Aims: Rising temperature and declining summer precipitation due to the 1970s-climate shift in southern South America have reduced forest productivity at dry sites. Here, we worked with the most widespread Southern Hemisphere treeline species, Nothofagus pumilio, across contrasting climatic conditions and determined whether rising atmospheric CO2 concentrations as well as warmer and drier climatic conditions provoked by the 70s-climatic shift have been causing systematic changes in treeline growth rates and intrinsic water-use efficiency (iWUE).Location: 36–54° S, southern Andes. Time period: 1950–2010. Major taxa studied: Nothofagus pumilio. Methods: We worked at five disparate climatic treeline locations, spanning 18 degrees of latitude; at each location, we sampled trees at four different elevations, including treeline elevation. We quantified the variation of annual tree-ring width (TRW) as a function of climate, elevation, tree age, size, annual CO2 concentrations, and location, using linear mixed-effects models and interpreted TRW trends in relation to iWUE and isotope (δ13C and δ18O) signalling. Results: Across locations, the patterns of treeline growth occurring in the 1980–2010 period exhibited a clear and significant negative trend, in contrast to the previous 1950–1980 period. We found an increase of iWUE and δ18O across time and locations. Given that an increase in δ18O indicates a decrease in stomatal conductance, we assert that drought-induced stomatal closure appears to be causing the reduction in growth. Main conclusions: We show unequivocal evidence that warmer and drier summer conditions translated into a decrease of growth rates along the elevational treeline of the southern Andes, reverting previous growth improvements linked to climate warming. An improvement in iWUE at all locations is most likely explained by decreased stomatal conductance given the rising δ18O signal. An iWUE–growth decoupling may act as an ecological strategy to respond to drought.