Can tree-rings inform assisted migration? Revisiting provenance trials across Atlantic Canada to compare local adaptation between red spruce populations

As climate niches of most tree species are projected to shift rapidly in the coming decades, forest-assisted migration (FAM) of populations from warmer sites is a promising silvicultural tool to help forests adapt to global changes. However, additional knowledge on species-specific local genetic adaptation and phenotypic plasticity is needed to inform the deployment of FAM in forest management. Here we applied a novel dendroecological approach to a unique network of ten 60 year-old provenance trials covering a large climate gradient in eastern Canada to assess the FAM potential of red spruce, a species unique to the Acadian-Wabanaki forest region projected to decline under climate change. We first controlled for non-climatic growth drivers by applying a Bayesian hierarchical model to individual, annual tree growth records extracted from tree-rings. Non-climatic variables explained 75.6 % and 92 % (posterior mean Bayesian R²) of tree-level and site-level growth, respectively. Across populations and sites, residual annual growth displayed the strongest correlations with summer climate, with growth declines during warm and dry summer conditions. In contrast with summer patterns, warmer and drier spring conditions were generally favorable to growth, while winter temperatures had only marginal associations with growth. To test the hypothesis that red spruce populations from warmer locations are good candidates for assisted migration, we assessed significant changes in climate-growth correlations as a function of climate transfer distance, calculated as the differences between population origin climate and climate at each trial site. We found that trees moved to colder sites, which simulates standard FAM practices, were significantly less sensitive to summer water deficit and changes in precipitation. However, additional variation in climatic sensitivity to summer temperature suggests potential risks at elevated transfer distances. Results from our application of dendroecology to existing provenance trials provides support for red spruce as a candidate species for FAM.