Forest water use efficiency (WUE), the ratio of gross primary productivity (GPP), and evapotranspiration (ET) is important variable to understand the coupling between water and carbon cycles, and to assess resource use, ecosystem resilience, and commodity production. Here, we determined WUE for managed loblolly pine plantation forests over the course of a rotation on the coastal plain of North Carolina in the eastern U.S. We found that forest annual GPP, ET, and WUE increased until age ten, which stabilized thereafter. WUE varied annually (2 - 44%), being higher at young plantation (YP, 3.12 ± 1.20 g C kg H2O d-1 ) compared to a mature plantation (MP, 2.92 ± 0.45 g C kg H2O d-1 ), with no distinct seasonal patterns. Stand age was strongly correlated with ET (R2 = 0.71) and GPP (R2 = 0.64). ET and GPP were tightly coupled (R2 = 0.86). Radiation and air temperature showed a significant effects on GPP and ET (R2 = 0.71 – R2 = 0.82) at a monthly scale, but not WUE. Drought affected WUE (R2 = 0.35) more than ET (R2 = 0.25) or GPP (R2 = 0.07). A drought enhanced GPP in MP (i.e., 6% – 9 %) and YP (i.e., 20% – 53%), but reduced ET (i.e., 8 – 11 %) and (i.e., 30 – 43 %) in MP and YP respectively, conferring that drought resulted in higher WUE by 8 – 30%. Minor seasonal and interannual variations in forest WUE of MP (age >10) suggested that forest WUE became stable as stands matured. Our study concluded that carbon and water cycles in loblolly pine plantations were tightly coupled with different characteristics in different ages and hydrologic regimes. WUE can be used to quantify water (carbon) flux from carbon (water) flux under varying environmental conditions. The tradeoffs between water and carbon resources should be recognized in forest management to achieve multiple ecosystem services (i.e., water supply and carbon sequestration).