Assessing Future Maladaptation Risk in the Keystone Species Populus tremuloides.

Understanding the adaptive potential of widespread trees is critical for predicting species resilience under climate change and informing their conservation strategies. We assessed the genomic offset in Populus tremuloides Michx. (aspen) using 10,937 genotyping-by-sequencing derived, high-quality SNPs from 891 individuals sampled across the species' range, from Mexico to both the eastern and western regions of North America. Gradient forest models were applied using multiple SNP datasets and climate scenarios, with the genomic offset standardized by aspen population genetic clusters and ecological regions to identify future habitat suitability (recipient importance) and potential sources of reproductive material (donor importance) for assisted migration. Moisture-related variables exerted consistently stronger selective pressure on genetic variation than temperature, reflecting the riparian ecology of aspen. Populations from Mexico and the southwestern U.S.A. exhibited the highest offsets, suggesting elevated vulnerability despite apparent drought adaptations, whereas northern populations showed the lowest offsets, consistent with projected northward range shifts. Traits not captured by genomic offset models, such as polyploidy, clonality, hybridization, and epigenetics, may buffer environmental stress and enhance resilience, underscoring the limitations of purely genomic predictions. To complement offset estimates, we tested linear models using phenotypic data (growth, polyploidy), but the limited representation of aspen populations in common garden trials restricted validation. Nevertheless, a positive association between polyploidy and offset emerged, suggesting a potential adaptive or buffering mechanism that warrants further investigation. Together, our results illustrate both the promise and the limitations of genomic offset as a predictive framework for widely distributed species. By presenting the first approach to scale the genomic offset to a broadly-distributed forest tree, this study provides a foundation for integrating genomic, phenotypic, and ecological data to inform climate-resilient forest management. Range-wide common garden and provenance trials remain essential, yet they remain scarce for broadleaf species such as aspen. Integrating multiple data sources is critical for developing effective climate-resilient forest management strategies.