Provenance effect on the fine-root metabolome and structure of mature Scots pine revealed in a long-term common garden experiment.

Rapid changes in temperature and precipitation present a major challenge for long-lived forest trees, as their long generation times limit the required rate of adaptive responses within populations. Common garden experiments provide valuable insights into local adaptation, but belowground responses remain understudied. We investigated fine root metabolomes, biochemistry, and structure in 54-year-old Pinus sylvestris originating from two contrasting environments: a local lowland site and a colder high-elevation mountain region, both grown in a common garden. Untargeted GC-MS/MS profiling identified 160 metabolites and revealed pronounced effects of both season and tree origin. In spring, roots from high-elevation trees exhibited a distinct metabolomic profile characterized by lower concentrations of nitrogen-containing compounds and elevated levels of lipid derivatives, including fatty acids and sterols. In contrast, autumn roots from both origins showed broadly similar metabolite abundances. According to biochemical analyses, seasonal shifts were also evident in carbohydrate dynamics, with higher concentrations of non-structural carbohydrates in spring and increased structural carbohydrates deposition in autumn. Nitrogen compounds, particularly amino acids, increased in autumn roots of both tree origins, most likely reflecting nutrient storage. Differences related to tree origin extended to root morphology, as roots from high-elevation trees had higher cell wall carbohydrate content and altered root branching patterns compared to those from lowland trees. Our findings suggest that tree origin leaves a clear imprint on the fine root metabolome and structure of mature Scots pine, with most pronounced differences occurring in early spring. These results highlight how inherited adaptation to origin climates interacts with seasonal soil processes. High-resolution metabolomics thus offers a powerful tool for detecting subtle belowground provenance-related belowground effects that may be overlooked by traditional trait-based approaches, providing new perspectives on possible tree adaptation under climate change.