Woody tissue photosynthesis reduces stem CO2 efflux by half and remains unaffected by drought stress in young Populus tremula trees

A substantial portion of locally respired CO2 in stems can be assimilated by chloroplast‐containing tissues. Woody tissue photosynthesis (Pwt) therefore plays a major role in the stem carbon balance. To study the impact of Pwt on stem carbon cycling along a gradient of water availability, stem CO2 efflux (EA), xylem CO2 concentration ([CO2]), and xylem water potential (Ψxylem) were measured in 4‐year‐old Populus tremula L. trees exposed to drought stress and different regimes of light exclusion of woody tissues. Under well‐watered conditions, local Pwt decreased EA up to 30%. Axial CO2 diffusion (Dax) induced by distant Pwt caused an additional decrease in EA of up to 25% and limited xylem [CO2] build‐up. Under drought stress, absolute decreases in EA driven by Pwt remained stable, denoting that Pwt was not affected by drought. At the end of the dry period, when transpiration was low, local Pwt and Dax offset 20% and 10% of stem respiration on a daily basis, respectively. These results highlight (a) the importance of Pwt for an adequate interpretation of EA measurements and (b) homeostatic Pwt along a drought stress gradient, which might play a crucial role to fuel stem metabolism when leaf carbon uptake and phloem transport are limited. Woody tissue photosynthesis remains constant during drought, highlighting its importance as reliable carbon source when leaf assimilation is compromised. In addition, stem CO2 efflux was substantially lowered by local woody tissue photosynthesis, altering its temperature sensitivity.