Résumé
Modeling work of the isotopic composition of tree ring cellulose (δ18 Ocell ) relies on the isotopic equilibrium assumption between atmospheric water vapor and tree source water, frequently assimilated to integrated precipitation. Here, we explore the veracity of this assumption based on observations collected during a field campaign in Río Negro province (Argentina) in February–March 2017. We examine how the observed isotopic composition of water vapor deviates from equilibrium with precipitation. This deviation, named isotopic disequilibrium (Δ18 Ovap_eq), is low (between −2.0‰ and 4.1‰) and a significant relationship is observed between the isotopic composition of water vapor and its expected value at equilibrium. Negative Δ18 Ovap_eq can be explained by evaporation of small raindrops (from 1% to 5% of initial droplet mass). Positive Δ18 O vap_eq can result from vegetation transpiration with transpired water accounting for 14% to 29% to ambient water vapor. The low Δ18 O vap_eq at the study site may be due to the high level of relative humidity (from 70% to 96%) favoring isotopic diffusive exchanges between the two water phases and thus promoting the isotopic equilibrium. We examine the impact of the isotopic equilibrium assumption on the calculation of δ18O cell . A perfect agreement is shown between observed and calculated δ18O cell provided that the isotopic composition of source water is significantly higher than the expected averaged isotopic composition of precipitation over the tree growing period.