The Impact of Late Holocene Land Use Change, Climate Variability, and Sea Level Rise on Carbon Storage in Tidal Freshwater Wetlands on the Southeastern United States Coastal Plain

Abstract

This study examines Holocene impacts of changes in climate, land use, and sea level rise (SLR) on sediment accretion, carbon accumulation rates (CAR), and vegetation along a transect of tidal freshwater forested wetlands (TFFW) to oligohaline marsh along the Waccamaw River, South Carolina (four sites) and along the Savannah River, Georgia (four sites). We use pollen, plant macrofossils, accretion, and CAR from cores, spanning the last 1,500–6,000 years to test the hypothesis that TFFW have remained stable throughout the late Holocene and that marshes transitioned from TFFW during elevated SLR during the Medieval Climate Anomaly, with further transformation resulting from colonial land use change. Results show low and stable accretion and CAR through much of the Holocene, despite moderate changes associated with Holocene paleoclimate. In all records, the largest observed change occurred within the last ~400 years, driven by colonial land clearance, shifting terrigenous sediment into riparian wetlands, resulting in order‐of‐magnitude increases in accretion and C accumulation. The oligohaline marshes transitioned from TFFW ~300–500 years ago, coincident with colonial land clearance. Postcolonial decreases in CAR and accretion occur because of watershed reforestation over the last century. All sites show evidence of recent (decades to century) swamp forest decline due to increasing salinity and tidal inundation from SLR. This study suggests that allochthonous sediment input during colonialization helped maintain TFFW but that current SLR rates are too high for TFFW to persist, although higher accretion rates in oligohaline marshes increase the resilience of tidal wetlands as they transition from TFFW to marsh.

Modeling Soil Porewater Salinity Response to Drought in Tidal Freshwater Forested Wetlands

“…Long‐term field studies also provided evidence that the lower sites are actively converting to marsh because trees are dying, and the understory marsh environment is becoming more dominant (Krauss, Doyle, & Howard, ). The lower sites (heavily salt‐impacted) at both rivers were found to begin degradation during elevated SLR in the late Holocene (Jones et al, ).…”

“…For the two marsh sites along the two rivers, mean annual soil porewater salinity also increased significantly under drought‐induced saltwater intrusion, reaching ≥5 psu, compared to the level of 2–4 psu. This stands to reason because the two marsh sites were converted from tidal forest sites in the last several centuries due to long‐term SLR‐induced saltwater intrusion as well as conversion to rice field (Jones et al, ). Our modeling simulation showed that under a nondrought year, mean porewater salinity would be 2.1 ± 1.4 psu at the Waccamaw site and 3.6 ± 2.4 psu at the Savannah site.…”

“…When tidal freshwater soils were exposed to salinity of 9.9 psu for over 12 days, the soil microbial community was altered significantly, and sulfate reduction became the dominant metabolic pathway in the soil (Weston et al, ). TFFW also tend to have low sedimentation rates compared to oligohaline marshes, though the time scale of the evaluation matters (Noe et al, ), and are more vulnerable to saltwater intrusion and transitioning quickly to oligohaline marshes (Jones et al, ; Stagg et al, ). Considering the relatively higher contemporary accretion rates in marsh than in the forest sites, as more sediment can be delivered, settled, and captured by emergent grasses (e.g., Ensign et al, ), and considering also the accelerated rate of sea level rise and increased storm events in the future, the lower forest sites along the two rivers may be more vulnerable than marsh sites and likely will convert to oligohaline marsh in the foreseeable future.…”

“…TFFW are also sensitive to coastal droughts through declines in direct precipitation, perennially lower riverine base flows, and human altered incoming tidal level that affect tidal range and salinity along many rivers (Conrads & Darby, ). For example, the combination of dredging (deepening) the navigation channel and the tidal gate along the Savannah River and its tributaries in the southeastern United States have affected flow paths and resulted in increased river salinity upstream, which led to shifts in vegetation structure (Jones et al, ; Krauss, Doyle, & Howard, ). It was found that soil porewater salinity increased from average salinity 1.2 ± 0.2 psu (range: 0.49–2.38) at a moderately impacted TFFW site to average salinity 4.3 ± 0.3 psu (range: 2.53–6.63) at a highly salt‐impacted TFFW site along the Savannah River (Krauss et al, ).…”

The Role of the Upper Tidal Estuary in Wetland Blue Carbon Storage and Flux

“…Root biomass decomposed by only 12%–32% of original biomass annually at our sites, and for the Upper, Middle, Lower, and Marsh sites along these two rivers, this rate was related to shifts in lignin:N ratios, salinity, and relative flooding that can change quickly with environmental modification (Stagg et al, ). Over time, the net effect of environmental change facilitated soil C burial concomitant with forest‐to‐marsh habitat shifts, and vice versa, as soil surface elevations adjusted to Holocene sea level shifts (Jones et al, ).…”