Plant Effective Conductance Fails to Predict Soil–Plant Hydraulics in Sandy Soils

Abstract. Atmospheric and soil dryness impose increasing limitations in the hydraulic pathway of the soil–plant–atmosphere continuum (SPAC), leading to a decrease in the hydraulic conductivity of many elements, such as soil, soil–root interface, xylem, and leaves. Many SPAC models simplify the flow through this series of hydraulic conductances to a single effective conductance that varies as a function of the xylem water potential. This effective conductance is expected to implicitly account for soil characteristics, including soil texture-specific hydraulic limitations. However, it is unclear whether such an approach can properly reproduce the non-linear effects of soil texture-specific conductivities. This paper aims to explore under what conditions reducing the SPAC to a function of internal plant water status accurately captures the relation between transpiration rate and leaf and soil water potential, and when larger model complexity is required. For this purpose, a plant-only model, where the conductances along the SPAC solely depend on plant water potential, was compared to a more detailed model incorporating the hydraulics of varying soils. We demonstrate that the former simplification works well in fine-grained soils, such as clay and silt loam, but fails in more coarse-textured soils like sandy loam, where the pronounced non-linearity of its hydraulic functions requires explicit description. Additionally, a sensitivity analysis of key parameters within the full SPAC model reveals that the soil hydraulic properties have a significantly stronger impact on plant transpiration in sandy soils, as opposed to finer soils, where internal plant traits are more influential on plant–water relations.