A global three-dimensional leaf functional space reveals partial decoupling between carbon investment and water regulation

Plant-atmosphere exchanges of CO2 and water are commonly parameterised using trait-based assumptions that implicitly couple leaf economic strategies with hydraulic and stomatal regulation. Yet the extent to which leaf carbon-water economy traits and hydraulic traits constitute a unified versus independent constraint remains unclear at the global scale. We compiled a global dataset of seven leaf functional traits across 3,179 species to assess the dimensional architecture and ecological coordination of leaf carbon and water regulation. Our analyses revealed that carbon-water economy traits and hydraulic traits of leaves collectively forming a three-dimensional functional space. Stepwise incorporation of leaf mass per area, leaf nitrogen concentration and δ13C markedly increased the effective number of dimensions in the hydraulic functional space-by up to 195%-but had minimal impact on the internal coordination, especially in non-woody species. Furthermore, patterns of trait divergence were strongly conserved at the family and biome levels, while no consistent alignment was observed across climatically similar regions. Together, these results suggest that leaf carbon-water economy and hydraulics are partially decoupled but linked through integrative water-use strategies, with implications for improving trait-based parameterization and vegetation responses to changing evaporative demand and drought.