Inclusion of a cold hardening scheme to represent frost tolerance is essential to model realistic plant hydraulics in the Arctic-boreal zone in CLM5.0-FATES-Hydro

As temperatures decrease in autumn, vegetation of temperate and boreal ecosystems increases its tolerance to freezing. This process, known ashardening, results in a set of physiological changes at the molecular level that initiate modifications of cell membrane composition and thesynthesis of anti-freeze proteins. Together with the freezing of extracellular water, anti-freeze proteins reduce plant water potentials and xylemconductivity. To represent the responses of vegetation to climate change, land surface schemes increasingly employ "hydrodynamic" models thatrepresent the explicit fluxes of water from soil and through plants. The functioning of such schemes under frozen soil conditions, however, ispoorly understood. Nonetheless, hydraulic processes are of major importance in the dynamics of these systems, which can suffer from, e.g., winter"frost drought" events. In this study, we implement a scheme that represents hardening into CLM5.0-FATES-Hydro. FATES-Hydro is a plant hydrodynamics module in FATES, acohort model of vegetation physiology, growth, and dynamics hosted in CLM5.0. We find that, in frozen systems, it is necessary to introducereductions in plant water loss associated with hardening to prevent winter desiccation. This work makes it possible to use CLM5.0-FATES-Hydro tomodel realistic impacts from frost droughts on vegetation growth and photosynthesis, leading to more reliable projections of how northern ecosystemsrespond to climate change.