Coupled allometric scaling in biomass allocation and hydraulic resistance partitioning sustains water homeostasis throughout ontogeny

Water homeostasis is essential for plant survival, yet how size-dependent shifts in hydraulic resistance between stems and leaves, coupled with allometric biomass allocation to sustain water homeostasis, remains unresolved. Here, using 141 poplar saplings spanning a size gradient in different ontogenetic stages, we quantified aboveground biomass (AGB) (a proxy for tree size), biomass allocation, and hydraulic resistance partitioning between stems and leaves, as well as leaf gas exchange and water potential. We found that leaves scaled with stems following nonlinear allometries: exponent of 0.80 for biomass and 1.35 for absolute hydraulic resistance, both significantly differing from isometric scaling. Consequently, both the leaf-to-stem biomass ratio and hydraulic resistance ratio declined with tree size, while hydraulic resistance on a leaf-area basis increased. Increasing resistance under stable leaf water potentials explained the decline in leaf-level transpiration with tree size. Asymptotic models fitted all traits better than power-law models, indicating biomass allocation and hydraulic properties approached constant values as trees grow. Our results demonstrate that the coupled allometric scaling between AGB allocation and hydraulic resistance partitioning biomass is synchronized with stomatal closure that collectively maintains constant leaf water potential with size during early tree ontogeny, providing empirical scaling rules for large-scale vegetation models.