## 4 Discussion
### Drought Effects on GPP, ET, and WUE  
Our study sites had a high groundwater table common to most lower coastal plain areas. They were capable of providing sufficient water for evapotranspiration. However, the years 2007 and 2008 were extremely dry. During these years, the depth of the groundwater table fell below 50 to 80 cm below the soil surface [43; 46; 48; 70]. During the 2007/2008 drought period, ET was reduced due to a reduced canopy interception brought about by low precipitation [45]. However, other studies have reported greater actual ET during drought [97; 98] due to increased available energy and a vapor pressure deficit. It appears that the increased atmospheric demand did not overcome the decreased precipitation and water supply during drought at our study sites. The decline in ET at YP was higher compared to MP during the drought periods. This result is consistent with a coastal study reporting that drought had significantly affected young stands [81]. Low leaf biomass/LAI, shallow root systems, and a reduction in plant hydraulic conductivity may have induced drought-related reduction in transpiration [47]. Clearly, drought affected young plantations more than matured plantations, which supports our hypothesis. Reductions in tree interception, plant transpiration, and soil evaporation may have slightly reduced ET during drought at the MP site. Hydraulic redistribution due to deep roots may have supplied sufficient soil water to the upper soil layers, thus, sustaining ET at the MP site during the drought [38]. An increase in GPP with reduced soil water content during the drought indicates that the impact of low soil water availability was less compared to the significant effect of light and temperature on GPP. A previous study at the site reported that the drought effect was overshadowed by higher LAI and light availability [48]. At sites with high groundwater tables, rainfall-induced soil anoxia limits decomposition and nutrient mineralization [46], thereby limiting GPP, and, thus, drier conditions may enhance root function and carbon gain. Additionally, soil characteristics at our sites (a mix of porous organic matter and sand) allowed the production of deep roots and the exploitation of a greater soil volume [38]. As a consequence, large water potential gradients existed within the soil-root-plant continuum; thus, triggering the redistribution of water via roots from deep and wet soil to shallow and dry soil. This phenomenon likely mitigated the effect of soil drying and positively influenced carbon assimilation [38]. Higher GPP in YP during drought may have been due to efficient root systems that can withstand extreme dry conditions, keeping fine roots hydrated for longer period of time. The ability of these roots to keep roots hydrated over a longer period delayed the drying of the upper soil surfaces [37; 38; 47]. In our study, drought induced higher WUE which supports our hypothesis. This enhancement in WUE was due to an increase in GPP and a decline in ET. This is inconsistent with a study [45] where a similar rise in evapotranspiration was not accompanied by an increase in productivity. Our result of an enhancement in WUE due to a larger decline in ET than an increase in GPP was also found in a long-term drought experiment [99]. However, other studies (e.g., drought-prone subtropical and semi-arid/sub-humid ecosystems) reported lower WUE during drought [57; 100]. The asynchronous response of GPP and ET during drought suggests that decoupling between carbon and water cycles in this wetland ecosystem occurred due in part to the differential forcing of biotic (e.g., stomatal responses) and abiotic (e.g., surface water evaporation) components of the system. This result does not support our hypothesis of a tight coupling between GPP and ET during drought and is not consistent with the strong GPP-ET coupling reported in other studies [6; 23; 99; 100].