Patterns of microbial diversity along salinity gradients: Conductors or passengers?
Justin Wright
Duke University
Significant areas of freshwater forested coastal wetlands in the North Atlantic Coastal Plain have experienced mortality, creating landscapes commonly referred to as “ghost forests.” The most likely cause of this mortality are environmental changes associated with salt water intrusion and sea level rise (SWISLR). At our research site in coastal North Carolina, ghost forests are largely a transitional landscape feature, and are commonly replaced by shrublands - a land cover type that was previously relatively rare. One underexplored mechanism that might underlie the formation and maintenance of shrublands is differential responses of plants and microbes to the multivariate changes associated with SWISLR which alter plant-microbe interactions. To explore this hypothesis, we surveyed plant and microbial communities along transects reflecting salinity exposure gradients in the Alligator River National Wildlife Refuge. We found that understory plant and bacterial community composition were best predicted by soil salinity, pH, and inundation risk concurrently (multiplicative model), woody plant communities were best predicted by inundation risk alone, and fungal communities were most sensitive to changing soil pH. Plant, soil microbial, and leaf endophyte communities experienced rapid turnover across relatively low levels of soil salinity and showed similar patterns of change across a salt gradient. However, patterns of community turnover across soil pH differed among community types, with fungal communities experiencing the greatest rate of change between 4.5-5.5, and understory plants between 5-6. This suggests that biota may shift more synchronously to changes in soil salinity, but that indirect effects of saltwater exposure (increasing pH) may lead to divergent community responses. We also conducted a plant-soil feedback experiment testing the effect of forest and shrubland soils on dominant tree and shrub species in a greenhouse setting. We found that forest soils may be microbially primed for the growth of shrubs.
Session 4A: Ghost Forests and Marsh Migration in the Coastal Zone
June 12
12:45 - 2:15 pm
Choptank Ballroom
Tidal swamp microtopography reflects shifting balances of shallow subsidence and root zone expansion along lower river salinity gradients
Ken Krauss
U.S. Geological Survey
Tidal freshwater forested wetlands (TFFWs) are in an active phase of transition to tidal marsh with sea-level rise and salinity incursion along the Atlantic and Gulf Coasts of the United States (U.S.). A prominent feature of TFFWs is hummock/hollow microtopography where hollows represent the flat, base-elevation of the floodplain where inundation occurs relatively frequently. Hummocks provide elevated soil surfaces that often harbor relatively greater numbers and species of trees and shrubs. Hummocks first appear at the landscape river boundary just seaward of bottomland hardwoods where tides and salinity reach, then these persist for many years, and disappear as TFFWs eventually transition to marsh. We studied TFFW surface elevation processes along four Atlantic coastal landscape river gradients by using surface elevation tables and marker horizons. Shallow subsidence between trees, equating to as much as 5.5 mm/year, was an important process in hollow formation and maintenance as roots held hummock elevations relatively more stable. However, hummocks were actively subsiding on all sites with little sign of root zone expansion within hummocks, despite hummock elevation gain on some sites. For down-river transitions, hollow infilling through increasing sediment accretion and root zone expansion were predominant processes driving the loss of microtopography as marshes replaced TFFWs closer to the estuarine interface; hollows gained elevations to meet hummocks. While these results do not preclude the importance of healthy root zone processes to the maintenance (and formation) of hummocks, our results indicate that reductions in critical sediment supplies to offset natural shallow subsidence explain persistence and eventual loss of hummock and hollow microtopography in TFFWs with salinization.
Forest responses to saltwater flooding in the Delmarva peninsula
Keryn Gedan
George Washington University
Ghost forests are a visible sign of sea level rise and saltwater intrusion and have become a common occurrence in low elevation, coastal areas within the Delmarva region. We have intensively studied forest dynamics within a dying coastal forest in the Eastern Shore of Virginia and report on significant changes to forest structure. Over five years of monitoring (2019-2023), the forest canopy gap portion has increased and above-background rates of tree mortality have been detected. Species turnover favors a shift from mixed species composition towards gymnosperms. Loblolly pine, Pinus taeda, and redcedar, Juniperus virginiana, are the most persistent tree species prior to complete forest mortality. Trees exhibited reduced sap flux in salt-inundated forest prior to their death. In the understory, shrub basal area, canopy area, and occupancy increased significantly at the ecotone frontier, and Phragmites australis density increased at the lagging boundary of the ecotone adjacent to marsh.
Impacts of sea level rise and repeated storm surge on the marsh-forest ecotone in South Carolina
Tom O’Halloran
Clemson University
The South Atlantic Bight serves as a climatological hotspot for land-falling tropical storms in North America. Over the past decade, the rate of relative sea level rise in this region has accelerated to 10 mm yr-1, primarily driven by coupled ocean-atmosphere forcing and accentuated by land subsidence in certain areas. This phenomenon has created a hotspot for saltwater intrusion and storm surge inundation. Recent remote sensing analyses of the entire Atlantic coast identified a peak in coastal wetland forest mortality in South Carolina. In this talk, we present results from Hobcaw Barony, a research reserve situated at the mouth of the greater Winyah Bay watershed, adjacent to the hotspot identified in previous work. Researchers from Clemson and the University of South Carolina have systematically monitored marsh and forest dynamics here for decades. Utilizing aerial photographs, we have documented the spatial extent of forest mortality post-Hurricane Hugo in 1989, and subsequently established forest survey plots to track recovery from 1994 to the present day. These forest plots have recently encountered repeated storm surges due to an elevated tropical storm frequency. In 2022, Hurricane Ian created a storm surge reaching 2.32 meters above sea level, marking the highest since Hurricane Hugo. Consequently, mortality in loblolly / longleaf pine forest along the marsh-forest boundary has ensued, exacerbated by pine beetle infestation. Here we compare the historical record of forest mortality from Hurricane Hugo to recent formation of ghost forests and use a spatial statistical model to analyze the geomorphic factors that explained (or did not explain) spatial variation in mortality between the two events.