Forecasts of species’ climate change responses often make the simplifying assumptions that (1) all populations have the same climatic tolerances, and (2) those tolerances can be inferred from species-wide responses. These assumptions underlie the use of climate envelope approaches for forecasting range dynamics. However, recent empirical work has shown that population-level climate responses can vary widely and even be opposite in sign to species-wide responses. Thus, the question is whether species’ fates in future climates will be determined by species-wide responses, as forecasted by climate envelope approaches, or locally differentiated population-level responses. Here we use tree-ring data to examine the consequences of contrasting species-wide and population climate responses, and test which better predicts observed growth responses to recent climate change. Using ponderosa pine as a model species and case study, we focus on 24 populations across the western US that have experienced varying degrees of warming over the past century. We divided growth time series data from each population into pre-warming (1900-1982) and post-warming (1983-2015) periods, defined empirically with breakpoint regression of climate time series data. We modeled pre-warming growth variation as a function of spatially-varying climate normals, capturing the species-wide response, and annually-varying seasonal climate, capturing population-specific responses.
Results/Conclusions
We used this model to (1) project post-1982 growth following species-wide versus population-specific responses through the end of the 21st century under future climate scenarios, and (2) compare predicted post-warming (1983-2015) growth to observed growth. Based on species-wide climate responses, growth increases of up to 200% were predicted across all populations, whereas population-specific climate responses predicated growth decreases of up to 75%. Observed post-warming growth correlated poorly with species-wide predictions, with correlations between predicted and observed growth trending increasingly negative at sites that experienced more warming. Taken together, our results indicate that trees have responded to recent warming according to population-specific climate sensitivities, and that future climate change impact projections that ignore these population-level processes will be increasingly wrong as the climate continues to change.
