Opportunities, challenges and pitfalls in characterizing plant water-use strategies

Classifying the diverse ways that plants respond to hydrologic stress into generalizable ‘water-use strategies’ has long been an eco-physiological research goal. While many schemes for describing water-use strategies have proven to be quite useful, they are also associated with uncertainties regarding their theoretical basis and their connection to plant carbon and water relations. In this review, we discuss the factors that shape plant water stress responses and assess the approaches used to classify a plant's water-use strategy, paying particular attention to the popular but controversial concept of a continuum from isohydry to anisohydry. A generalizable and predictive framework for assessing plant water-use strategies has been historically elusive, yet recent advances in plant physiology and hydraulics provide the field with a way past these obstacles. Specifically, we promote the idea that many metrics that quantify water-use strategies are highly dynamic and emergent from the interaction between plant traits and environmental conditions, and that this complexity has historically hindered the development of a generalizable water-use strategy framework. This idea is explored using a plant hydraulics model to identify: (a) distinct temporal phases in plant hydraulic regulation during drought that underpin dynamic water-use responses, and (b) how variation in both traits and environmental forcings can significantly alter common metrics used to characterize plant water-use strategies. This modelling exercise can bridge the divide between various conceptualizations of water-use strategies and provide targeted hypotheses to advance the understanding and quantification of plant water status regulation across spatial and temporal scales. Finally, we describe research frontiers that are necessary to improve the predictive capacity of the plant water-use strategy concept, including further investigation into the below-ground determinants of plant water relations, targeted data collection efforts and the potential to scale these concepts from individuals to whole regions. A free Plain Language Summary can be found within the Supporting Information of this article.