## 4 Discussion  
### Inter-Annual Variation in GPP, ET, and WUE  
Several studies have reported a seasonal variation in GPP and ET [18]. However, less literature is available for inter-annual variations in GPP and ET [59], especially over an extended period. Changes in GPP accelerated during the early growth of the stand in the recently harvested YP, in contrast to MP. Clearcutting removed the forest cover causing a low post-harvest GPP at YP in comparison with MP. The GPP at MP was always higher than at YP due in part to the recovery of the leaf area, 15 years after a harvest [46]. A thinning operation in 2009 at MP decreased leaf area index (LAI) [37], but interestingly GPP in this study appears to have been robust to this change. This could have been due to high leaf level physiology rates under higher light, water, and nutrient availability to the remaining trees (after thinning) that compensated for the loss of leaf area. Our result supports our hypothesis that GPP at the MP site would be higher than at the YP site. However, the inter-annual rate of the increase in GPP slowed when the stand attained canopy closure, confirming the notion that GPP is at its fastest rate when the trees are young but will tend to plateau before it later declines as the stand matures [30].  
Annual ET at our sites ranged from 1.23 kg H\({}_{2}\)O m\({}^{-2}\) d\({}^{-1}\) to 2.92 kg H\({}_{2}\)O m\({}^{-2}\) d\({}^{-1}\), consistent with a range of ET reported for the southeastern US (1.33 to 4.12 kg H\({}_{2}\)O m\({}^{-2}\) d\({}^{-1}\)[43; 47; 60; 61; 62; 63; 64]). Other hydrological study sites worldwide observed an annual ET of 0.76 to 2.81 kg H\({}_{2}\)O m\({}^{-2}\) d\({}^{-1}\)[8; 34; 65; 66; 67; 68; 69]. The small inter-annual variability in annual ET at our sites must have been due to deeper roots and a shallow groundwater table in the coastal loblolly pine ecosystems [70] although ET at the MP site was higher than at the YP site supporting our hypothesis.  
Literature reporting long-term scaled WUE is scarce, making a comparison to other studies difficult. It was reported that a decline in WUE has been observed in the Northern  
Figure 8: Daily average evapotranspiration (ET), gross primary productivity (GPP), and water use efficiency (WUE) at YP and MP. Daily values during the growing season (April–September) were averaged during 2007 and 2008 (drought years) and the rest of the years (non-drought years). Colored bars represent the drought and non-drought periods.  
Amazonian forest since the 1950s due to an increasing atmospheric CO\({}_{2}\) concentration [71]. However, this decreasing trend in WUE did not occur in our study. Since the ecosystem-scale WUE results from the integrated mean leaf-level WUE [8], interspecific variation in vegetation response to these changes in CO\({}_{2}\) concentrations over the years may explain why we did not observe a decreasing pattern of WUE at our sites. The observed inter-annual variability in WUE in our study indicated that, even though the annual ET and GPP responded to similar environmental drivers, the extent of their responses did not vary much among years except when exposed to severe climate conditions.