Outpaced by Drought: Weak Local Adaptation and Limited Potential for Drought Adaptation in a Deciduous Conifer

As drought impacts on forests intensify, evaluating the potential for adaptation is critical for predicting forest responses and the impacts of management strategies. We assessed phenotypic and genomic variation in western larch (Larix occidentalis Nutt.) populations to evaluate the potential for drought adaptation across its range. First, we established a seedling common garden experiment outside the natural range with 52 populations subjected to two drought treatments. Second, we analyzed pooled targeted exome-sequence capture data from 44 populations of an adult provenance trial to evaluate the relative ability of climate, geography, and neutral genetic structure to predict landscape genomic variation. We found that population differentiation for drought resistance was low. Drought reduced population differentiation for growth and bud set, and weakened clinal associations for growth. We found no antagonistic correlations between drought resistance and growth or phenology, indicating that trade-offs are unlikely to constrain adaptation. Further, landscape genomic variation was primarily explained by neutral genetic structure, with little additional variation explained by climate or geography. Together, these findings demonstrate weak local adaptation to drought and reduced genetic variation in traits under drought indicate that natural populations of western larch may have limited potential to adapt to future drought conditions.