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The rate of metabolic processes demanding energy in tree stems changes in relation with prevailing climatic conditions. Tree water availability can affect stem respiration through impacts on growth, phloem transport or maintenance of diverse cellular processes, but little is known on this topic. Here we monitored seasonal changes in stem CO 2 efflux (F s), radial growth, sap flow and non-structural carbohydrates in trees of Quercus ilex in a Mediterranean forest stand subjected since 2003 to either partial (33%) throughfall exclusion (E) or unchanged throughfall (C). F s increased exponentially during the day by an effect of temperature, although sap flow attenuated the increase in F s during the day time. Over the year, F s also increased exponentially with increasing temperatures, but F s computed at a standard temperature of 15° C (F s 15) varied by almost 4-fold among dates. F s 15 was the highest after periods of …