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SignificanceThe metabolism of North America’s oldest boreal trees (Thuja occidentalis L.) is strongly affected by rising anthropogenic normalCnormalO2 emissions. Intrinsic water use efficiency (italiciWUE) increased dramatically, although nonlinearly, since the beginning of the industrial Era. Our study shows that while T. occidentalis L. acclimated to rising [normalCnormalO2], no stem growth was observed, suggesting that trees could not benefit from the increased italiciWUE.

Dendrochronology (i.e., the study of annually dated tree-ring time series) has proved to be a powerful technique to understand tree-growth. This paper intends to show the interest of using ecophysiological modeling not only to understand and predict tree-growth (dendroecology) but also to reconstruct past climates (dendroclimatology). Process models have been used for several decades in dendroclimatology, but it is only recently that methods of model-data fusion have led to significant progress in modeling tree-growth as a function of climate and in reconstructing past climates. These model-data fusion (MDF) methods, mainly based on the Bayesian paradigm, have been shown to be powerful for both model calibration and model inversion. After a rapid survey of tree-growth modeling, we illustrate MDF with examples based on series of Southern France Aleppo pines and Central France oaks. These examples show that if plants experienced CO 2 fertilization, this would have a significant effect on tree-growth which in turn would bias the climate reconstructions. This bias could be extended to other environmental non-climatic factors directly or indirectly affecting annual ring formation and not taken into account in classical empirical models, which supports the use of more complex process-based models. Finally, we conclude by showing the interest of the data assimilation methods applied in climatology to produce climate re-analyses.