A unified framework for sapwood and stem water content in plants: structure, function, and measurement

Stem water content is a fundamental parameter governing key plant physiological processes, including transpiration, stomatal conductance, sap flow, and hydraulic capacitance. Although extensively studied in the context of timber, construction, and engineering, its physiological significance in living plants remains comparatively underdeveloped. This review synthesises the physiological literature to provide a clear and consistent definition of stem water content, partitioning it into bark, phloem, sapwood, and heartwood components, with particular emphasis on sapwood water content. A theoretical framework is developed to define the maximum and minimum bounds of sapwood water content based on porosity, basic wood density, and the fibre saturation point. These bounds are supported by measurements compiled from the published literature including 3395 samples across 742 species. The diel range of sapwood water content is further examined in relation to wood density, hydraulic capacitance, atmospheric vapour pressure deficit, and species-specific stomatal regulation (isohydric versus anisohydric behaviour). Measurement techniques, including wood sampling, dielectric sensors, heat-pulse methods, and multiple tomographic techniques, are critically evaluated with respect to their underlying principles, advantages, and limitations. This review provides a unified conceptual framework for sapwood water content, improving clarity, comparability, and application in plant physiological research.