Tree-Ring and Sediment Analyses Reveal Processes of Bald Cypress Ghost Forest Formation From Dredging in the Cape Fear River, North Carolina, USA.

Forested, freshwater tidal wetlands in the southeastern US are dominated by bald cypresses (), which tolerate low levels of salinity. However, the response of old-growth bald cypress trees to prolonged increases in salinity remains uncertain. Bald cypress ghost forests occur along Smith Creek, a tributary of the Cape Fear River, North Carolina which has been dredged multiple times since 1871. Atop relative sea-level rise, dredging may be accelerating saltwater intrusion and ghost forest formation. To examine the effects of increased salinity on bald cypress and improve understanding of the process of ghost forest formation, we sampled trees and sediments along a salinity gradient in Smith Creek. We developed tree-ring chronologies for a high-salinity (8-12 ppt) and low-salinity (< 5 ppt) site. We quantified growth suppression patterns and climate-growth relationships at each site. Agglutinated benthic foraminifera and thecamoebians found in sediment samples served as proxies for increasing salinity over time. At the high salinity site, foraminifera began to appear at 25 cm depth in the sediment profile (indicating elevated salinity since c.1950). A lack of foraminifera at the low-salinity site indicated no major salinity increase. We identified five growth suppression events in bald cypress at the high-salinity site that were not observed at the low-salinity site (1859-1860, 1877-1887, 1946-1960, 1971-1983, 1985-2022), three of which are consistent with stress following years of dredging in the Cape Fear River (1946, 1970, 2000). The length and severity of the most recent suppression event at the high-salinity site suggests that those bald cypress are experiencing permanently stressful conditions. Additionally, monthly correlation analyses indicated that these trees may have increased sensitivity to drought and temperature. Saltwater intrusion into forested freshwater tidal wetlands induces stress in bald cypress, facilitating ghost forest formation and the loss of important ecosystem services.