Abstract
Plant resistance to drought has long been thought to be associated with the ability to maintain transpiration and photosynthesis longer during drought, through the opening of stomata. This premise is at the root of most current framework used to assess drought impacts on land plants in vegetation models. We examined this premise by coupling a meta-analysis of functional traits of stomatal response to drought (i.e. the water potential causing stomatal closure, ψclose) and embolism resistance (the water potential at the onset of embolism formation, Ψ12), with simulations from a soil-plant hydraulic model. We found that ψclose and Ψ12 were equal (isometric) only for a restricted number of species, but as Ψ12 decreases, the departure from isometry increases, with stomatal closure occurring far before embolism occurs. For the most drought resistant species (Ψ12<-4.2 MPa), Ψclose was remarkably independent of embolism resistance and remained above -4.5 MPa, suggesting the existence of a restrictive boundary by which stomata closure must occur. This pattern was supported by model simulations. Indeed, coordinated decrease in both ψclose and ψ12 leads to unsuspected accelerated death under drought for embolism resistant species, in contradiction with observations from drought mortality experiments. Overall our results highlight that most species have similarity in stomatal behavior, and are highly conservative in terms of their water use during drought. The modelling framework presented here provides a baseline to simulate the temporal dynamic leading to mortality under drought by accounting for multiple, measurable traits.
