Mechanistic drivers of stem respiration: A modelling exercise across species and seasons.

Stem respiration (R S ) plays a crucial role in plant carbon budgets. However, its poor understanding limits our ability to model woody tissue and whole-tree respiration. A biophysical model of stem water and carbon fluxes (TReSpire) was calibrated on cedar, maple and oak trees during spring and late summer. For this, stem sap flow, water potential, diameter variation, temperature, CO 2 efflux, allometry and biochemistry were monitored. Shoot photosynthesis (P N ) and nonstructural carbohydrates (NSC) were additionally measured to evaluate source-sink relations. The highest R S and stem growth was found in maple and oak during spring, both being seasonally decoupled from P N and [NSC]. Temperature largely affected maintenance respiration (R M ) in the short term, but temperature-normalized R M was highly variable on a seasonal timescale. Overall, most of the respired CO 2 radially diffused to the atmosphere (>87%) while the remainder was transported upward with the transpiration stream. The modelling exercise highlights the sink-driven behaviour of R S and the significance of overall metabolic activity on nitrogen (N) allocation patterns and N-normalized respiratory costs to capture R S variability over the long term. These insights should be considered when modelling plant respiration, whose representation is currently biased towards a better understanding of leaf metabolism.