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Both increases in temperature and changes in precipitation may limit future tree growth, but rising atmospheric CO2 could offset some of these stressors through increased plant Water Use Efficiency (WUE). The net balance between the negative impacts of climate change and positive effects of CO2on tree growth will be most important for systems already at plant physiological limits, where increased climate stress could drive mortality and shifts in range distribution. Here, we quantify the effects of climate, stand structure, and rising CO2on both annual tree-ring growth increment and WUE at a savanna-forest boundary in the Upper Midwest United States. Taking a Bayesian hierarchical modelling approach, we find that plant WUE increased by ~13-25% over the course of the 20thcentury, but on average, tree-ring growth increments do not significantly increase. Consistent with higher WUE under increased CO2 and recent wetting, we observe a decrease in sensitivity of tree growth to annual precipitation, leading to 25-65% higher growth under dry conditions compared to trees of similar age and size in the past. However, an emerging interaction between summer maximum temperatures and annual precipitation diminishes the water-savings benefit under hot and dry conditions. Both the decrease in precipitation sensitivity, and the interaction between temperature and precipitation are strongest in open canopy microclimates, suggesting that stand structure may modulate response to future changes. Overall, we find that while higher WUE may provide some water savings benefits to growth under normal drought conditions, near-term future temperature increases combined with drought events could drive growth declines of over 50%. These products are used in the manucript, Heilman et al., 2020, Increased water use efficiency leads to decreased precipitation sensitivity of tree growth, but is offset by high temperatures. Submitted for review. The tree rings in this data package and those in msb-paleon packages 36-43 correspond with the isotope data in msb-paleon package 34. This material is based upon work supported by the National Science Foundation under grants #DEB-1241874, 1241868, 1241870, 1241851, 1241891, 1241846, 1241856, 1241930.