## 4 Discussion
### Major Climatic Effects on C Fluxes and Balance  
The effect of solar irradiance, atmospheric air temperature, and soil water availability on GPP, RE, and NEE has been widely reported [39; 40; 87; 104; 105; 106]. It is important to determine the influence of these key climate drivers on forest C cycling because the climate-C cycle feedbacks control ecosystem-level C dynamics in response to the changing climate [45; 53; 52]. However, a deeper understanding of these climate effects on C balance remains limited, especially for coastal forested wetlands.  
The combined effect of climatic drivers (Tair, Rn, GWT, and \(p\)) only explained 29% of the variability in NEE, suggesting greater biological control (e.g., LAI, mortality, etc.) on NEE than the climate drivers. Determining the response of carbon fluxes and balance to climate and vegetative factors is essential to understand the mechanisms behind the variations in GPP, RE, and NEE. However, obtaining biological data is more challenging than obtaining the climate variables [107]. Previous studies showed that biological effects could be larger [107; 108], equivalent to [109], or less than [107; 108] climatic effects at an interannual scale. Other studies also studied the correlations between climatic and biological effects [107; 109], reflecting the responses of ecosystem C cycling to the changing climate.  
Sustained favorable light availability resulting in increased GPP is well established in many studies [39; 87; 110; 111; 105] and is primarily a function of higher LAI [80; 90]. However, at our site, GPP, RE, and NEE were not mainly controlled by net radiation but rather by temperature. This result suggests that, in this coastal wetland setting, caution must be taken in using solar irradiance to model C balance as other biological and environmental factors may contribute to the observed variability.  
Air temperature is the major climatic control for RE for many studies [56; 111; 112]. Increased temperature enhanced litter decomposition in areas with abundant water [110; 113; 114; 115; 116; 117; 118]. The sensitivity of RE to temperature may lead to continuing losses of C to the atmosphere due to hydrologic forcing coupled with a future warmer climate.  
Surprisingly, GWT contributed more to the RE variability than Rn (Table 2). The microtopography of our site influences the local hydrologic conditions. The hummocks surrounding tree bases are usually above the water table, while the hollows (usually non-vegetated) are submerged more than 70% of the year [55]. Site conditions such as this do not necessarily improve the GPP and RE. Instead, GPP and RE were enhanced during times of decreased groundwater level. Since NEE is the function of its component GPP and RE, the absence of variation in NEE with GWT suggests that even though GWT secondarily controls the variations in GPP and RE, they converge towards a relatively stable NEE. Therefore, modeling must be cognizant of using GWT to parameterize NEE in this mostly submerged wetland site.  
During the non-growing season, the water table at our site is at full storage capacity [44], due to the low topographic setting and distinct microtopography. The site is a basin for the water from the nearby uplands. However, the water is difficult to drain because of the proximity of our site to the sea. The poor drainage characteristic of the site and poorly defined pathways for run-off decreased the overland flow. However, a deeper water table is attained during the growing seasons when precipitation is low. Thus, seasonal change in hydrology must be considered when modeling a unique coastal plain forest such as ours.