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B10: Controlling saltwater intrusion: aiding the recovery of freshwater ecosystems
As sea levels rise, it is imperative to understand the effects of saltwater intrusion on freshwater ecosystems. This is especially true in the coastal wetlands of the SE US, where freshwater forests are dying from exposure to salinity. One management strategy that could prevent salinity intrusion in surface waters are water control structures (WCS). The Great Reserve, located outside of Georgetown, SC, represents a biodiverse freshwater forested wetland affected by saltwater intrusion from the adjacent tidal Black River. Several years ago, scientists and landowners observed the degradation of the ecosystem. The owners of the property installed a WCS at the outflow of the Black River to mitigate and reduce the saltwater intrusion. This project provided a unique opportunity to study how the installation of a WCS affects the growth of forest tree species.
Seven 25 m x 50 m forested plots were established in the Great Reserve. Trees greater than 10 cm diameter at breast height (DBH) within each plot were tagged and measured. Yearly growth rates were calculated based on the increment changes in DBH.
The number of tree species varied between plots. The plot closest to the Black River only had 1 species (cypress), while the furthest plot had 4 tree species. While diversity did not change during the study, average growth rates did. Cypress trees in all plots had increased growth rates after the installation of the WCS. Cypress trees close to the river increased nearly 3-fold (0.09 cm/yr to 0.26 cm/yr based on DBH). As a reference, we compared these growth rates to cypress trees in another impacted wetland system without a WCS.
Fewer tree species and more standing dead trees closer to the WCS illustrates the impact of salinity on the Great Reserve. The increase in cypress growth rate after the installation of the WCS is likely due to a reduction of salinity entering during the tidal cycle. This indicates the potential recovery of these freshwater ecosystems. If trees continue to respond positively, the construction of WCS should be promoted to other private landowners for the widespread conservation of freshwater wetlands.