###### Abstract  
Forest water use efficiency (WUE), the ratio of gross primary productivity (GPP) to evapotranspiration (ET), is an 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 plantations over the course of a rotation on the coastal plain of North Carolina in the eastern U.S. We found that the 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 \(\pm\) 1.20 g C kg\({}^{-1}\) H\({}_{2}\)O d\({}^{-1}\)) compared to a mature plantation (MP, 2.92 \(\pm\) 0.45 g C kg\({}^{-1}\) H\({}_{2}\)O d\({}^{-1}\)), with no distinct seasonal patterns. Stand age was strongly correlated with ET (\(R^{2}\) = 0.71) and GPP (\(R^{2}\) = 0.64). ET and GPP were tightly coupled (\(R^{2}\) = 0.86). Radiation and air temperature significantly affected GPP and ET (\(R^{2}\) = 0.71 - \(R^{2}\) = 0.82) at a monthly scale, but not WUE. Drought affected WUE (\(R^{2}\) = 0.35) more than ET (\(R^{2}\) = 0.25) or GPP (\(R^{2}\) = 0.07). A drought enhanced GPP in MP (19%) and YP (11%), but reduced ET 7% and 19% in MP and YP, respectively, resulting in a higher WUE (27-32%). Minor seasonal and interannual variation in forest WUE of MP (age > 10) suggested that forest functioning became stable as stands matured. We conclude that carbon and water cycles in loblolly pine plantations are tightly coupled, with different characteristics in different ages and hydrologic regimes. A stable WUE suggests that the pine ecosystem productivity can be readily predicted from ET and vice versa. The tradeoffs between water and carbon cycling should be recognized in forest management to achieve multiple ecosystem services (i.e., water supply and carbon sequestration). evapotranspiration; gross primary productivity; eddy covariance; coastal plain; loblolly pine plantation; forested wetland; carbon and water coupling 202121212312  
(e.g., low light intensity, drought), this balance depends on the trade-off between maintaining a high amount of CO\({}_{2}\) absorption (benefit) and a low transpiration rate (cost) at the ecosystem scale. Whether or not forested wetlands can regulate WUE over the long run in a drier environment is a question that ecosystem modelers must consider. Studying WUE is important to determine the carbon-water coupling amidst climate change and extreme weather events [4; 5]. However, multiple-year information about the response of forested wetland ecosystems to global warming and drought is scarce. Ecosystem WUE (expressed as GPP/ET) [5; 6; 7; 8] is determined in many eddy covariance flux studies. Other studies used intrinsic WUE which take into account the GPP and surface conductance [9; 10] or inherent WUE, which considers the vapor pressure deficit in the ratio between GPP and ET [11; 12; 1]. Other studies even calculated WUE as the fraction of GPP and ET obtained from the Breathing Earth System Simulator (BESS) model [13; 14; 15; 16]. Many authors have reported that radiation, temperature, and soil water induce variation in inter-annual [17] or seasonal ET [18; 19] and GPP rates [20; 21; 22]. However, evaluating what influences WUE is rather complicated since the variation in WUE results from changes in evapotranspiration and ecosystem productivity, which sometimes are decoupled. WUE was reported to be associated with light availability [6], soil water [23], and temperature [24]. However, long-term observations of radiation and temperature do not necessarily change over time. Thus, the patterns and trends in soil water dynamics closely associated with groundwater level depth at a seasonal scale may become one of the critical factors in WUE. Climate change predictions indicate a frequent occurrence of dry spells along the US southern coastal plains [25].