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As rising sea levels alter coastal ecosystems, there is a pressing need to examine the effects of saltwater intrusion on coastal communities. Using 16S Illumina profiling, we characterized the communities of baldcypress tree (Taxodium distichum) root endosphere and rhizosphere soil bacteria. Our study utilized established sites along salinity and flooding gradients in the United States of Georgia, Louisiana, and South Carolina. We hypothesized that environmental variables, namely salinity and water level, as well as distance between sites would be correlated with baldcypress-associated rhizosphere and root endosphere bacterial communities. We found that geographic distance correlated with rhizosphere but not root endosphere bacterial communities, suggesting that the trees may have stabilized their endosphere communities via recruitment of a more specific suite of taxa from the surrounding soil. Mean water level, mean salinity, and the volume of woody debris were associated with both endosphere and rhizosphere bacterial communities in baldcypress trees. The density of host trees was also associated with endosphere community composition. Our study is the first to use 16S Illumina sequencing to characterize bacterial communities in baldcypress trees— a key restoration species in coastal swamp ecosystems under threat from rising sea levels.

Decomposing foliar litter organic matter (LOM) in forested watersheds is an essential terrestrial source of dissolved organic matter (DOM) to aquatic ecosystems. To evaluate the relationship of chemical composition between LOM and DOM, we collected freshly fallen leaves of white oak (Quercus alba) and conducted an 80-week field decomposition experiment along a small elevation gradient in a floodplain within the Congaree National Park, South Carolina. Foliar litters were collected for water extraction and instrumental analyses using pyrolysis GC/MS, UV/VIS and fluorescence spectrophotometry. Factor analyses of pyrolysates showed that fresh LOM was mainly composed of lignin compounds (LgC), phenolic compounds (PhC), and carbohydrate (Carb), and the change in composition was relatively small throughout the decomposition process. In contrast, there were two distinct chemical compositions of DOM in early (between 0–8 weeks) and late phases (between 16–80 weeks). The early phase had a higher percentage of PhC, but the late phase contained higher percentages of Carb, unsaturated hydrocarbon (UnSaH), aromatic hydrocarbon (ArH), and nitrogen-containing compounds (Ntg). The fluorescence emission-excitation matrix showed there was an increasing trend in humic and fulvic-like fractions in DOM over time, matching well with increases of UnSaH and ArH fractions from the results of pyrolysis GC/MS. Our study illustrated that the changes of chemical components in LOM and water extractable DOM were not parallel during the decomposition process and the degradation of lignin and phenolic compounds was one of the controlling factors on the production of DOM.

Occupancy patterns of herpetofauna in most tidal freshwater swamps are unknown. Tidal freshwater swamps currently face multiple threats, including salinization, which can influence their associated plant and animal communities. The impacts of salinization to herpetofauna communities in tidal freshwater swamps have not been assessed. To improve predictions regarding these herpetofauna, we conducted surveys in tidal freshwater swamps of the Savannah National Wildlife Refuge located in South Carolina, USA, from March to June, 2016 and 2017, using a variety of methods. Goals included inventorying species, determining communities, examining microhabitat associations, and modeling occupancy to predict the impacts of salinity changes. We detected 8 species of amphibians and 12 species of reptiles in our surveys. Community analyses failed to detect patterns related to measured environmental variables. Species richness and diversity declined along the salinity gradient, but the observed patterns did not match our predictions and may instead be related to site-level heterogeneity. Microhabitat associations were detected for two amphibian species via occupancy analyses. Occupancy and regression analyses indicated soil salinity may be a factor affecting nine species’ occurrences. Amphibian detections may be affected by water depth, pH values, and weather conditions. These results expand our understanding of herpetofauna within an understudied, and threatened, wetland type.

Riverine forests support high rates of plant productivity, yet total wood carbon (C) stocks in these systems remain understudied. We measured C concentrations and dry mass of live and dead detrital wood to understand their importance relative to total ecosystem C storage across an elevation gradient on the floodplain of the Congaree River. Our study sites included a low elevation bald cypress swamp and three higher elevation mature mixed bottomland hardwood communities. Average wood C concentration was 45.9 ± 0.07% and estimated cumulative C pools (sum of downed and standing dead wood plus live tree, sapling, and shrub wood) ranged from 6850 to 17,200 g C/m2 for bottomland hardwood forests and 17,700 g C/m2 for the bald cypress swamp. Most of the aboveground C was stored in living wood (83.1–90.5% total aboveground pools). Our results indicate that the mass and C stored in live and dead wood are at the high end of both measured and modeled ranges reported in the literature for temperate zone forests. As such, the mature bottomland forests of the Congaree River represent a substantial and important store of sequestered C.

The objectives of this study were to identify processes that contribute to resilience of coastal wetlands subject to rising sea levels and to determine whether the relative contribution of these processes varies across different wetland community types. We assessed the resilience of wetlands to sea-level rise along a transitional gradient from tidal freshwater forested wetland (TFFW) to marsh by measuring processes controlling wetland elevation. We found that, over 5 years of measurement, TFFWs were resilient, although some marginally, and oligohaline marshes exhibited robust resilience to sea-level rise. We identified fundamental differences in how resilience is maintained across wetland community types, which have important implications for management activities that aim to restore or conserve resilient systems. We showed that the relative importance of surface and subsurface processes in controlling wetland surface elevation change differed between TFFWs and oligohaline marshes. The marshes had significantly higher rates of surface accretion than the TFFWs, and in the marshes, surface accretion was the primary contributor to elevation change. In contrast, elevation change in TFFWs was more heavily influenced by subsurface processes, such as root zone expansion or compaction, which played an important role in determining resilience of TFFWs to rising sea level. When root zone contributions were removed statistically from comparisons between relative sea-level rise and surface elevation change, sites that previously had elevation rate deficits showed a surplus. Therefore, assessments of wetland resilience that do not include subsurface processes will likely misjudge vulnerability to sea-level rise.

Downed woody debris is an important component of the forest floor, responsible for long-term storage of nutrients in many ecosystems. In large floodplain forests of the southeastern United States, wood loads are relatively low, suggesting these landscapes may promote rapid decomposition and nutrient turnover rates. The goals of this study were to identify the major factors responsible for wood decomposition and associated nutrient dynamics in an old-growth floodplain forest at Congaree National Park, South Carolina (USA). In situ decomposition and nutrient dynamics were quantified for 700 days (April 2011–March 2013) using wood from a common overstory species, red maple (Acer rubrum L.). During the study period, regional drought conditions allowed red imported fire ant (RIFA, Solenopsis invicta Buren) colonization and infestation of the study wood after 183 days in the field. Carbon (C) content decreased throughout the study, indicating net mineralization was occurring year-round. Invertebrate activity contributed to extremely fast decomposition rates (mean k 0.650 ± 0.02 year−1) and wood turnover times (mean 4.79 ± 0.15 years). In contrast to C, wood nitrogen (N) and phosphorus (P) content increased following RIFA infestation, displaying net nutrient immobilization. The rapid turnover rates measured at CONG indicate that the C storage function of woody debris may be diminished, but availability of mineralized N and P may also have increased the importance of wood stocks to support forest net primary productivity.

Alterations to Louisiana’s coastal landscape during the last 200 years have resulted in major changes to the natural hydrologic regime. Hurricane and tornado winds further impact coastal forests by exacerbating conditions that affect community structure, growth, and regeneration. Twenty-three years of study of forested wetlands in the Lake Verret watershed provide an opportunity to relate changes in forest communities to precipitation, drought, storms, and rising water levels. The area had two communities in 1986, but four communities were present by 2009. A major change in community composition occurred in drier sites after Hurricane Andrew (1992), when Chinese tallow (Triadica sebifera) became established. Swamp plots were generally not affected by hurricanes, but composition continued to change as a result of flooding. After Hurricane Andrew, net primary productivity (NPP) declined dramatically until 2001. Subsequent hurricanes in 2004 and 2005 also caused reductions in NPP, but recovery was rapid. Low-lying and transition areas are trending from bottomland communities toward more of a deepwater swamp as baldcypress (Taxodium distichum) and water tupelo (Nyssa aquatica) increase their stature, likely due to the system-wide flooding. Natural ridges are flooded less often and have remained primarily bottomland, though Chinese tallow (Triadica sebiferum) numbers increased following Hurricane Andrew.

Little is known of the ability of baldcypress (Taxodium distichum (L.) L.C. Rich.) in a suppressed sub-canopy position to respond to increases in light. We measured 3 years of diameter growth response of long-suppressed baldcypress saplings to canopy gap creation in a hydrologically altered, water tupelo-dominated backswamp of the lower Roanoke River floodplain in eastern North Carolina. Baldcypress saplings responded strongly to gap creation. Saplings showed a significant positive growth trend, with mean basal area growth in gaps expanding from 2.5 cm2 in year one, 4 times that of control saplings, to 8.5 cm2 in year three, 20 times that of control saplings. Within canopy gaps, a positive trend between growth and initial sapling diameter was evident, while such a trend among controls was non-existent. Mortality was similar between gap and control saplings after 1 year, but by year three gap sapling mortality had declined to 2 %, while control mortality increased to 16 %. It is clear that long-suppressed baldcypress, sometimes at the brink of death, are able to respond vigorously to an increase in light from canopy gaps with decreased mortality and increased growth. This has important implications for wetlands requiring restoration or baldcypress management within the framework of two-age or uneven-aged silvicultural systems.

Hummock and hollow microtopography is pervasive in tidal freshwater swamps. Many tree species grow atop hummocks significantly more than in hollows, leading to the hypothesis that hummocks provide preferred locations for maximizing physiological proficiency of inhabiting trees that experience repeated flooding. We used thermal dissipation probes to measure the ecophysiological proficiency of a very flood-tolerant tree, Taxodium distichum, as manifested through in-situ changes in sapflow (a proxy for transpiration) in 11 trees on hummocks and 11 trees in hollows. Overall, sapflow increased significantly by 3.3 g H2O m−2 s−1 (11 %) in trees on both hummocks and hollows during flooding, contrary to our expectations. We found no significant differences in sapflow rates between T. distichum trees positioned on hummocks versus hollows in relation to discrete flood events. Coincidentally, hummock elevations were equivalent to the flood depths that promoted greatest physiological proficiency in T. distichum, suggesting a physiological role for the maintenance of hummock height in tidal swamps. While we reject our original hypotheses that flooding and positioning in hollows will reduce sapflow in T. distichum, this research reveals a potentially important feedback between hummock height, flood depth, and maximum tree physiological response.

Tidal freshwater wetlands are sensitive to sea level rise and increased salinity, although little information is known about the impact of salinification on nutrient biogeochemistry in tidal freshwater forested wetlands. We quantified soil nitrogen (N) and phosphorus (P) mineralization using seasonal in situ incubations of modified resin cores along spatial gradients of chronic salinification (from continuously freshwater tidal forest to salt impacted tidal forest to oligohaline marsh) and in hummocks and hollows of the continuously freshwater tidal forest along the blackwater Waccamaw River and alluvial Savannah River. Salinification increased rates of net N and P mineralization fluxes and turnover in tidal freshwater forested wetland soils, most likely through tree stress and senescence (for N) and conversion to oligohaline marsh (for P). Stimulation of N and P mineralization by chronic salinification was apparently unrelated to inputs of sulfate (for N and P) or direct effects of increased soil conductivity (for N). In addition, the tidal wetland soils of the alluvial river mineralized more P relative to N than the blackwater river. Finally, hummocks had much greater nitrification fluxes than hollows at the continuously freshwater tidal forested wetland sites. These findings add to knowledge of the responses of tidal freshwater ecosystems to sea level rise and salinification that is necessary to predict the consequences of state changes in coastal ecosystem structure and function due to global change, including potential impacts on estuarine eutrophication.

Seasonally flooded, freshwater cypress-tupelo wetlands, dominated by baldcypress (Taxodium distictum), water tupelo (Nyssa aquatica), and swamp tupelo (Nyssa sylvatica var. biflora) are commonly found in coastal regions of the southeastern United States. These wetlands are threatened due to climate change, sea level rise, and coastal urban development. Understanding the natural biogeochemical cycles of nutrients in these forested wetlands as ecosystems services such as carbon sequestration and nitrogen processing can provide important benchmarks to guide conservation plans and restoration goals. In this study, surface water and soil pore water samples were collected weekly from a cypress-tupelo wetland near Winyah Bay, South Carolina and analyzed for dissolved organic carbon (DOC), dissolved organic nitrogen (DON), inorganic nitrogen, and phosphate during its flooding period between October 2010 and May 2011. DOC was further characterized by specific ultra-violet absorbance at 254 nm, spectral slope ratio (SR) (ratio of two spectral slopes between 275–295 nm and 350–400 nm), E2/E3 ratio (ratio between A254 and A365), and fluorescence excitation-emission matrix. In addition, litterfall was collected on a monthly basis for a year while the biomass of the detritus layer (i.e., decomposed duff lying on the wetland floor) was determined before and after the flooding period. Results of the field study showed that concentrations of DOC, DON, NH4 +–N, and (NO2 − + NO3 −)–N in the surface water were generally higher during the fall, or peak litterfall season (October to December), than in the spring season (March to May). Highest concentrations of 54.8, 1.48, 0.270, and 0.0205 mg L−1, for DOC, DON, NH4 +–N, and (NO2 − + NO3 −)–N respectively, in surface waters were recorded during October. Lower SUVA, but higher SR and E2/E3 ratios of DOC, were observed at the end of the flooding season comparing to the initial flooding, suggesting the wetland system converts high aromatic and large DOC molecules into smaller and hydrophilic fractions possibly through photochemical oxidation. A similar trend was observed in soil pore water, but the pore water generally had greater and relatively stable concentrations of dissolved nutrients than surface water. No obvious temporal trend in phosphate concentration and total nitrogen to total phosphorus ratio (N:P) were found. Results of the laboratory extraction and mass balance calculation suggested fresh litter was a major source of DOC whereas decomposed duff was the source of dissolved nitrogen in surface water. In summary, the biogeochemistry of this isolated cypress-tupelo wetland is not only driven by the vegetation within the wetland system but also by hydrology and weather conditions such as groundwater table position, precipitation, and temperature.

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.

This report documents changes in forest structure and growth potential of dominant trees in salt-impacted tidal and non-tidal baldcypress wetlands of the southeastern United States. We inventoried basal area and tree height, and monitored incremental growth (in basal area) of codominant baldcypress (Taxodium distichum) trees monthly, for over four years, to examine the inter-relationships among growth, site fertility, and soil physico-chemical characteristics. We found that salinity, soil total nitrogen (TN), flood duration, and flood frequency affected forest structure and growth the greatest. While mean annual site salinity ranged from 0.1 to 3.4 ppt, sites with salinity concentrations of 1.3 ppt or greater supported a basal area of less than 40 m2/ha. Where salinity was < 0.7 ppt, basal area was as high as 87 m2/ha. Stand height was also negatively affected by higher salinity. However, salinity related only to soil TN concentrations or to the relative balance between soil TN and total phosphorus (TP), which reached a maximum concentration between 1.2 and 2.0 ppt salinity. As estuarine influence shifts inland with sea-level rise, forest growth may become more strongly linked to salinity, not only due to salt effects but also as a consequence of site nitrogen imbalance.

Little is known about the effectiveness of mangroves in suppressing water level heights during landfall of tropical storms and hurricanes. Recent hurricane strikes along the Gulf Coast of the United States have impacted wetland integrity in some areas and hastened the need to understand how and to what degree coastal forested wetlands confer protection by reducing the height of peak water level. In recent years, U.S. Geological Survey Gulf Coast research projects in Florida have instrumented mangrove sites with continuous water level recorders. Our ad hoc network of water level recorders documented the rise, peak, and fall of water levels (± 0.5 hr) from two hurricane events in 2004 and 2005. Reduction of peak water level heights from relatively in-line gages associated with one storm surge event indicated that mangrove wetlands can reduce water level height by as much as 9.4 cm/km inland over intact, relatively unchannelized expanses. During the other event, reductions were slightly less for mangroves along a river corridor. Estimates of water level attenuation were within the range reported in the literature but erred on the conservative side. These synoptic data from single storm events indicate that intact mangroves may support a protective role in reducing maximum water level height associated with surge.

Coastal baldcypress (Taxodium distichum) forests are being killed in the southern United States as a result of saltwater intrusion. Tank studies have indicated substantial intraspecific variation in salt tolerance within baldcypress populations, but only one field study has been conducted and it used a limited seed source. The major objective of this project was to expand the collection range of baldcypress across the southeastern United States and determine if there are baldcypress populations that can survive and grow in saltwater damaged areas. Seeds were collected in 1996 from eight estuarine areas (James River = VA; Cape Fear River = NC; Winyah Bay = SC; Ogeechee River = GA; Ochlockonee River = FL; Mobile Bay = AL; Biloxi River=MS; Chalmette = LA). Seeds were stratified and planted, and seedlings were grown for 2 years before planting in two abandoned ricefields on Hobcaw Barony near Georgetown, South Carolina in 1999. Salinity levels reached 18.5ppt during 2001 at the peak of the worst drought on record. By 2002, the only seedlings surviving were those from LA, AL, and FL. After 5 years in the field, LA seedlings were the best performers with a mortality rate of only 27%. More detailed analyses of the biologic and genetic characteristics of trees growing in the LA site need to be completed to determine if they represent a source of baldcypress seed possessing greater tolerance to saline conditions. If so, seeds need to be collected and nurseries established to grow seedlings to help restore degraded wetland swamp areas along the Gulf and Atlantic coasts.

Insufficient sedimentation, coupled with high rates of relative sea-level rise (subsidence plus eustatic sea-level rise), are two important factors contributing to wetland loss in coastal Louisiana, USA. We hypothesized that adding nutrient-rich, secondarily treated wastewater effluent to subsiding wetlands in Louisiana could promote vertical accretion in these systems through increased organic matter production and subsequent deposition and allow accretion to keep pace with estimated rates of relative sea-level rise (RSLR). However, we also hypothesized that nutrient enrichment could stimulate the decomposition of organic matter, thus negating any increase in accretion due to increased organic matter accumulation. To test these hypotheses, we measured leaf-litter decomposition, litter nutrient dynamics, and sediment accretion in a permanently flooded and subsiding forested wetland receiving wastewater effluent and in an adjacent control site, both before and after effluent applications began. We...

Paired plots were established across a soil moisture gradient (dry, periodically flooded, flooded) in three forested wetland watersheds in Louisiana and South Carolina, USA in 1986–1987. All trees greater than 10-cm diameter at breast height were tagged and measured annually through 1999 to determine density, basal area changes, ingrowth, and mortality. A greater number of tree species was found in South Carolina dry and periodically flooded sites than in Louisiana. Flooded sites in both states were dominated by water tupelo (Nyssa aquatica) and baldcypress (Taxodium distichum). The overall trend in density in both states was flooded > periodically flooded > dry. From 1987 to 1999, density decreased in all of the Louisiana sites except one, while in South Carolina, density remained the same or increased in four of the sites and decreased in the other four. The greatest changes in density occurred in those sites where water-level changes were occurring and in areas where storm winds struck. Basal area in 1987 was similar in both states, ranging from 20.8 to 55.2 m2/ha in Louisiana and 24.0 to 49.5 m2/ha in South Carolina. Flooded sites had the greatest basal area, and periodically flooded and dry sites had similar basal areas. Mortality rates in Louisiana and South Carolina forested wetlands are typically low (around 2%/year) in areas that have not been altered hydrologically. Annual mortality in Louisiana plots with increased water levels rose from 4% in 1987 to 16% in 1997. Wind storm events significantly increased mortality rates, and mortality rates remained high for years after the event, as damaged trees died. Of special concern are areas like the Verret Basin site where the exotic, invasive species Chinese tallow (Sapium sebiferum) invaded after Hurricane Andrew and has the potential to become a dominant canopy tree in the future.

Lake Chicot, in south central Louisiana, USA, was created in 1943 by the impoundment of Chicot Bayou. Extensive establishment of woody seedlings occurred in the lake during a 1.5 year period, including the growing seasons of both 1986 and 1987, when the reservoir was drained for repair work on the dam. Study plots were established in September 1986 to document woody vegetation establishment and to provide a baseline by which to monitor survival and growth after flooding resumed.Taxodium distichum seedlings were the dominant species after one growing season, with a maximum density of 50 seedlings/m2, an average of about 2/m2, and an average height of 75 cm. The lake was reflooded at the end of 1987, bringing water depths at the study plots up to about 1.4 m. Temporary drawdowns were again conducted during the fall of 1992 and 1996. In December 1992, the site was revisited, new plots established, and saplings counted and measured. There was an average of 2.1T. distichum stems/m2, and the average height was 315 cm. After the 1996 growing season, there was still an average of about 1.9 stems/m2, and the average height had increased to 476 cm. Preservation ofT. distichum forests in relatively shallow but continuously flooded areas such as Lake Chicot may be a simple matter of draining the lake after a good seed crop and maintaining the drawdown long enough for the seedlings to grow taller than the typical growing season water level. In the case of Lake Chicot, this period was two growing seasons. This action will mimic natural, drought-related drawdowns of the lake and will allow the seedlings to establish themselves and grow tall enough to survive normal lake water levels.

Buttonbush (Cephalanthus occidentalis) and swamp tupelo (Nyssa sylvatica var.biflora) seedlings were exposed to flooding and salinity conditions simulating the chronic exposure of sea-level rise and the acute exposure of hurricane storm surge. Chronically exposed seedlings were either watered or flooded with 0, 2, or 10 ppt salinity. Those in the acute experiment were watered or flooded with freshwater until exposed to a 21-ppt salinity surge for 48 hours. Freshwater flooding reduced photosynthesis (A), water pressure potential (Ψ), height, and stem and root biomass for swamp tupelo while biomass of buttonbush roots decreased and stem increased. Chronic watering with 2 ppt salinity reduced height, basal diameter, and stem and root biomass for swamp tupelo but did not significantly affect buttonbush. Both species were negatively affected by watering with 10 ppt salinity, with total mortality of swamp tupelo. Flooding with 2 ppt salinity also caused a 100% mortality for swamp tupelo. All buttonbush seedlings survived with reducedA, Ψ, and stem and root biomass. Plants chronically flooded with 10 ppt salinity were more affected, with drastically decreasedA, stomatal conductance (g s ), and Ψ within the first day of treatment and all dying within three weeks. Watering with 21 ppt salinity reducedA andg, of both species during the second day of the surge, but buttonbush recovered within the next 20 days. Flooding with 21-ppt saline water also greatly impacted buttonbush, which did not recover as when watered with 21 ppt salinity. Swamp tupelo was already stressed due to flooding and showed no further reduction in A org s due to the 21-ppt salinity flood surge. Buttonbush seedlings were more tolerant of salinity and flooding conditions than swamp tupelo seedlings, although it is unlikely that either species could survive long-term exposure to 10-ppt salinity flooding.

The survival, growth, and biomass of baldcypress (Taxodium distichum (L.) Rich.), water tupelo (Nyssa aquatica L.), Chinese tallow (Sapium sebiferum (L.) Roxb.), and green ash (Fraxinus pennsylvanica Marsh.) seedlings were examined in an experiment varying water levels (watered, flooded) and salinity levels (0, 2, and 10 ppt, plus a simulated storm surge with 32 ppt saltwater). All seedlings, except for those flooded with 10 ppt saltwater, survived to the end of the experiment. In 10 ppt saltwater, flooded baldcypress, water tupelo, and green ash survived two weeks whereas Chinese tallow survived for 6 weeks. However, a second set of slightly older baldcypress, water tupelo, and Chinese tallow seedlings survived eight weeks of flooding with 10 ppt saltwater. When carried through the winter to the beginning of the second growing season, flooded baldcypress and Chinese tallow seedlings from the 0 and 2 ppt treatments leafed out, but only Chinese tallow recovered from the saltwater surge treatment. The diameter and growth (height) of each species was not affected when watered with 2 ppt saltwater, except for the effects of the height growth of baldcypress. Growth was reduced for all species when watered with 10 ppt saltwater. The diameter growth of green ash was reduced by freshwater flooding. The diameter growth of baldcypress and water tupelo was greater when flooded with fresh water. Flooding with 2 ppt saltwater caused a significant reduction in diameter growth in water tupelo, green ash, and Chinese tallow, but not in baldcypress. Root and stem biomass values were not significantly different for any species between the 0 and 2 ppt salinity watering treatments. However, seedlings watered with 10 ppt saltwater had significantly lower root and stem biomass values, except for baldcypress roots and green ash stems. Baldcypress was least affected by flooding with 0 and 2 ppt saltwater, although there were slight reductions in root biomass with increasing salinity. Because of the susceptibility of the seedlings of these four species to increases in flooding and salinity, their regeneration may be limited in the future, thereby causing shifts in species composition.

The influence of flooding and salinity on photosynthesis and water relations was examined for four common coastal tree species [green ash (Fraxinus pennsylvanica Marshall), water tupelo (Nyssa aquatica L.). Chinese tallow (Sapium sebiferum (L.) Roxb.), and baldcypress (Taxodium distichum (L.) Richard)]. Both chronic (as might be associated with sea level rise) and acute (similar to hurricane storm surges) exposures to these stresses were examined. Chronic freshwater flooding of green ash, water tupelo, and Chinese tallow seedlings reduced photosynthesis (A) relative to that of watered seedlings, while baldcypress was unaffected. Chinese tallow A declined with increasing length of flooding. A salinity increase of the floodwater to 2 ppt decreased A of baldcypress and water tupelo, but not A of green ash and Chinese tallow, which was already severely reduced by freshwater flooding. All seedlings of the four species died within 2 to 6 weeks when flooded with 10 ppt saltwater. Photosynthesis of all four species did not differ between 0 and 2 ppt watering. Watering with 10 ppt salinity initially reduced A of all four species, but the seedlings recovered over time. Photosynthesis was severely decreased for all species when flooded with 21 ppt salinity for 48 hours. Reduced A continued following the treatment. Photosynthesis of only green ash and water tupelo was reduced by watering with 21 ppt salinity for 48 hours. Flooding of low-lying areas with increased salinity would lead to shifts in species composition of coastal forests due to these differential tolerances.

In the interest of increasing productivity of forested wetlands for timber production and/or wildlife value, management schemes that deal mainly with water-level control have been developed. The three forest types in the southeastern U.S. most commonly affected are cypress/tupelo forests, bottomland hardwood forests, and wet pine sites (including pocosins). In forested wetlands, hydrology is the most important factor influencing productivity. In bottomland and cypress/tupelo forests, water-level control can have mixed results. Alterations in natural hydrologic patterns leading to increased flooding or drainage can cause decreased growth rates or even death of the forest. Bottomland hardwoods respond favorably in the short term to water-level management, but the long-term response is currently under study. In wet pine sites, timber volume can be increased significantly by water-level management, but the impact upon other ecological functions is less understood. It is difficult to adequately describe productivity relations in wetland forests because of the great diversity in habitat types and the lack of data on how structure and function might be affected by forestry operations. There is a definite need for more long-term, regional studies involving multidisciplinary efforts.

The Verret basin was formerly an overflow area between the Mississippi and Atchafalaya rivers and contains about 41,000ha of forested wetlands. Water levels are rising at the rate of over 1 cm/year in this area, and the forests are subjected to longer and deeper flooding. Tree growth, litterfall, and species composition were monitored across a flooding gradient during January 1985–December 1986. The driest area was only 20 cm higher in elevation than the wettest area, but the structure of the forest changes greatly over this range. The drier area was dominated by sweetgum (Liquidambar styraciflua L.), oaks (Quercus spp.), and sugarberry (Celtis laevigata Willd.), while green ash (Fraxinus pennsylvanica Marsh.), red maple (Acer rubrum L.), and baldcypress (Taxodium distichum (L.) Rich.) were dominant in the wetter area. Green ash and bitter pecan (Carya aquatica (Michaux. f.) Nutt.) were found in all plots, but these two species are under severe stress in the more flooded area as evidenced by dead and dying trees. Stem wood production increased from 1985 to 1986 in the driest (392 to 473 g/m2/yr) and wettest (199 to 399 g/m2/yr) plots, but remained relatively unchanged in the transitional area (386 to 380g/m2/yr). Leaf litter production decreased across the gradient from dry to flooded plots during both years. Over 40% of the litterfall in the drier plot was from flood-tolerant shrub species. In the flooded plots, red maple and baldcypress were major contributors to total litterfall. Increased flooding of dry bottomland forests in the future could lead to decreased litterfall and increased tree death over the entire watershed.

Six-month-old and eighteen-month-old baldcypress and loblolly pine seedlings were subjected to a simulated saltwater surge (salinity=30 parts per thousand) for 0 to 5 days followed by fiushing and daily watering with fresh water. Growth, biomass, and survival were monitored for 9 weeks following inundation. Six-month-old seedlings of both species were extremely susceptible to saltwater flooding, and survival percentages declined rapidly beyond I day of saltwater flooding. Eighteen-month-old seedlings were better able to survive saltwater flooding, with some pine seedlings surviving up to 3 days of flooding and some baldcypress up to 4 days of flooding. The general response of surviving baldcypress seedlings to saltwater flooding was to die back and then resprout. For loblolly pine, growth was slowed until a threshold level was reached, at which point the seedlings died. There was a direct correlation between duration of saltwater flooding and diameter growth with six-month-old seedlings but not with the eighteen-month-old seedlings. There was a steady decline in diameter with increasing days of saltwater flooding in surviving eighteen-month-old baldcypress, whereas eighteen-month-old loblolly seedlings showed some diameter growth even after 3 days of saltwater flooding Root:shoot biomass partitioning in surviving seedlings was not appreciably affected by treatments.

Litter decomposition was studied for one year in three Louisiana forested wetland sites with different flooding regimes. Decomposition was significantly higher in a crayfish pond where flooding was manipulated by pumping. Only 20% of the original dry mass remained after 46 weeks versus over 40% in the natural and impounded wetland forests. Nitrogen was immobilized in the natural and impounded areas but was mineralized during the spring and summer in the managed area. Phosphorus was not immobilized in the natural and impounded areas like nitrogen but was mineralized much more slowly in these areas than it was in the managed area. Because of the slower decomposition in the natural and impounded areas, falling leaf litter buries the old, partially decomposed material. Thus, there tends to be a net accumulation of organic matter, nitrogen, and phosphorus in stagnant, more flooded areas and mineralization and/or export from free-flowing areas.

As global climate changes, sea level rise and increased frequency of hurricanes will expose coastal forests to increased flooding and salinity. Quercus species are frequently dominant in these forest, yet little is known about their salinity tolerance, especially in combination with flooding. In this study, 1-year-old seedlings of Quercus lyrata Walt. (overcup oak), Q. michauxii Nutt. (swamp chestnut oak), Q. nigra L. (water oak), and Q. nuttallii Palmer (Nuttall oak) were chronically (simulating sea level rise) and acutely (simulating hurricane storm surge) exposed to increased flooding and salinity, individually and in combination. The four species demonstrated two response patterns of photosynthesis (A), conductance, and leaf water potential, apparently related to their relative flood tolerance. In Q. lyrata, Q. nuttallii, and Q. nigra (moderately flood-tolerant), A was not immediately reduced after the initiation of the freshwater flooding, but was reduced as the duration of flooding increased. In the second pattern, demonstrated by the weakly flood-tolerant Q. michauxii, A was immediately reduced by freshwater flooding with an increasing impact over time. Watering with 2 parts per thousand (ppt) saline water did not consistently reduce A, but flooding with 2 ppt reduced A of all species, similar to the response with freshwater flooding. Photosynthesis of all species was reduced by 6 ppt watering or flooding, with the latter treatment killing all species within 8 weeks. When acutely exposed to 30 ppt salinity, A was quickly and severely reduced regardless of whether the seedlings were watered or flooded. Acutely flooded seedlings exposed to high salinity died within 2 weeks, but seedlings watered with 30 ppt saline water recovered and A was not reduced the following spring. As saline flooding of coastal areas increases due to sea level rise, photosynthesis of these species will be differentially affected based primarily on their flood tolerance. This suggests that increased flooding associated with sea level rise will impact these tree species to a greater extent than small increases in soil salinity. High salinity accompanying storm surges will be very harmful to all of these species.

Available literature indicates that hurricanes do not generally produce long-term detrimental impacts to unmodified coastal systems and that they often provide net benefits along the U.S. Gulf Coast. While there is normally initial erosion from hurricanes, they also often result in a large influx of inorganic sediments, creating new wetlands and contributing to the maintenance of existing wetlands. The formation of washover deposits is disastrous where cultural development has occurred, but in natural areas these deposits are part of the natural cycle of shoreline development and contribute to habitat diversity and productivity. Abundant rainfall typically associated with hurricanes often results in large increases of sediment and nutrient inputs into coastal estuaries, leading to both short-term and long-term increases in productivity. Rainfall during tropical disturbances accounts for a significant part of total precipitation along the northern gulf. The immediate impact of hurricanes may be to reduce populations of some species but these populations generally recover rapidly. Overall, productivity in natural systems seems to be increased by periodic hurricanes. Hurricane impacts are often severe and long lasting in wetlands that have been modified by human impacts such as semi- or complete impoundments.

A field and laboratory study was conducted to determine decomposition rates and nutrient flux in an impounded, managed swamp forest in the headwater swamp forests of the Barataria Basin. The role of redox potential and heterotrophic immobilization in determining floodwater nutrient concentrations were examined in the field and in laboratory microcosms. Twenty-six percent of the nitrogen and 40 percent of the phosphorus introduced into the system were retained in teh swamp impoundment, mainly as a result of the settlement of particulate matter. Weight loss of leaf litter in the microcosms closely followed field samples, but decomposition was more rapid and complete in the field. This greater efficiency of breakdown in the field may be the result of the exclusion of macrofauna in the microcosms and greater turbulence under field conditions. Overall results indicate that the area is a long-term sink for both nitrogen and phosphorus via the burial of partially decomposed organic matter under reduced conditions and denitrification.

One-year-old baldcypress (Taxodium distichum [L.] Rich.) seedlings were planted across a flooding gradient in a Louisiana bottomland forest in March and September 1985. One half of the seedlings were protected with a chickenwire fence. Survival and height growth were monitored from 1985 through 1987. First year survival was high in all areas for both plantings, except for the unprotected March-planted seedlings, which were destroyed by animals. After three growing seasons, survival of March-planted seedlings in the flooded and intermittently flooded plots was still over 70%. Natural mortality of March-planted seedlings in the unflooded plot was high, with only 40% of the seedlings surviving to the end of the study. Survival of the September-planted seedlings was also best in the flooded and intermittently flooded plots and least in the unflooded plot. Height growth, like survival, was best for seedlings in the flooded and intermittently flooded plots. It seems that baldcypress seedlings can be planted in a swamp and grow well if animal damage is controlled.

An important factor contributing to the deterioration of wetland forests in Louisiana is increasing water levels resulting from eustatic sea-level rise and subsidence. Analyses of long-term water level records from the Barataria and Verret watersheds in southeastern Louisiana indicate an apparent sea level rise of about 1-m per century, mainly the result of subsidence. Permanent study plots were established in cypress-tupelo stands in these two watersheds. The tree, water level, and subsidence data collected in these plots were entered into the U.S. Fish and Wildlife Service’s FORFLO bottomland hardwood succession model to determine the long-term effects of rising water levels on forest structure. Analyses were made of 50–100 years for a cypress-tupelo swamp site in each basin and a bottomland hardwood ridge in the Verret watershed. As flooding increased, less flood tolerant species were replaced by cypress-tupelo within 50 years. As flooding continued, the sites start to become nonforested. From the test analyses, the FORFLO model seems to be an excellent tool for predicting long-term changes in the swamp habitat of south Louisiana.