This study is the first presenting annually resolved chronologies of both δ¹⁸O and δ¹³C in Nothofagus pumilio and Fitzroya cupressoides trees from Northern Patagonia. Interannual variability in δ¹⁸O and δ¹³C was assessed over the period 1952-2011. Based on these chronologies, we determined the primary climatic controls on stable isotopes and tree physiological responses to changes in atmospheric CO₂ concentrations (c_a), temperature and humidity. Changes in specific intrinsic water use efficiency (iWUE) were inferred from variations in δ¹³C whereas the effects of CO₂ increase on stomatal conductance were explored using δ¹⁸O. Over the 60-year period, iWUE increased significantly (by ca. 25%) in coincidence with the rise of c_a. The two species appear to have different strategies of leaf gas-exchange. Whereas iWUE variations were likely driven by both stomatal conductance and photo-synthetic assimilation rates in Fc, they were largely related to stomatal conductance in Np. After removing the low-frequency trends related to increasing c_a, significant relation-ships between δ¹³C and summer temperatures were recorded for both species.  However, δ¹³C variations in Fc were more strongly influenced by summer temperatures than in Np Our results advocate for an indirect effect of summer temperatures on stable isotope ratios, which are mostly influenced by sunlight radiation in Fc and relative humidity/soil moisture in Np. δ¹³C variations in Fc were spatially correlated to a large area south of 35°S in southern South America. These promising results encourage the use of δ¹³C variations in Fc for reconstructing past variations in temperature and large-scale circulation indexes such as the Southern Annular Mode.

Authors : A. Lavergne, V. Daux V., R. Villalba , M. Pierre, M. Stievenard, A. Srur