Phytochemical divergence between above- and belowground tree organs along elevational gradients.

Plants produce diverse metabolites to defend against biotic and abiotic stresses. However, quantification of plant chemical defenses along environmental gradients has largely focused on aboveground organs, leaving belowground patterns poorly understood. To address how leaf and root defense chemistry shifts with changing biotic and abiotic conditions, we compared untargeted metabolomics of 183 species across elevational gradients spanning tropical, subtropical, and subalpine forests. Leaves and roots showed distinct chemical compositions across elevations, with phytochemical diversity declining from tropical to subalpine forests, corresponding with shifts in herbivore pressure and soil conditions. These patterns suggest that local variation in biotic and abiotic pressures across elevations may be associated with gradients in phytochemical diversity. We also uncovered contrasting patterns of chemical strategies between organs. Leaf chemistry shifts from polar, fast-turnover, low-cost metabolites in tropical forests to less polar, aromatic, and energetically costly metabolites in subtropical and subalpine forests, whereas roots display contrasting metabolic adjustments. Our results suggest that elevation filters species by their distinct metabolomic profiles, with organ-specific and context-dependent chemical investment, highlighting the importance of integrating above- and belowground organs to understand plant chemical responses across gradients. We reveal that organs of the same tree tell distinct chemical stories, illuminating the chemical life of trees.