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

Here, using 141 poplar sapling individuals spanning a size gradient in different ontogenetic stages, we quantified aboveground biomass (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 leaf mass scaled with stem mass with an exponent of 0.80, higher than the theoretical 3/4 but below isometry. The ratio of leaf to stem hydraulic resistance decreased with tree size following an exponent of –0.38. Shoot hydraulic resistance on a leaf-area basis increased with tree size following a 0.24 power law, but tended to level off in larger individuals, while the relative contribution of leaves declined to a steady value of 57% of the total aboveground resistance. Stomatal conductance and transpiration rate also declined with tree size in an allometric way, coordinated with hydraulic reconfiguration to stabilize leaf water potential.