Causes of climate predictions’ uncertainty include wide spread in modeled gross primary productivity (GPP) for evergreen broadleaf forests. Deterministic predictions inherently lack the portion of variability that a regression’s error term summarizes. Omitted predictors’ contribution to error represent simulations’ necessary underestimation of real variability. Earth system model outputs with high variability relative to reference data warrant skeptical examination. We compare three statistical and 15 process models to site-level means, seasonal amplitude and driver responsiveness of GPP as calculated at six Amazon eddy covariance (EC) towers. Current month’s weather determines only 12% of the variability in EC GPP, implying that models whose predicted GPP’s variability approaches that of EC GPP probably are substantially hypersensitive to weather drivers. Roughly half the models have stronger seasonal GPP variability than ECs show, and inaccurately identify the timing of annual minimum GPP. Responses to temperature and light for some highly seasonal models are of the opposite sign as EC GPP’s. Strongly seasonal models’ deepest dip in photosynthesis both occurs later in the dry season and is more severe than EC estimates. Excessive reactivity to drivers appears to cause the high simulated variability of the strongly seasonal models.

Carbon allocation to non-structural carbohydrates (NSC) is essential for plant metabolism playing an important role in tree responses to drought. It is still unclear if and how interspecific hydraulic trait variation modulates NSC concentration dynamics in different plant organs, particularly in tropical tree species. We investigated whether drought-resistance strategies (inferred from hydraulic traits) explain seasonal and interannual NSC dynamics in leaves, branches, trunks, and roots in seasonal eastern Amazon tree species in Brazil. We measured NSC concentration in eight abundant species during three years, including the end of the wet and dry seasons of the typical regular years (2013-2014) and the extreme drought induced by El Niño–Southern Oscillation in 2015 (ENSO). Organs have an important contribution to explain the starch (ST), soluble sugar (SS), and NSC variance among trees. We showed seasonal and year-to-year homeostasis in ST and SS concentrations in a majority of organs during 2013 and 2014, but SS increased in all organs during the extreme ENSO drought, while the ST concentration did not. The increase in SS concentration was more evident in woody organs from species with intermediate and tolerant drought strategies. The drought-tolerant species maintain higher root starch concentrations and mobilize more SS during extreme drought.