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

Oxygen isotopes in tree rings (δ¹⁸OTR) 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 δ¹⁸OTR 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 δ¹⁸OTR 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 δ¹⁸OTR values, (2) identifying the physical processes controlling δ¹⁸OTR 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 (δ¹⁸OP) may need to be improved in the future, the resulting daily δ¹⁸OP values adequately reproduce observed (from weather stations) and simulated (by global circulation model) δ¹⁸OP series. The δ¹⁸OTR values of the two species are correctly simulated using the δ¹⁸OP estimation as MAIDENiso input, although some offset in mean δ¹⁸OTR levels is observed for the South American site. For both species, the variability in δ¹⁸OTR 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 δ¹⁸OTR values is necessary to interpret the recorded climate signal more reliably. [ABSTRACT FROM AUTHOR]