Forest system hydraulic conductance: partitioning tree and soil components

Soil–leaf hydraulic conductance determines canopy–atmosphere coupling in vegetation models, but it is typically derived from ex-situ measurements of stem segments and soil samples. Using a novel approach, we derive robust in-situ estimates for whole-tree conductance (k_tree), ‘functional’ soil conductance (k_soil), and ‘system’ conductance (k_system, water table to canopy), at two climatically different tropical rainforest sites. Hydraulic ‘functional rooting depth’, determined for each tree using profiles of soil water potential (Ψ_soil) and sap flux data, enabled a robust determination of k_tree and k_soil. k_tree was compared across species, size classes, seasons, height above nearest drainage (HAND), two field sites, and to alternative representations of k_tree; k_soil was analysed with respect to variations in site, season and HAND. k_tree was lower and changed seasonally at the site with higher vapour pressure deficit (VPD) and rainfall; k_tree differed little across species but scaled with tree circumference; r_soil (1/k_soil) ranged from 0 in the wet season to 10× less than r_tree (1/k_tree) in the dry season. VPD and not rainfall may influence plot-level k; leaf water potentials and sap flux can be used to determine k_tree, k_soil and k_system; Ψ_soil profiles can provide mechanistic insights into ecosystem-level water fluxes.