Aim Rising atmospheric CO 2 and climate warming have induced changes in tree growth and intrinsic water-use efficiency (iWUE) world-wide, but the long-term impact of such changes on terrestrial productivity remains unknown. Based on a synthesis of the literature, here we investigate the net impact of recent atmospheric changes across forest biomes. Location A range of sites covering major forest biomes. Methods We use dendrochronological and isotopic records to provide an integrated analysis of changes in growth and iWUE, evaluating the impacts of atmospheric changes in tree growth. In our analysis, positive relationships between changes in growth and iWUE reflect CO 2 stimulation, while neutral effects yield inflections in growth curves (plotted against iWUE), and negative relationships indicate the prevalence of stressors. To estimate net effects (since 1960) and compare responses across biomes, we use a response contrast (RC) index, based on the ratio between cumulative changes in growth and iWUE. Results In 37 recently published case studies changes in iWUE were consistently positive, increasing by between 10 and 60%, but shifts in growth varied widely within and among forest biomes. Positive RC values were observed in high latitudes (>?40 degrees N), while progressively lower (always negative) responses were observed toward lower latitudes. Growth rates declined between 15 and 55% in tropical forests. In subtropical sites growth declined by between 7 and 10%, while mixed responses occurred in other regions. Main conclusions Over the past 50 years, tree growth decline has prevailed despite increasing atmospheric CO 2. The impact of atmospheric changes on forest productivity is latitude dependent (R 2?=?0.9, P?<?0.05), but our results suggest that, globally, CO 2 stimulation of mature trees will not counteract emissions. In most surveyed case studies warming-induced stress was evoked to explain growth decline, but other factors, such as nutrient limitation, could have overridden the potential benefits of rising CO 2 levels.
