Landscape Genomics and Climate Projections Reveal Genomic Offset and Provenance Vulnerability in Picea sitchensis

Sitka spruce (Picea sitchensis (Bong.) Carr.) is the dominant plantation conifer in Atlantic Europe, yet the genomic basis of provenance-level climate adaptation remains poorly resolved. We applied gradient forest analysis to 31,049 genome-wide SNPs from 1106 individual trees representing 79 native-range provenances in the IUFRO collection, using three environmental predictors retained after collinearity screening: Longitude, Temperature Seasonality, and Annual Precipitation. Longitude was the dominant driver of genomic turnover (mean R2 = 0.01168), followed by Temperature Seasonality (0.00672) and Annual Precipitation (0.00228), reflecting the long-distance coastal gradient of the native range. A redundancy analysis conditioned on ancestry principal components confirmed a significant multivariate genotype–environment association (F = 8.679, p = 0.001). Genomic offset was negatively correlated with all three provenance-level performance traits measured at the IUFRO common garden after 50 years of growth: height, stem diameter and stem quality, providing empirical validation of the genomic-climatic framework. Projecting the fitted model onto European planting sites using an 8-GCM CMIP6 ensemble showed mean offset increasing from 0.0021 (SSP2-4.5, 2041–2060) to 0.0041 (SSP5-8.5, 2061–2080), with the most climate-exposed cells under the high-emission late-century scenario approaching the upper tail of the source population offset distribution. The European planting region showed a higher projected offset than the North American source range under all scenarios. This supports the hypothesis that provenances from Oregon to southern British Columbia are most suited for planting in regions under future Atlantic European conditions.