Tidal Freshwater Forested Wetlands Research Articles

Modeling impacts of saltwater intrusion on methane and nitrous oxide emissions in tidal forested wetlands.

Emissions of methane (CH4 ) and nitrous oxide (N2 O) from soils to the atmosphere can offset the benefits of carbon sequestration for climate change mitigation. While past study has suggested that both CH4 and N2 O emissions from tidal freshwater forested wetlands (TFFW) are generally low, the impacts of coastal droughts and drought-induced saltwater intrusion on CH4 and N2 O emissions remain unclear. In this study, a process-driven biogeochemistry model, Tidal Freshwater Wetland DeNitrification-DeComposition (TFW-DNDC), was applied to examine the responses of CH4 and N2 O emissions to episodic drought-induced saltwater intrusion in TFFW along the Waccamaw River and Savannah River, USA. These sites encompass landscape gradients of both surface and porewater salinity as influenced by Atlantic Ocean tides superimposed on periodic droughts. Surprisingly, CH4 and N2 O emission responsiveness to coastal droughts and drought-induced saltwater intrusion varied greatly between river systems and among local geomorphologic settings. This reflected the complexity of wetland CH4 and N2 O emissions and suggests that simple linkages to salinity may not always be relevant, as non-linear relationships dominated our simulations. Along the Savannah River, N2 O emissions in the moderate-oligohaline tidal forest site tended to increase dramatically under the drought condition, while CH4 emission decreased. For the Waccamaw River, emissions of both CH4 and N2 O in the moderate-oligohaline tidal forest site tended to decrease under the drought condition, but the capacity of the moderate-oligohaline tidal forest to serve as a carbon sink was substantially reduced due to significant declines in net primary productivity and soil organic carbon sequestration rates as salinity killed the dominant freshwater vegetation. These changes in fluxes of CH4 and N2 O reflect crucial synergistic effects of soil salinity and water level on C and N dynamics in TFFW due to drought-induced seawater intrusion.

Modeling impacts of drought-induced salinity intrusion on carbon dynamics in tidal freshwater forested wetlands.

Tidal freshwater forested wetlands (TFFW) provide critical ecosystem services including an essential habitat for a variety of wildlife species and significant carbon sinks for atmospheric carbon dioxide. However, large uncertainties remain concerning the impacts of climate change on the magnitude and variability of carbon fluxes and storage across a range of TFFW. In this study, we developed a process-driven Tidal Freshwater Wetlands DeNitrification-DeComposition model (TFW-DNDC) that has integrated new features, such as soil salinity effects on plant productivity and soil organic matter decomposition to explore carbon dynamics in the TFFW in response to drought-induced saltwater intrusion. Eight sites along the floodplains of the Waccamaw River (USA) and the Savannah River (USA) were selected to represent the TFFW transition from healthy to moderately and highly salt-impacted forests, and eventually to oligohaline marshes. The TFW-DNDC was calibrated and validated using field observed annual litterfall, stem growth, root growth, soil heterotrophic respiration, and soil organic carbon storage. Analyses indicate that plant productivity and soil carbon sequestration in TFFW could change substantially in response to increased soil pore water salinity and reduced soil water table due to drought, but in interactive ways dependent on the river simulated. These responses are variable due to nonlinear relationships between carbon cycling processes and environmental drivers. Plant productivity, plant respiration, soil organic carbon sequestration rate, and storage in the highly salt-impacted forest sites decreased significantly under drought conditions compared with normal conditions. Considering the high likelihood of healthy and moderately salt-impacted forests becoming highly salt-impacted forests under future climate change and sea-level rise, it is very likely that the TFFW will lose their capacity as carbon sinks without up-slope migration.

Watershed and Estuarine Controls Both Influence Plant Community and Tree Growth Changes in Tidal Freshwater Forested Wetlands along Two U.S. Mid-Atlantic Rivers

The tidal freshwater zone near the estuarine head-of-tide is potentially sensitive to both sea-level rise and associated salinity increases as well as changing watershed inputs of freshwater and nutrients. We evaluated the vegetation response of tidal freshwater forested wetlands (TFFW) to changes in nontidal river versus estuarine controls along the longitudinal gradient of the Mattaponi and Pamunkey rivers in the Mid-Atlantic USA. The gradient included nontidal freshwater floodplain (NT) and upper tidal (UT), lower tidal (LT), and stressed tidal forest transitioning to marsh (ST) TFFW habitats on both rivers. Plot-based vegetation sampling and dendrochronology were employed to examine: (1) downriver shifts in plant community composition and the structure of canopy trees, understory trees/saplings/shrubs and herbs, tree basal-area increment (BAI) and (2) interannual variability in BAI from 2015 dating back as far as 1969 in relation to long-term river and estuary monitoring data. With greater tidal influence downstream, tree species dominance shifted, live basal area generally decreased, long-term mean BAI of individual trees decreased, woody stem mortality increased, and live herbaceous vegetative cover and richness increased. Acer rubrum, Fagus grandifolia, Ilex opaca, and Fraxinus pennsylvanica dominated NT and UT sites, with F. pennsylvanica and Nyssa sylvatica increasingly dominating at more downstream tidal sites. Annual tree BAI growth was positively affected by nontidal river flow at NT and UT sites which were closer to the head-of-tide, positively influenced by small salinity increases at LT and ST sites further downstream, and positively influenced by estuarine water level throughout the gradient; nutrient influence was site specific with both positive and negative influences. The counterintuitive finding of salinity increasing tree growth at sites with low BAI is likely due to either competitive growth release from neighboring tree death or enhanced soil nutrient availability that may temporarily mitigate the negative effects of low-level salinization and sea-level increases on living TFFW canopy trees, even as overall plant community conversion to tidal marsh progresses.

Long-term Salinity, Hydrology, and Forested Wetlands Along a Tidal Freshwater Gradient

Tidal freshwater forested wetlands (TFFWs) commonly occur along coastal rivers; however, sea level rise and changes to river discharge may impact these wetlands. Information that characterizes the hydrology and salinity regime of the TFFW zone is needed to assess risk and predict future viability. A combination of field data and modeling were used to characterize TFFW hydrology, salinity, and vegetation along two distributaries (the East River and St. Marks River) of the Apalachicola River in west Florida, USA. Six gaging stations were established (three per river) roughly equidistant along the TFFW tidal gradient to monitor salinity and water levels at the river-wetland interface. Eighteen 500-m2 forest survey stations (nine per river) were also established roughly equidistant between (and including) the upper and lower gaging stations to measure canopy trees (> 2.54 cm DBH) and calculate species importance values (IV200). Field measures, along with other monitoring and model data, were used to develop a 30-year salinity record (1985–2015) for each gaging station based on an artificial neural network (ANN) model. Optimal ANN models for each TFFW gaging station were selected based on Akaike’s Information Criteria, and 30-year mean daily salinity was interpolated to all forest survey stations based on river distance. Important input variables for the ANN models included daily Apalachicola River discharge, Apalachicola Bay tidal stage, and bay salinity, among others. Based on 30-year salinity models, mean daily salinity ranged between 0.30 and 0.63 ppt at the downriver stations of East and St. Marks Rivers, respectively, to 0.14 and 0.14 ppt at the upriver stations, respectively. Results showed a predictable reduction in mean salinity and salinity ranges further upriver at both rivers; however, the St. Marks River showed a distinct inflection point in reduced salinity compared with the East River. Evaluating tree species IV200, there was a shift to species indicative of non-tidal conditions midway up the study reach of the St. Marks River while tidal species remained prominent throughout the East River study reach. Comparison of salinity, tidal reach, and species IV200 for each river suggests hydrology may be the most important contributor to the downriver extent of TFFWs while salinity may be an important driver of TFFW community composition.

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

AbstractThere is a growing concern about the adverse effects of saltwater intrusion via tidal rivers, streams, and creeks into tidal freshwater forested wetlands (TFFW) due to sea level rise (SLR) and intense and extended drought events. However, the magnitude and duration of porewater salinity in exceedance of plant salinity stress threshold (2 practical salinity units, psu) and the controlling factors remain unclear. In this study, we developed a TFFW soil porewater salinity model, in which the feedback mechanisms between soil salinity and evapotranspiration and hydraulic conductivity were incorporated. We selected sites (upper, middle, lower tidal freshwater forest sites, and oligohaline marsh site) along the coastal floodplains of two rivers, the Waccamaw River (SC, USA) and the Savannah River (GA and SC, USA), that represent landscape salinity gradients from tidal influence of the Atlantic Ocean. The model results agreed well with field measurements and revealed that with drought‐induced saltwater intrusion, the mean annual soil porewater salinity and duration of elevated soil porewater salinity (>2 psu) increased significantly compared to the normal (nondrought) condition, posing a threat to the health and ecosystem services of TFFW even in the absence of SLR. Model results also showed more severe salinity stress under drought for the lower forest sites along the two rivers, where soil salinity values have already been at or in exceedance of the 2 psu threshold.

Patterns of Denitrification Potential in Tidal Freshwater Forested Wetlands

Limited evidence for spatial patterns of denitrification in tidal freshwater forested wetlands (TFFWs), seemingly due to high spatial variability in the process, is surprising considering the various spatial gradients of its biogeochemical and hydrogeomorphic controls in these ecosystems. Because certain physical environmental gradients may be useful for the prediction of denitrification in TFFWs, we measured denitrification and ecosystem attributes in hummock-hollow microtopography of TFFWs along longitudinal riverine positions (upper, middle, and lower tidal river sites, and nearby upstream nontidal forested floodplains) of the adjoining Pamunkey and Mattaponi Rivers, Virginia. We tested differences by river, site, and plot in denitrification enzyme activity (DEA) and substrate limitations of denitrification potential (DP). The Pamunkey River carries greater river nitrate concentrations, and we found less nitrate limitation of DP and greater soil nitrate in hollows of this river. DEA in tidal hummocks was positively correlated with soil organic matter, nitrogen, and carbon, with the highest rates in lower tidal sites. Hummocks also promoted greater oxygen-controlled substrate limitation of DP, whereby experimental aeration stimulated DP under subsequent inundation more in hummocks than hollows. Additionally, tidal sites had greater DEA than nontidal sites, inferred to be caused by a combination of higher moisture, organic, and nutrient content. Our results indicate that the increasing nitrogen concentrations in these rivers will increase denitrification more on the Mattaponi River by alleviating its greater nitrogen limitation compared to the Pamunkey River, and modification to sedimentation, inundation, or microtopography from sea level rise may alter denitrification gradients in TFFWs and upstream low-elevation nontidal floodplains.

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

AbstractThis 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.

Forest composition and growth in a freshwater forested wetland community across a salinity gradient in South Carolina, USA

Tidal freshwater forested wetlands (TFFW) of the southeastern United States are experiencing increased saltwater intrusion mainly due to sea-level rise. Inter-annual and intra-annual variability in forest productivity along a salinity gradient was studied on established sites. Aboveground net primary productivity (ANPP) of trees was monitored from 2013 to 2015 on three sites within a baldcypress (Taxodium distichum) swamp forest ecosystem in Strawberry Swamp on Hobcaw Barony, Georgetown County, South Carolina. Paired plots (20×25-m) were established along a water salinity gradient (0.8, 2.6, 4.6PSU). Salinity was continuously monitored, litterfall was measured monthly, and growth of overstory trees ⩾10cm diameter at breast height (DBH) was monitored on an annual basis. Annual litterfall and stem wood growth were summed to estimate ANPP. The DBH of live and dead individuals of understory shrubs were measured to calculate density, basal area (BA), and important values (IV). Freshwater forest communities clearly differed in composition, structure, tree size, BA, and productivity across the salinity gradient. The higher salinity plots had decreased numbers of tree species, density, and BA. Higher salinity reduced average ANPP. The dominant tree species and their relative densities did not change along the salinity gradient, but the dominance of the primary tree species differed with increasing salinity. Baldcypress was the predominant tree species with highest density, DBH, BA, IV, and contribution to total ANPP on all sites. Mean growth rate of baldcypress trees decreased with increasing salinity, but exhibited the greatest growth among all tree species. While the overall number of shrub species decreased with increasing salinity, wax myrtle (Morella cerifera) density, DBH, BA, and IV increased with salinity. With rising sea level and increasing salinity levels, low regeneration of baldcypress, and the invasion of wax myrtle, typical successional patterns in TFFW and forest health are likely to change in the future.

Contemporary Deposition and Long-Term Accumulation of Sediment and Nutrients by Tidal Freshwater Forested Wetlands Impacted by Sea Level Rise

Contemporary deposition (artificial marker horizon, 3.5 years) and long-term accumulation rates (210Pb profiles, ~150 years) of sediment and associated carbon (C), nitrogen (N), and phosphorus (P) were measured in wetlands along the tidal Savannah and Waccamaw rivers in the southeastern USA. Four sites along each river spanned an upstream-to-downstream salinification gradient, from upriver tidal freshwater forested wetland (TFFW), through moderately and highly salt-impacted forested wetlands, to oligohaline marsh downriver. Contemporary deposition rates (sediment, C, N, and P) were greatest in oligohaline marsh and lowest in TFFW along both rivers. Greater rates of deposition in oligohaline and salt-stressed forested wetlands were associated with a shift to greater clay and metal content that is likely associated with a change from low availability of watershed-derived sediment to TFFW and to greater availability of a coastal sediment source to oligohaline wetlands. Long-term accumulation rates along the Waccamaw River had the opposite spatial pattern compared to contemporary deposition, with greater rates in TFFW that declined to oligohaline marsh. Long-term sediment and elemental mass accumulation rates also were 3–9× lower than contemporary deposition rates. In comparison to other studies, sediment and associated nutrient accumulation in TFFW are lower than downriver/estuarine freshwater, oligohaline, and salt marshes, suggesting a reduced capacity for surface sedimentation (short-term) as well as shallow soil processes (long-term sedimentation) to offset sea level rise in TFFW. Nonetheless, their potentially large spatial extent suggests that TFFW have a large impact on the transport and fate of sediment and nutrients in tidal rivers and estuaries.

Hydrological modification, saltwater intrusion, and tree water use of a Pterocarpus officinalis swamp in Puerto Rico

Tidal freshwater forested wetlands occupy a narrow ecological space determined by the balance between saltwater and freshwater inputs to the system. However, this balance is not well understood. In the Caribbean, tidal freshwater-forested wetlands dominated by Pterocarpus officinalis are vulnerable to changes in tidal influence and freshwater inputs. In this setting, the seasonal interactions of saltwater and freshwater inputs create less than ideal conditions for these forests to survive. Hence, it is crucial to have a better understanding of the hydrologic context of these and other tidal freshwater forested wetlands. We examined the extent of tidal forcing and saltwater influence in the largest Pterocarpus swamp of Puerto Rico by installing automated water level and conductivity recorders across a tidal creek transect at four different distances from the ocean, and by using water stable isotopes ratios (δD, δ18O) as natural tracers to determine the most important freshwater sources for tree transpiration. Records of water level and salinity revealed that the amount of rainfall was most influential on saltwater wedge migration in the creek for locations at the front and back of the tidal network, but that tidal dynamics were most influential at the middle section of the tidal network. Saltwater intrusion into the deepest parts of the tidal network was most prominent during sustained dry periods. Isotopic ratios of the surface water samples in the forest revealed that most of the water there was derived from freshwater runoff, but there was a seasonal change in its relative contribution to the forest hydrology. During the dry season, high δ values suggested the presence of runoff-derived water that had undergone evaporation, and saline influences were found in locations where past deforestation created preferential pathways for this water. During both seasons, δ 18O values of groundwater revealed the influence of saline water at depths 60 cm and greater near the adjacent tidal creek. Nonetheless, 18O-enriched stem water and low d-excess values suggested that Pterocarpus officinalis relies primarily on water accessed from unsaturated soil mounds that surround the tree and that are subject to evaporation, as opposed to groundwater at depth. This observation provides further insight to the adaptation mechanisms of Pterocarpus officinalis to survive in flooded ecosystems. In summary, our work demonstrates that for preventing saltwater intrusion, the amount of rainfall and freshwater inflow is more important than the sea level or the existence of an overland hydrological connection to the ocean.

Linking channel hydrology with riparian wetland accretion in tidal rivers

AbstractThe hydrologic processes by which tide affects river channel and riparian morphology within the tidal freshwater zone are poorly understood yet are fundamental to predicting the fate of coastal rivers and wetlands as sea level rises. We investigated patterns of sediment accretion in riparian wetlands along the nontidal through oligohaline portion of two coastal plain rivers in Maryland, U.S., and how flow velocity, water level, and suspended sediment concentration (SSC) in the channel may have contributed to those patterns. Sediment accretion was measured over a 1 year period using artificial marker horizons, channel hydrology was measured over a 1 month period using acoustic Doppler current profilers, and SSC was predicted from acoustic backscatter. Riparian sediment accretion was lowest at the nontidal sites (mean and standard deviation = 8 ± 8 mm yr−1), highest at the upstream tidal freshwater forested wetlands (TFFW) (33 ± 28 mm yr−1), low at the midstream TFFW (12 ± 9 mm yr−1), and high at the oligohaline (fresh‐to‐brackish) marshes (19 ± 8 mm yr−1). Channel maximum flood and ebb velocity was twofold faster at the oligohaline than tidal freshwater zone on both tidal rivers, corresponding with the differences in in‐channel SSC: The oligohaline zone's SSC was more than double the tidal freshwater zone's and was greater than historical SSC at the nontidal gages. The tidal wave characteristics differed between rivers, leading to significantly greater in‐channel SSC during floodplain inundation in the weakly convergent than the strongly convergent tidal river. High sediment accretion at the upstream TFFW was likely due to high river discharge following a hurricane.

Periodicity in Stem Growth and Litterfall in Tidal Freshwater Forested Wetlands: Influence of Salinity and Drought on Nitrogen Recycling

Many tidally influenced freshwater forested wetlands (tidal swamps) along the south Atlantic coast of the USA are currently undergoing dieback and decline. Salinity often drives conversion of tidal swamps to marsh, especially under conditions of regional drought. During this change, alterations in nitrogen (N) uptake from dominant vegetation or timing of N recycling from the canopy during annual litter senescence may help to facilitate marsh encroachment by providing for greater bioavailable N with small increases in salinity. To monitor these changes along with shifts in stand productivity, we established sites along two tidal swamp landscape transects on the lower reaches of the Waccamaw River (South Carolina) and Savannah River (Georgia) representing freshwater (≤0.1 psu), low oligohaline (1.1–1.6 psu), and high oligohaline (2.6–4.1 psu) stands; the latter stands have active marsh encroachment. Aboveground tree productivity was monitored on all sites through monthly litterfall collection and dendrometer band measurements from 2005 to 2009. Litterfall samples were pooled by season and analyzed for total N and carbon (C). On average between the two rivers, freshwater, low oligohaline, and high oligohaline tidal swamps returned 8,126, 3,831, and 1,471 mg N m−2 year−1, respectively, to the forest floor through litterfall, with differences related to total litterfall volume rather than foliar N concentrations. High oligohaline sites were most inconsistent in patterns of foliar N concentrations and N loading from the canopy. Leaf N content generally decreased and foliar C/N generally increased with salinization (excepting one site), with all sites being fairly inefficient in resorbing N from leaves prior to senescence. Stands with higher salinity also had greater flood frequency and duration, lower basal area increments, lower tree densities, higher numbers of dead or dying trees, and much reduced leaf litter fall (103 vs. 624 g m−2 year−1) over the five study years. Our data suggest that alternative processes, such as the rate of decomposition and potential for N mineralization, on tidal swamp sites undergoing salinity-induced state change may be more important for controlling N biogeochemical cycling in soils than differences among sites in N loading via litterfall.

Research Insight from Tidal Freshwater Forested Wetlands

Research Insight from Tidal Freshwater Forested Wetlands