Employing a Hysteresis Approach to Analyze Shifts in Tree Physiological Thresholds in Response to Drought

Drought-induced tree mortality underscores the need for improved physiological models to predict tree responses to water stress. We employed a hysteresis modelling approach analyzing diel water absorption and desorption cycles to understand drought-induced shifts in physiological thresholds. We applied this approach to sap flow (Js), stem diameter fluctuation (SDF), stem volumetric water content fluctuation (SVWCF), and vapour pressure deficit (VPD), datasets from two throughfall reduction experiments in contrasting ecosystems: a cloud forest (Peru) and a temperate forest (USA). Using a generalised transcendental equation, we quantified five diurnal hysteresis loops (Js:VPD, Js:SDF, Js:SVWCF, SDF:VPD, SVWCF:VPD) and derived hysteresis parameters (centroids, area, lag and angle of rotation) and physiological thresholds, i.e. transitions from water absorption to desorption and from water desorption to absorption. The model captured nonlinear physiological responses and highlighted the hysteresis strength and out-of-phase dynamics in each hysteresis loop. Droughted trees exhibiting contrasting patterns of stem shrinkage and moisture depletion revealed distinct water-use strategies: a stem water-extractive strategy under stress, relying on internal water storage, versus a conservative drought-avoidance strategy, maintaining a positive stem water balance. Our findings suggest that hysteresis-derived parameters serve as diagnostic tools and early indicators of drought stress, improving predictions of tree hydraulic resilience and water use strategies.