Decorrelating carbon and water fluxes in the Amazon Basin.

Tropical carbon and water fluxes are assumed tightly coupled - exhibiting strong positive correlations as plants exchange water for carbon through stomata. However, analysis of 40 years of satellite-based estimates of gross primary production (GPP) and evapotranspiration (ET) across the Amazon Basin reveals pronounced spatial and temporal divergence in that coupling. Here we show that 64% of the Basin exhibits weakly positive ( R < 0.3 ) or negative GPP-ET correlations, particularly in forests with high baseline water availability ( R = - 0.03 ± 0.14 ) . Weak coupling is driven by divergent GPP and ET behavior during the dry season, implying physiological and hydrologic drivers decouple carbon and water fluxes in forests where water stress is limited. Moreover, correlation strength has declined since 1982, especially in intact forests, concurrent with rising vapor pressure deficit (VPD). We hypothesize that rising VPD suppresses canopy conductance and GPP, while elevating atmospheric demand for ET. Continued decoupling could increase water loss relative to carbon gain, undermining the Amazon capacity as a sustained carbon sink.