Unveiling the integration of above- and below-ground tree carbon-hydraulic traits in Amazonian trees across hydrological niches.

Understanding trait coordination and trade-offs along the root-to-leaf hydraulic pathway is critical for assessing forest functioning, as these traits significantly impact ecosystem carbon allocation and water use. Here, we investigated the relationship between carbon and hydraulic traits in 11 Amazonian tree species distributed across vertically structured hydrological niches. Using a carbon-hydraulic framework, we tested the hypothesis that interspecific differences arise from the optimization of xylem hydraulic efficiency, reflecting how tropical trees balance water transport efficiency with the carbon costs of maintaining transport tissues across vertical canopy positions. Our results show that above-ground traits were largely explained by canopy position (vertical stratification), whereas below-ground carbon-hydraulic traits were predominantly influenced by interspecific differences. Upper canopy trees exhibited lower and less variable specific root length (SRL) than shallow-rooted understory trees, indicating divergent carbon allocation strategies. Thicker terminal roots had higher hydraulic conductivity (Ks) than finer roots, but Ks declined from roots to terminal branches in most species. Additionally, branch and leaf Ks increase with tree size, indicating greater hydraulic efficiency in larger canopy species. Below-ground, we presented evidence that an increase in SRL is linked to decreased hydraulic conductivity and is influenced by root diameter. Above-ground, branch and leaf hydraulic conductivity tend to be higher in species with higher wood density, which are also more prevalent in upper canopy layers. Together, our findings reveal a coordinated above- and below-ground carbon-hydraulic trait framework across Amazonian trees. Species that occupy different vertical above-ground hydrological niches in lowland Amazon forests exhibit different carbon allocation strategies, which helps explain variation in species dominance and resource use throughout the vertical forest profile.