Daytime depression in temperature‐normalised stem CO 2efflux in young poplar trees is dominated by low turgor pressure rather than by internal transport of respired CO 2

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Summary Daytime decreases in temperature‐normalised stemCO2efflux (EA_D) are commonly ascribed to internal transport of respiredCO2(FT) or to an attenuated respiratory activity due to lowered turgor pressure. The two are difficult to separate as they are simultaneously driven by sap flow dynamics. To achieve combined gradients in turgor pressure andFT, sap flow rates in poplar trees were manipulated through severe defoliation, severe drought, moderate defoliation and moderate drought. Turgor pressure was mechanistically modelled using measurements of sap flow, stem diameter variation, and soil and stem water potential. A mass balance approach considering internal and externalCO2fluxes was applied to estimateFT. Under well‐watered control conditions, both turgor pressure and sap flow, as a proxy ofFT, were reliable predictors ofEA_D. After tree manipulation, only turgor pressure was a robust predictor ofEA_D. Moreover,FTaccounted for < 15% ofEA_D. Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause ofEA_Din young poplar trees. Turgor pressure is determined by both carbohydrate supply and water availability, and should be considered to improve our widely used but inaccurate temperature‐based predictions of woody tissue respiration in global models.