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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.

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.

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.

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.

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.

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.

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.

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.