Current climate change scenarios indicate warmer temperatures and the potential for more extreme droughts in the tropics, such that a mechanistic understanding of the water cycle from individual trees to landscapes is needed to adequately predict future changes in forest structure and function. In this study, we contrasted physiological responses of tropical trees during a normal dry season with the extreme dry season due to the 2015-2016 El Niño-Southern Oscillation (ENSO) event. We quantified high resolution temporal dynamics of sap velocity (V_s), stomatal conductance (g_s) and leaf water potential (Ψ_L) of multiple canopy trees, and their correlations with leaf temperature (T_leaf) and environmental conditions [direct solar radiation, air temperature (T_air) and vapor pressure deficit (VPD)]. The experiment leveraged canopy access towers to measure adjacent trees at the ZF2 and Tapajós tropical forest research (near the cities of Manaus and Santarém). The temporal difference between the peak of g_s (late morning) and the peak of VPD (early afternoon) is one of the major regulators of sap velocity hysteresis patterns. Sap velocity displayed species-specific diurnal hysteresis patterns reflected by changes in T_leaf. In the morning, T_leaf and sap velocity displayed a sigmoidal relationship. In the afternoon, stomatal conductance declined as T_leaf approached a daily peak, allowing Ψ_L to begin recovery, while sap velocity declined with an exponential relationship with T_leaf. In Manaus, hysteresis indices of the variables T_leaf-T_air and Ψ_L-T_leaf were calculated for different species and a significant difference (p < 0.01, α = 0.05) was observed when the 2015 dry season (ENSO period) was compared with the 2017 dry season ("control scenario"). In some days during the 2015 ENSO event, T_leaf approached 40°C for all studied species and the differences between T_leaf and T_air reached as high at 8°C (average difference: 1.65 ± 1.07°C). Generally, T_leaf was higher than T_air during the middle morning to early afternoon, and lower than T_air during the early morning, late afternoon and night. Our results support the hypothesis that partial stomatal closure allows for a recovery in Ψ_L during the afternoon period giving an observed counterclockwise hysteresis pattern between Ψ_L and T_leaf.

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