The lack of delimitation between the South American Monsoon System (SAMS) and the Intertropical Convergence Zone (ITCZ) has led to problematic calibration of archives in paleoclimate studies, particularly in northern South America. We show for the first time recorded in a paleoclimate archive that the ITCZ is the primary controller of oxygen (δ18OTR) and carbon (δ13CTR) isotopes in Cedrela nebulosa tree-rings (1864–2018). In contrast, a monsoonal pattern is not observed at this latitude. Spatial correlations revealed that δ18OTR better reflects months of higher precipitation (Mar–Jun) in the western Amazon than local rainout processes at decadal time scale, owing to the strong convection in the basin at this time of the year. Similarly, this study identified cloud cover as a vital controller in sunshine duration, which influences the phenology of Cedrela nebulosa. We interpret the variability of the δ13CTR as an enhancement in the photosynthetic rate during light-increased months (Jul–Sep), strongly regulated by cloudiness reduction when the ITCZ rain band retreats to northwestern South America in austral winter. Overall, these results reveal that Cedrela nebulosa is well-adapted to wet environments, and its cellulose-based stable isotopic signals reflect the direct influence of the ITCZ excursions from austral autumn to winter (Mar–Sep) with minimal SAMS control.
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