Ghost cypress as indicators of sea-level rise in the Neuse River, North Carolina, USA

Abstract

“Ghost cypress”—standing trees killed by increased salinity—indicate sea-level rise (SLR) effects along lower reaches of many coastal plain rivers. Mature cypress can survive indefinitely in permanently flooded sites, but experience mortality at salinities as low as 2 to 3 ppt. Thus, ghost trees in permanently inundated sites can indicate mortality due to increased salinity. Ghost cypress were mapped along the margins of the Neuse River estuary and fluvial-estuarine transition zone (FETZ), along with co-indicators of salinity as a potential cause of death. The distribution was compared with other indicators of upstream propagation of SLR effects; all occurred within a 25 km river reach. Many ghost cypress are consistent with SLR-driven mortality, but in the upper FETZ the co-indicators argue against it, and throughout the study area some ghost cypress lack co-indicators of salinity effects and may have been killed by other factors. The upstream limit of ghost cypress with co-indicators suggesting possible SLR-driven mortality, and the downstream limit of Nyssa aquatica and N. biflora, whose habitats and niches overlap almost entirely with Taxodium except for less salinity tolerance, occur downstream of other indicators of the leading edge of SLR. The furthest upstream is the hydraulic impact of backwater effects on river flow. Downstream, other effects are encountered: a transition from occasionally to frequently flooded wetlands, sedimentary burial of Pleistocene alluvial terraces, and a shift from dominantly mineral floodplain soils to Histosols. The ecological indicators of cypress and tupelo are furthest downstream. Hydraulic (backwater) effects are the leading edge of SLR impacts on the Neuse, trailed by geomorphological, sedimentological, and pedological indicators. Though biota often respond more rapidly to changes than landforms and soils, ecological indicators such as ghost cypress and forest-to-marsh transitions that are salinity dependent are the downstream-most sentinels of sea-level encroachment in rivers.