Nature-based solutions to the climate crisis increasingly focus on forests. In this research, we address the question “how much CO2 will trees remove from the atmosphere in the future?”, by projecting the future absolute growth of trees under future climate conditions. We confront the problem of extrapolation – i.e., that we must predict the response of trees to climate that is increasingly different from historical conditions. We evaluate first the predictability of average ring width and interannual climate sensitivity across environmental gradients, and second the appropriateness of using this spatial variation in tree response to climate for space-for-time substitution, using data spanning the entire geographic distribution of Douglas-fir (Pseudotsuga menziesii). Ring-width response to spatial versus temporal temperature variation was opposite in sign, such that spatial variation in productivity, caused by local adaptation and other slow processes, cannot be used for space-for-time substitution. We treat climate sensitivities as substitutable, to generate projections of future Douglas-fir growth, and discuss the need to determine reaction norms from tree-ring time series data to improve forecasts of tree growth under future climates.