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

Understanding the effects of intensification of Amazon basin hydrological cycling manifest as increasingly frequent floods and droughts on water and energy cycles of tropical forests is essential to meeting the challenge of predicting ecosystem responses to climate change including forest tipping points Here we investigated the impacts of hydrological extremes on forest function using 12 years of observations between 2001 2020 of water and energy fluxes from eddy covariance along with associated ecological dynamics from biometry at the Tapaj s National Forest Measurements encompass the strong 2015 2016 El Ni o drought and La Ni a 2008 2009 wet events We found that the forest responded strongly to El Ni o Southern Oscillation ENSO Drought reduced water availability for evapotranspiration ET leading to large increases in sensible heat fluxes H Partitioning ET by an approach that assumes transpiration T is proportional to photosynthesis we found that water stress induced reductions in canopy conductance G sub s sub drove T declines partly compensated by higher evaporation E By contrast the abnormally wet La Ni a period gave higher T and lower E with little change in seasonal ET Both El Ni o Southern Oscillation ENSO events resulted in changes in forest structure manifested as lower wet season leaf area index However only during El Ni o 2015 2016 we observed a breakdown in the strong meteorological control of transpiration fluxes via energy availability and atmospheric demand because of slowing vegetation functions via shutdown of G sub s sub and significant leaf shedding Drought reduced T and G sub s sub higher H and E amplified by feedbacks with higher temperatures and vapor pressure deficits signaled that forest function had crossed a threshold from which it recovered slowly with delay post drought Identifying such tipping point onsets beyond which future irreversible processes may occur at local scale is crucial for predicting basin scale threshold crossing changes