When do plant hydraulics matter in ecosystem modelling?

 The dynamics of the ascent of water from the soil to the leaves of vascular plants determine ecosystem responses to environmental forcing and their recovery from periods of water stress. Recently several models that describe the dynamics of plant hydraulics have been proposed. In this study we introduce four different configurations of a plant hydraulics model in an existing terrestrial biosphere model T&C. The model configurations in increasing order of complexity introduce the basics of the cohesion-tension theory, plant water storage dynamics and long-term damage and repair of the plant's water conducting system. Using the model configurations at six case studies spanning semi-arid to tropical ecosystems we quantify how plant hydraulics can modulate overall ecosystem responses to environmental forcing. As droughts develop, models with plant hydraulics predict a slower onset of plant water stress and can reproduce diurnal patterns of water and carbon fluxes that models that incorporate empirical stomatal conductance only cannot capture. However, when the complex variability of the environmental forcing (i.e., observed hourly meteorological forcing driving the models) is considered, plant hydraulics alone cannot significantly improve model performance. Models that only have simple empirical stomatal conductance models can adequately capture most of the variability of the observed ecosystem responses without explicitly simulating plant hydraulics. Most of the time, the gain from introducing plant hydraulics in ecosystem modelling is limited compared to the possible model improvements from correct representation of other processes such as plant phenology. Nevertheless, during periods of water stress, only models that explicitly simulate plant hydraulics can reproduce observed ecosystem responses to stress and the dynamics of ecosystem recovery. Finally, sensitivity analyses highlight that accurately modelling plant hydraulics relies on good knowledge of plant hydraulics traits, particularly at the leaf level, as stomata are usually the hydraulic bottleneck in the water flow from the soil to the atmosphere.