Increased water use efficiency leads to decreased precipitation sensitivity of tree growth, but is offset by high temperatures.

Both increases in temperature and changes in precipitation may limit future tree growth, but rising atmospheric CO 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 CO on tree growth is crucial for ecotones, where increased climate stress could drive mortality and shifts in range. Here, we quantify the effects of climate, stand structure, and rising CO on both annual tree-ring growth increment and intrinsic WUE (iWUE) at a savanna-forest boundary in the Upper Midwest United States.