Emergence of a maximum soil flux under drought governs stomatal responsiveness to vapor pressure deficit over sub-daily and daily timescales

Drought and heat waves have synergistic effects on mortality as plants experience both supply and demand-side water stress. Which of these stresses ultimately controls stomatal conductance (gs) and whether patterns of regulation represent biological strategies or are imposed on the plant are addressed by way of a mechanistic hydraulic "null"model. Dynamic losses of soil hydraulic conductivity are modeled in a soil domain fed by a maximum water flux from a deep-water source. A root-uptake plane extracts water from the soil in a two-node (root and leaf) plant model, with gs a function of leaf water potential. Multi-day simulations reproduce previously published experimental observations of anisohydric to isohydric transitions, driven by the balance of soil supply and atmospheric demand. Feedforward control of transpiration E emerges from steep declines in soil hydraulic diffusivity that confine diurnal variation in moisture gradients and water discharge/recharge cycles to a shallow region at the root plane that thins with increased demand. Empirical models of VPD sensitivity are compared to the full model to provide mechanistic insight into empirical parameters. Apparent responses of gs to VPD are shown to emerge from plant, soil and atmospheric feedbacks that are both time and scale dependent.