TR eSpire – a biophysical TR ee Stem respiration model

Summary Mechanistic models of plant respiration remain poorly developed, especially in stems and woody tissues where measurements of CO 2 efflux do not necessarily reflect local respiratory activity. We built a process‐based model of stem respiration that couples water and carbon fluxes at the organ level ( TR eSpire). To this end, sap flow, stem diameter variations, xylem and soil water potential, stem temperature, stem CO 2 efflux and nonstructural carbohydrates were measured in a maple tree, while xylem CO 2 concentration and additional stem and xylem diameter variations were monitored in an ancillary tree for model validation. TR eSpire realistically described: (1) turgor pressure to differentiate growing from nongrowing metabolism; (2) maintenance expenditures in xylem and outer tissues based on Arrhenius kinetics and nitrogen content; and (3) radial CO 2 diffusivity and CO 2 solubility and transport in the sap solution. Collinearity issues with phloem unloading rates and sugar–starch interconversion rates suggest parallel submodelling to close the stem carbon balance. TR eSpire brings a breakthrough in the modelling of stem water and carbon fluxes at a detailed (hourly) temporal resolution. TR eSpire is calibrated from a sink‐driven perspective, and has potential to advance our understanding on stem growth dynamics, CO 2 fluxes and underlying respiratory physiology across different species and phenological stages.