Daytime depression in temperature-normalised stem CO2 efflux in young poplar trees is dominated by low turgor pressure rather than by internal transport of respired CO2

Daytime decreases in temperature-normalised stem CO2 efflux (E A_D) are commonly ascribed to internal transport of respired CO2 (F T) 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 and F T, 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 external CO2 fluxes was applied to estimate F T. Under well-watered control conditions, both turgor pressure and sap flow, as a proxy of F T, were reliable predictors of E A_D. After tree manipulation, only turgor pressure was a robust predictor of E A_D. Moreover, F T accounted for < 15% of E A_D. Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause of E A_D in 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.