Modelling tree ring cellulose δ¹⁸O variations in two temperature-sensitive tree species from North and South America

Abstract. Oxygen isotopes in tree rings (δ¹⁸O_TR) are widely used to reconstruct past climates. However, the complexity of climatic and biological processes controlling isotopic fractionation is not yet fully understood. Here, we use the MAIDENiso model to decipher the variability in δ¹⁸O_TR of two temperature-sensitive species of relevant palaeoclimatological interest (Picea mariana and Nothofagus pumilio) and growing at cold high latitudes in North and South America. In this first modelling study on δ¹⁸O_TR values in both northeastern Canada (53.86° N) and western Argentina (41.10° S), we specifically aim at (1) evaluating the predictive skill of MAIDENiso to simulate δ¹⁸O_TR values, (2) identifying the physical processes controlling δ¹⁸O_TR by mechanistic modelling and (3) defining the origin of the temperature signal recorded in the two species. Although the linear regression models used here to predict daily δ¹⁸O of precipitation (δ¹⁸O_P) may need to be improved in the future, the resulting daily δ¹⁸O_P values adequately reproduce observed (from weather stations) and simulated (by global circulation model) δ¹⁸O_P series. The δ¹⁸O_TR values of the two species are correctly simulated using the δ¹⁸O_P estimation as MAIDENiso input, although some offset in mean δ¹⁸O_TR levels is observed for the South American site. For both species, the variability in δ¹⁸O_TR series is primarily linked to the effect of temperature on isotopic enrichment of the leaf water. We show that MAIDENiso is a powerful tool for investigating isotopic fractionation processes but that the lack of a denser isotope-enabled monitoring network recording oxygen fractionation in the soil–vegetation–atmosphere compartments limits our capacity to decipher the processes at play. This study proves that the eco-physiological modelling of δ¹⁸O_TR values is necessary to interpret the recorded climate signal more reliably.