# Analyzing the Long-Term Phenological Trends of Salt Marsh Ecosystem across Coastal LOUISIANA
## 4 Discussion
### Mann-Kendall Trend Analysis  
Most of the salt marsh ecosystem showed a progressive increase in the base GBM value, indicating that a considerable amount of carbon had been accumulated over time and stored as aboveground biomass in the salt marsh ecosystem. Interestingly, the maximum GBM value did not show as much progressive increase as the base GBM value, indicating that the maximum biomass as well as maximum photosynthesis rate had reached a level of saturation. Therefore, strong negative changes in seasonal amplitude and small seasonal integral were observed, which indicated a progressive decline in the levels of the net seasonal photosynthesis rate over the 16-year time period. Normally, changes in the levels of photosynthesis are attributed to changes in the levels of temperature and precipitation, as they influence the photoperiod and salinity of the ecosystem [100; 101]. However, neither the temperature nor the precipitation anomalies for either southeast or southwest LA showed any particular trend during the 16-year time period (Figure 9). The salt marsh ecosystem of these regions has experienced periodic diebacks; however, since salt marshes are resilient ecosystems, they are capable of recovering from short-term environmental stress if environmental conditions become normal again [73]. Atmospheric CO\({}_{2}\) levels rose from 369 ppm to 404 ppm during the period 2000-2015 [1], while sea levels were observed rising at a rate of 9-12 mm\(\cdot\)year\({}^{-1}\) in coastal LA [102]. Elevated CO\({}_{2}\) levels in the atmosphere have been known to induce photosynthetic saturation in C\({}_{4}\) plants; the saturation generally initiates at carbon dioxide concentrations beyond 350 ppm [103]. In salt marsh habitats, elevated atmospheric CO\({}_{2}\) levels have been observed to reduce both above and belowground biomass allocation in C\({}_{4}\) plant species [9; 10], as a result of a reduction in stomatal conductance [104], as well as the rate of water usage [9; 105; 10] ultimately reducing the rate of photosynthesis. [PERSON] et al. [9]  
Figure 9: (a) Temperature; (b) Precipitation; and (c) Palmer Drought Severity Index (PDSI) anomalies for all four coastal climatic zones of LA. observed a greater reduction in the aboveground biomass allocation of C\({}_{4}\) plants than C\({}_{3}\) plants in response to elevated CO\({}_{2}\) and sea level rise in a salt marsh habitat, even with altered nutrient levels in the environment. The salt marsh ecosystem in Louisiana is mostly dominated by the smooth cordgrass (_Spartina alterniflora_) and salt meadow cordgrass (_Spartina patens_), both of which are C\({}_{4}\) species. Therefore, in light of the observed rise in CO\({}_{2}\) levels and sea level rise, this observed reduction in the level of net photosynthesis and biomass allocation in the salt marshes of LA is not entirely unexpected. Although we haven't done any field experiments to test this, there have been many studies over the past decade confirming the phenomena using observed datasets. If the progressive decline in the rates of photosynthesis and biomass allocation continues, the carbon sequestration rate of the LA salt marsh ecosystem might not be able to keep up with the rate of rising atmospheric CO\({}_{2}\); rather, with periodic disturbances such as hurricanes, dieback, and anthropogenic disasters, marsh loss will turn the ecosystem to carbon sources [106]. On the other hand, such elevated levels of atmospheric CO\({}_{2}\) and sea levels will stimulate photosynthesis, water usage capacity, and more efficient biomass allocation in C\({}_{3}\) plants compared with C\({}_{4}\) plants [9; 10]. This might explain the invasion of _Phragmites australis_ in the coastal salt marsh landscape of LA [107]. _Phragmites_ has a variable photosynthetic pathway that is capable of exhibiting both C\({}_{3}\) and C\({}_{4}\) photosynthesis [108].