Stem CO2 efflux and its contribution to ecosystem CO2 efflux decrease with drought in a Mediterranean forest stand

• Diurnal and seasonal shifts in stem CO2 efflux are governed by temperature.
• Stem CO2 efflux at a constant temperature decreases with seasonal drought.
• The ratio stem to ecosystem CO2 efflux decreases with increasing seasonal drought.
• Soluble sugars do not limit stem CO2 efflux during drought.
• Stem CO2 efflux does not seem to acclimate to 9 years of increased drought.

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 CO2 efflux (Fs), 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). Fs increased exponentially during the day by an effect of temperature, although sap flow attenuated the increase in Fs during the day time. Over the year, Fs also increased exponentially with increasing temperatures, but Fs computed at a standard temperature of 15 °C (Fs15) varied by almost 4-fold among dates. Fs15 was the highest after periods of stem growth and decreased as tree water availability decreased, similarly in C and E   treatments. The decline in Fs15 was not linked to a depletion of soluble sugars, which increased when water stress was higher. The proportion of ecosystem respiration attributed to the stems was highest following stem growth (23.3%) and lowest during the peak of drought (6.5%). High within-year variability in Fs15 makes unadvisable to pool annual data of Fs vs. temperature to model Fs at short time scales (hours to months) in Mediterranean-type forest ecosystems. We demonstrate that water availability is an important factor governing stem CO2 efflux and suggest that trees in Mediterranean environments acclimate to seasonal drought by reducing stem respiration. Stem respiratory rates do not seem to change after a long-term increase in drought intensity, however.