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Phloem sustains maintenance and growth processes through transport of sugars from source to sink organs. Under low water availability, tree functioning is impaired, i.e., growth/photosynthesis decline and phloem transport may be hindered. In a 3-year throughfall exclusion (TE) experiment on mature European beech (Fagus sylvatica L.) we conducted (CO2)-C-13 branch labeling to investigate translocation of recently fixed photoassimilates under experimental drought over 2 years (2015 and 2016). We hypothesized (H1) that mean residence time of photoassimilates in leaves (MRT) increases, whereas (H2) phloem transport velocity (V-phloem) decreases under drought. Transport of carbohydrates in the phloem was assessed via C-13 of CO2 efflux measured at two branch positions following (CO2)-C-13 labeling. Pre-dawn water potential ((PD)) and time-integrated soil water deficit (iSWD) were used to quantify drought stress. The MRT increased by 46% from 32.1 +/- 5.4 h in control (CO) to 46.9 +/- 12.3 h in TE trees, supporting H1, and positively correlated (P < 0.001) with iSWD. Confirming H2, V-phloem in 2016 decreased by 47% from 20.7 +/- 5.8 cm h(-1) in CO to 11.0 +/- 2.9 cm h(-1) in TE trees and positively correlated with (PD) (P = 0.001). We suggest that the positive correlation between MRT and iSWD is a result of the accumulation of osmolytes maintaining cell turgor in the leaves under longer drought periods. Furthermore, we propose that the positive correlation between V-phloem and (PD) is due to a lower water uptake of phloem conduits from surrounding tissues under increasing drought leading to a higher phloem sap viscosity and lower V-phloem. The two mechanisms increasing MRT and reducing V-phloem respond differently to low water availability and impair trees' carbon translocation under drought.