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Abstract

As rapid climate change intensifies threats to forests, evaluating the strength, scale, and drivers of local adaptation to climate, as well as intraspecific genetic variation of climate adaptive traits, can improve predictions of forest responses and evaluating management strategies such as assisted migration. Additionally, assessing how tree breeding for growth affects climate adaptation could help mitigate potential impacts on forest health and productivity. I investigated local adaptation to climate in western larch (Larix occidentalis Nutt.) by combining extensive phenotyping from a seedling common garden experiment with 52 natural populations and genomic data from targeted exome-capture (pooled sequencing) of 44 populations from an adult provenance trial. I assessed adaptive and breeding potential by quantifying genetic variation and trait correlations at population and family levels under contrasting soil moisture conditions. I also evaluated the relative contributions of climate, geography, and neutral genetic structure to patterns of landscape genomic variation. Finally, I examined the effects of breeding for growth on climate adaptive traits by comparing 29 selectively-bred families and 23 natural populations from two breeding zones in British Columbia. Phenotypic and genomic evidence of local adaptation to climate was weak. Population differentiation was low for most traits, with weak clines associated with longitude and elevation rather than climatic variables expected to drive local adaptation. Partitioning of landscape genomic variation indicated that neutral genetic structure explained most of the genetic variance among populations, with climate and geography explaining little additional genetic variance. Genetic variation and heritability for drought resistance was limited, and genetic variation declined in growth and phenology under drought, indicating constraints to drought adaptation. Breeding for growth had minimal effects on climate adaptive traits, indicating that breeding has not likely altered risks associated with assisted migration. Overall, weak local adaptation suggests that assisted gene flow among populations may have limited benefits and risks for western larch. Combined with recent projected reductions in climatic suitability within the current range and expansion beyond the range based on climate niche models, these findings indicate that assisted migration beyond the natural range will likely be an important management strategy for western larch forests under future climates.