Coupled climate-carbon cycle models indicate that Amazon forests may be vulnerable to drought, with some predicting climate change-induced collapse of the Amazon forest and conversion to savanna, while others predict resilience. Much progress has been made in understanding tropical forest drought response, but a holistic picture encompassing both short-term physiological (e.g., photosynthesis) and longer term demographic responses (e.g., mortality) remains elusive, mainly due to the rarity of coinciding relevant measurements and drought events. Here we analyze the response of an eastern Amazonian forest at both timescales to the El Nino-induced drought of late 2009 / early 2010 using both eddy flux measurements of carbon exchange and periodic tree demographic surveys.
We partitioned response of GPP into environmental (light, vapor pressure deficit (VPD), diffuse light) and biological responses. Based on environmental conditions alone (high VPD), we expected GPP to be less than average during drought. In contrast, GPP was elevated above this expected negative response over a period of ~45 days, consistent with satellite observed green-up during some droughts. At the same time, drought significantly elevated 2009-2011 tree mortality above that during non-drought periods, consistent with a quantitative drought-mortality relationship reported for other Amazon forests. This work suggests that “green-up” of forest canopies during drought may be a drought stress response consistent with subsequent drought-induced tree mortality. More importantly, it highlights endogenous biological regulation of photosynthesis as an important mechanism, neglected by models, in mediating drought responses in tropical forests.