Functional traits and individual tree growth relationship weakens with stand development but strengthens with increasing temperature

Understanding spatiotemporal variation in individual tree growth–functional traits relationships (GTRs) is crucial to predicting forest growth responses to changing environments, aiding in long‐term forest planning and sustainability. Although weak GTRs have been frequently observed within individual forest site or across sites of similar climates and development stages, GTRs at large spatial scales remain uncertain. We hypothesize that GTRs at large scales are regulated by stand development and regional climate through their effects on competition intensity and tree mortality rates. We used forest inventory data in the United States (9828 plots and 228,981 trees) to investigate how stand age and regional climate (temperature and climate moisture index) regulate GTRs. We found an overall positive relationship between relative tree growth rate and stem traits associated with acquisitive strategies (greater height and lower wood density). However, leaf traits associated with acquisitive strategies (higher specific leaf area, leaf nitrogen and phosphorus content) exhibited divergent effects, promoting growth in angiosperms but reducing growth in gymnosperms. Importantly, GTRs weakened with stand age but strengthened with increasing mean annual temperature. Structural equation modelling showed that stand age indirectly weakened GTRs by increasing stand basal area and tree mortality. Synthesis. Our findings suggest that current efforts focusing on planting acquisitive tree species for rapid carbon sequestration may become less effective as forests mature, especially in conservation forests aimed at providing long‐term ecosystem services. Therefore, mixing conservative trees with longer growth cycles alongside acquisitive species could be a forest management strategy to enhance carbon sequestration over the long term.