Ubiquitous local differentiation in effects of climate on growth of temperate trees: Implications for forest carbon under climate change

Temperate tree species are notable for their broad distributions along climate gradients and their ability to tolerate highly variable conditions on diurnal to interannual time scales. This reflects the aggregate effects of genetic adaptation, morphological and physiological acclimation, and physiological tolerance. Understanding differentiation in the responses of local populations to spatial and temporal climate variability and incorporating its effects in models of forest dynamics remains a significant challenge in forecasting the effects of future climate change on tree species distributions and ecosystem processes. To test for and quantify the strength of local differentiation in the 50 most common tree species in forests of the eastern United States, we compiled data on tree growth from forest inventory plots and used imputation methods to fit statistical models for growth that incorporate both regional variation in long‐term mean temperature and precipitation, and local interannual variation in climate relative to the long‐term mean. The best supported models for 41 of the 50 species include local differentiation in response to climate. Most species showed roughly linear increases in growth as a function of precipitation along regional rainfall gradients, but with even stronger increases in growth when rainfall in a given year was greater than the long‐term mean at a location. Responses of species to regional and interannual variation in temperature were more varied, with regional variation often exceeding interannual variation in growth at a given location. Synthesis . Failure to incorporate these patterns of local differentiation in the SORTIE‐ND model of forest dynamics under an intermediate emissions pathway (RCP 4.5) results in a significant over‐estimate of future carbon stocks and sequestration. The degree of reduction in forest carbon stocks predicted when local differentiation is included depends on whether the differentiation reflects a rapid ability to acclimate to changing climate or a much slower genetic adaptation.