Trees assimilate carbon dioxide (CO2) through photosynthesis and release approximately half of it back to the atmosphere through respiration. Photosynthesis is a well-known process that has been mechanistically described in much detail. Contrastingly, respiration remains poorly understood, especially in woody tissues where measurements of gas exchange do not necessarily reflect respiration rates. Furthermore, most respiration models are still based on the growth-andmaintenance- respiration paradigm proposed in the early 1970s. This project aims to advance the mechanistic modelling of respiratory processes by integrating cutting-edge technology and a mechanistic model that couples water and carbon transport within the plant. The purpose is to answer critical questions about the significance of xylem CO2 in woody tissues, and to study how enriched atmospheric CO2 and drought affect tree growth, woody tissue respiration, and tree carbon balances. Cell turgor (the ultimate regulator of cell growth) and substrate supply (phloem transport, starch conversion, woody tissue photosynthesis) will be modelled as key drivers of respiration. Model simulations will be validated by monitoring phloem transport in vivo through novel techniques using isotopic labelling. This integrative approach is expected to bring a major breakthrough in the understanding of woody tissue respiration, which will improve our predictions of tree carbon cycling in scenarios of climate change.