Tree-Level Recovery from Hydraulic Stress: Insights from Experiments and Hydrodynamic Models

Modeling of plant hydraulics is at the forefront of development in vegetation and land-surface models. Numerical tools that consider water flow within the conductive system of plants, and particularly trees, have been developed and used in studies of hydraulic strategy and consequences of hydraulic behavior for drought tolerance. Several established land-surface models such as ED2, CLM, and E3SM have recently developed "hydro" versions and are ready to extrapolate the consequences of including tree hydraulic behaviors into large scale and global simulations. At the core of any plant hydrodynamic model is the assumption that xylem conductance is dependent on xylem water potential. This is typically represented using a Percent Loss of Conductivity (PLC) equation. Mathematically, these functions are reversible and do not include explicit hysteresis, i.e. as soon as water potential increases pre-stress levels of conductivity are regained. However, biological evidence suggests that post-stress recovery mechanisms are much slower, and the recovery of hydraulic conductance does not directly follow water potential.

We propose a range of potential recovery responses. At one end, a fully and quickly reversible recovery implies no tissue damage and is the path that corresponds to today's model formulations. Moderate recovery, driven by some tissue damage, can lead a slower return to maximal xylem and leaf conductivity in comparison to the recovery of soil water availability. At the other extreme, severe damage to conductive tissue may require wood growth to repair, and prevents return to pre-stress conductivity over the long-term.

We use observations of gas exchange, sap flow, soil moisture and evapotranspiration in a climate-controlled greenhouse and in a natural forest to characterize post-stress recovery. We propose a formulation of a time-dependent stress function that models the recovery of conductivity by introducing hysteresis to the values of the maximal xylem and leaf conductance parameters. We use a set of simulations in a plant hydraulic model, FETCH2, and an ecosystem model, Thetys-Chloris, to test the realism of the dynamics of post-stress recovery that result from these new formulations.