure when predicting the consequences of future drought stress in the tropics.

Declining Amazon biomass due to deforestation and subsequent degradation losses exceeding gains

“…In this study we sought to minimize these effects by careful filtering of L‐VOD datasets and comparing data from the wettest periods. Previous efforts to remove water‐content variations from L‐VOD yielded similar AGC change compared with using uncorrected L‐VOD data (Yang et al, 2022 ). Some remaining variations not due to biomass may remain, increasing the magnitude of anomalies, whereas longer‐term averages and trends remain more reliable.…”

“…Old‐growth forest areas showed an increase in AGC in 2015 relative to 2014 (Figure 3a ), despite including the onset of the 2015/2016 drought which reduced gross primary productivity in drought‐affected regions. The South Eastern Amazon did show a decrease in AGC which appeared to be compensated by increase in non‐drought affected regions of the South West and increase in the central Amazon (Figure S12 ) where enhanced growth may be due to greater radiation availability coordinated with flushing of more efficient young leaves at the beginning of the drought (Wu et al, 2016 ; Yang et al, 2022 ).…”

The detection and attribution of extreme reductions in vegetation growth across the global land surface

“…The residues in this coincidence analysis may have been partly due to the limited reliability of the climatic data (Harris et al, 2020), or extensive direct human activities that affect vegetation growth (Davis et al, 2020). Furthermore, part of any remaining inconsistency between NEGs and CEs could also be explained by the lagged impacts of CEs (Anderegg et al, 2015), for example in the epicenter of the 2005 and 2015/16 Amazonian droughts (Saatchi et al, 2013; Yang et al, 2022). In addition, natural disturbances such as wildfire and windthrow, or other biotic disturbance agents such as insect pests and pathogens, which are not completely aligned with meteorological conditions, may also affect vegetation growth (Balzter et al, 2007; Forzieri et al, 2021; Hicke et al, 2012).…”

Asymmetric response of Amazon forest water and energy fluxes to wet and dry hydrological extremes reveals onset of a local drought‐induced tipping point

“…Since the change was reversed, forest leaf area and ability to transpire water was eventually restored, a year or so after the end of the drought (see Figures 2 and 3), we conclude that the forest did not fully transition through a tipping point. However, the slow and prolonged post‐drought recovery suggests the forest is experiencing a “critical slowing down”—a behavior, that in dynamical systems theory is a warning sign that the forest may be approaching an irreversible critical transition or a tipping point (Hirota et al, 2021; Scheffer et al, 2009; Yang et al, 2022). Thus, we suggest that although a local tipping point transition has not yet occurred, the forest has likely reached a tipping point “onset” and that further or more frequent drought perturbations could result in sustained loss of leaf area and/or mortality that will be difficult to reverse as the water cycle feedback delineated here pushes vegetation collapse into a sustained alternative degraded state.…”