Amazon drought has great impacts on the inter-annual variations of regional ecosystem and global carbon cycling. However, the extent and controls on gross ecosystem productivity caused by the droughts are poorly resolved and inadequately represented. Significant debate has focused on whether tropical forest productivity is more limited by sunlight or rainfall. However, recent studies suggested the seasonality of gross ecosystem productivity (GEP) was mainly driven by endogenous canopy phenology instead of climate variability. Here, we ask an analogous question of whether, at the inter-annual time scale, endogenous canopy dynamics (as opposed to direct physiological responses to exogenous climate variation) also exerts significant control on tropical forest productivity? We use the intense El Niño droughts in Amazon tropical forest (in 2009-2010 and 2015-2016), to examine satellite-derived remote sensing responses. We used a novel approach, integrating measures of changes in canopy photosynthetic capacity (from the Enhanced Vegetation Index, EVI) with measures of realized photosynthetic activity (from remotely retrieved solar-induced chlorophyll fluorescence, SIF) to partition drought responses between canopy structure and canopy physiology.
Results/Conclusions
We found, consistent with previous reports of increased greening with drought, that overall greenness (EVI), presumably reflecting canopy scale structure (e.g. overall leaf quantity), increased with drought in Amazon forests during both El Niños. However, realized canopy photosynthesis (as captured by SIF) decreased, with greater decreases in the 2015/2016 El Nino than during the 2009/2010 El Nino. These results suggest that multifaceted, sometimes contrasting, mechanisms interact to determine whole forest drought responses. Because canopy structure and photosynthetic physiology apparently respond differently to drought at large scales, overall photosynthetic responses are structured by the interaction of mechanisms with the severity of droughts.