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 (psi(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 10x 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); psi(soil) profiles can provide mechanistic insights into ecosystem-level water fluxes.