Baldcypress Swamp Research Articles

Snapshots of Coastal Ecology Using Multiproxy Analysis Reveals Insights Into the Preservation of Swamp and Marsh Environments Since the Late Pleistocene

AbstractThe southeastern United States Coastal Plain ecosystem contains baldcypress (Taxodium distichum) swamps and grass‐dominated marshes. These ecosystems also occurred on the exposed continental shelf during lower sea levels but are rarely preserved due to the mechanically erosive nature of transgression and regression. Two presently marine sites on the northeastern Gulf of Mexico's continental shelf contain well‐preserved woody terrestrial sediments that were the subject of previous studies. This study continues the investigation using geochemical (δ13C, δ15N, δ34S) and palynological characteristics of these formerly terrestrial sediments to determine if swamps and/or marshes existed at the time of deposition. The first site is located ∼20 km southeast of Horn Island, Mississippi (MS) and the core has terrestrial sediments radiocarbon dated to 11,066–10,228 (2σ) calibrated years BP (early Holocene). The second site is the “Alabama Underwater Forest” located ∼13 km south of Gulf Shores, Alabama (AL) and the cores have terrestrial sediments optically stimulated luminescence dated to 63 ka (±10 ka, 2σ) to 72 ka (±16 ka, 2σ) (late Pleistocene). Geochemical results for the MS sediments indicate a swamp‐to‐freshwater marsh transitional series, whereas the AL sediments indicate a swamp‐to‐saltwater marsh transitional series, both supported by palynological results. Further exploration of the geochemical results using linear discriminant analysis, trained with published geochemical data, supports the swamp and marsh interpretations. We conclude that the near‐pristine preservation of these woody deposits is not solely due to physical mechanisms, such as rapid burial, but is also coupled with anoxia‐ and euxinia‐driven biogeochemical reactions promoting wood and woody debris preservation in swamp and marsh environments.

Germination potential of baldcypress (Taxodium distichum) swamp soil seed bank along geographical gradients

Changing environments of temperature, precipitation and moisture availability can affect vegetation in ecosystems, by affecting regeneration from the seed bank. Our objective was to explore the responses of soil seed bank germination to climate-related environments along geographic gradients. We collected seed banks in baldcypress (Taxodium distichum) swamps along the Mississippi River and the Gulf of Mexico Coast in the United States, which have distinct temperature and/or precipitation gradients, and germinated them in a greenhouse. The frequency, richness and seed density of species germinated from the seed bank were compared between various geographic locations, experimental water regimes (saturated, flooded) and wetland types (tidal, non-tidal and inland swamps). We also analyzed the relationship of seed density to the environment by using a Non-metric Multi-dimensional Scaling (NMDS) model. Sixty-one species germinated from the seed bank, differing in pattern by geographic location, experimental water regime and wetland type. The foundation species (i.e., T. distichum and Cephalanthus occidentalis) germinated with a niche affinity for the northern part of the latitudinal gradient (Tennessee and Illinois) and these species may shift northward with climate change. Some species had higher seed density in the locations that were subject to more persistent drought conditions (e.g., Texas) including Cyperus rotundus and Gratiola virginiana, indicating that these species may be better adapted to sites with high temperature and low precipitation. In contrast, certain species including Saururus cernuus and Ludwigia palustris were present throughout the range of these gradients, and so may be more resilient to any future climate shifts. We found that the regeneration potential of baldcypress swamps might be altered by changes in local and climate environment because of nuances of responses of seed banks to climates along latitudinal and longitudinal gradients. Our study can help predict vegetation regeneration potential to climate change environments depending on the ability of these species to disperse and maintain seed banks.

A 5200-year paleoecological and geochemical record of coastal environmental changes and shoreline fluctuations in southwestern Louisiana: Implications for coastal sustainability

This study presents a 5200-year history of coastal environmental changes and shoreline fluctuations based on a multi-proxy sedimentary record developed from the eastern end of Louisiana's Chenier Plain. Palynological, loss-on ignition, grain-size, and X-ray fluorescence data for a 525 cm sedimentary core indicate five stages of ecosystem development in the Rockefeller Wildlife Refuge since the mid-Holocene. Between 5190 and 4240 cal yr BP, the relative sea-level was much lower than the present level, and our study area was occupied by a baldcypress swamp. When the Sale-Cypremont subdelta complex formed at a western position approximately 4600 years ago, our study area was transformed to a maritime forest about 4240–3440 cal yr BP, suggesting shoreline progradation. When the Mississippi River delta lobe switched to more easterly positions during the Teche, St. Bernard, Lafourche, and Plaquemine-Balize complexes, our study site was transformed back to a freshwater swamp between 3440 and 1520 cal yr BP and then to a coastal marsh between 1520 and 300 cal yr BP. After the Atchafalaya-Wax Lake subdelta complex formed at a more westerly position again, the modern-day beach form at 300 cal yr BP. These coastal ecosystem developments suggest that shoreline retreat started at ~3440 cal yr BP and persisted to the present day. In addition, evidence of at least four possible paleohurricane events was found during the late Holocene. Our multi-proxy record demonstrates that shoreline fluctuation in southwestern Louisiana was tied to the Mississippi River Delta Switch since the mid-Holocene. Shoreline progradation occurred during the Sale-Cypremont subdelta phase ~5200–3500 years ago, when the sediment supply was high. Retrogradation occurred after ~3500 cal yr BP when the Mississippi River subdelta complexes occupied more easterly position, resulting in diminished sediment supply. A significant increase in the delivery of water and total suspended sediment load from the Atchafalaya River to the Gulf of Mexico is needed in order to sustain the southwestern Louisiana coastal wetlands.

Hydrologic Changes over 60 Years (1959-2019) in an Old-Growth Bald Cypress Swamp on a Rapidly Developing Landscape by Shawn E. Clem and Michael J. Duever

National Audubon Society’s Corkscrew Swamp Sanctuary (Naples, Florida, USA), an ecological relic of Old Florida, protects the largest remaining old-growth bald cypress swamp in the world. A 60-year record of daily water level measurements revealed substantial changes in Corkscrew Swamp’s hydrology as agricultural and residential development have grown to dominate the landscape surrounding the sanctuary. Between 1959 and 2019, surface water data indicate no decadal change in the timing or magnitude of peak wet season water levels or in the hydroperiods (days inundated per water year) of upland hammock and pine forests or wet prairies. Despite little change in rainfall patterns between the 1960s and 2010s, hydroperiods decreased 29% in marshes, 18% in old-growth bald cypress, and 17% in ponds, with the most notable change occurring between the 1990s and 2000s. Average dry season water level recession rate (decrease in water level over time) was 6.3 mm/d in the 2000s and 5.7 mm/d in the 2010s, 47% and 32% higher, respectively, than those seen in earlier decades. Analyses of groundwater data indicated a similar dry season decline between the 1990s and 2000s to that seen in surface water. Analyses of cumulative hydroperiod in surface and groundwater suggested markedly drier conditions began in the 2006 water year. While causes of the observed hydrologic changes are uncertain, likely interacting causes include reduced upstream wet season surface and groundwater storage, increased residential and agricultural extraction, increased downstream drainage, and increased evapotranspiration rates across the region as more deeply-rooted shrubs and trees replace more shallowly-rooted herbaceous plant communities. We discuss significant implications for plant and animal communities, particularly the historic Corkscrew wood stork colony, and discuss the role of long-term hydrologic monitoring for detecting and documenting ecological change.

Wood Biomass and Carbon Pools within a Floodplain Forest of the Congaree River, South Carolina, USA

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.

Palynological and Geochemical Records of Environmental Changes in a Taxodium Swamp near Lake Pontchartrain in Southern Louisiana (USA) during the Last 150 Years

Ryu, J.; Bianchette, T.A.; Liu, K.-B.; Yao, Q., and Maiti, K.D., 2018. Palynological and Geochemical Records of Environmental Changes in a Taxodium Swamp near Lake Pontchartrain in Southern Louisiana (USA) during the Last 150 Years. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 381–385. Coconut Creek (Florida), ISSN 0749-0208.The hydrological and environmental histories of estuarine wetlands in southern Louisiana are not well-documented. To better understand their local developmental processes, a 59 cm sediment core (WMA-1) was extracted from a bald cypress (Taxodium distichum) swamp located approximately 800 m west of Lake Pontchartrain. This area has been heavily influenced by human activities, and is susceptible to inundation from storm surges from Lake Pontchartrain as well as fluvial and lacustrine processes. 210Pb analysis indicates that WMA-1 has a sedimentation rate of 0.39 cm/year, consistent with the detection of a 137Cs peak at 18 cm depth. Sedimentological and geochemical analyses reveal two distinct sedimentological facies: an organic-rich dark brown peat unit from 0 to 29 cm containing low concentrations of terrestrial elements (e.g., Ti, and Fe), and a clay unit from 29 to 59 cm with elevated concentrations of most elements (e.g., Sr, Cl, Ti, Mn, and Fe). The pollen assemblage in the clay section is dominated by TCT (mainly Taxodium), but it is replaced by Salix and wetland herbaceous taxa in the overlying peat. The Taxodium swamp was present at the site at least 150 years ago, and was replaced by Salix and other bottomland hardwood trees and wetland herbs due to a decrease in water level around AD 1940. While climate may have played a role, this hydrological change was most likely caused by the significant regional human activities during the last 100 years. Two thin clay layers (at 3–5 cm, 14–19 cm) embedded in the upper peat section were likely deposited from hurricanes Isaac (2012) and Betsy (1969): two events responsible for heavy rain and significant storm-surge flooding. Our findings suggest that human activities are capable of altering local wetland hydrology and ecology, while the clay layers represent hurricane-induced floods at the site during the past century.

Repeated Drought Alters Resistance of Seed Bank Regeneration in Baldcypress Swamps of North America

Recurring drying and wetting events are likely to increase in frequency and intensity in predicted future droughts in the central USA and alter the regeneration potential of species. We explored the resistance of seed banks to successive droughts in 53 sites across the nine locations in baldcypress swamps in the southeastern USA. Along the Mississippi River Alluvial Valley and northern Gulf of Mexico, we investigated the capacity of seed banks to retain viable seeds after successive periods of drying and wetting in a greenhouse study. Mean differences in species richness and seed density were compared to examine the interactions of successive droughts, geographical location and water regime. The results showed that both species richness and total density of germinating seedlings decreased over repeated drought trials. These responses were more pronounced in geographical areas with higher annual mean temperature. In seed banks across the southeastern swamp region, most species were exhausted after Trial 2 or 3, except for semiaquatic species in Illinois and Tennessee, and aquatic species in Texas. Distinct geographical trends in seed bank resistance to drought demonstrate that climate-induced drying of baldcypress swamps could influence the regeneration of species differently across their ranges. Despite the health of adult individuals, lack of regeneration may push ecosystems into a relict status. Seed bank depletion by germination without replenishment may be a major conservation threat in a future with recurring droughts far less severe than megadrought. Nevertheless, the protection of moist refugia might aid conservation.

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.

Restoration and Management of a Degraded Baldcypress Swamp and Freshwater Marsh in Coastal Louisiana

The Central Wetlands Unit (CWU), covering 12,000 hectares in St. Bernard and Orleans Parishes, Louisiana, was once a healthy baldcypress–water tupelo swamp and fresh and low salinity marsh before construction of levees isolated the region from Mississippi River floodwaters. Construction of the Mississippi River Gulf Outlet (MRGO), which funneled saltwater inland from the Gulf of Mexico, resulted in a drastic ecosystem change and caused mortality of almost all trees and low salinity marsh, but closure of the MRGO has led to decreases in soil and surface water salinity. Currently, the area is open water, brackish marsh, and remnant baldcypress stands. We measured hydrology, soils, water and sediment chemistry, vegetation composition and productivity, accretion, and soil strength to determine relative health of the wetlands. Vegetation species richness is low and above- and belowground biomass is up to 50% lower than a healthy marsh. Soil strength and bulk density are low over much of the area. A baldcypress wetland remains near a stormwater pumping station that also has received treated municipal effluent for about four decades. Based on the current health of the CWU, three restoration approaches are recommended, including: (1) mineral sediment input to increase elevation and soil strength; (2) nutrient-rich fresh water to increase productivity and buffer salinity; and (3) planting of freshwater forests, along with fresh and low salinity herbaceous vegetation.

Nitrous Oxide Emissions from Soils in Louisiana’s Atchafalaya River Basin

The Atchafalaya River Basin functions as a net sink for nitrogen delivered by the Mississippi and Red Rivers. Continued influx of river-borne sediment drives rapid habitat change on the floodplain, and lakes are transitioning to seasonally flooded baldcypress swamps and bottomland hardwood forests. As flooding regimes change, leading to more intermittent wet and dry cycles, soil conditions become more conducive to incomplete denitrification resulting in increased N2O emissions. The objective of this laboratory study was to determine if sediments in the Basin are potential sources of N2O emissions, and if emission rates differ among habitats types. Results indicate that potential N2O emission rates are low, and differ by habitat type. Lake sediments had the highest potential denitrification rates (53.68 g N2-N ha−1d−1), while baldcypress sediments had the highest potential N2O emissions (2.31 g N2O-N ha−1d−1). Unflooded bottomland hardwood sediments had very low denitrification potential and were not a source of N2O emissions, but emissions of both N2 and N2O increased after bottomland hardwood sediments were flooded. These results suggest that overall N2O emissions are low, but management strategies aiming to improve water quality and reduce greenhouse gas emissions should consider each habitat separately when modeling nitrogen removal within large floodplains.

Carbon pools and fluxes in a tupelo (Nyssa aquatica)-baldcypress (Taxodium distichum) swamp 24-years after harvest disturbances

Bottomland forests are often productive ecosystems with large quantities of above- and belowground carbon, yet few long-term research projects relating to harvest effects in bottomland hardwood forests exist. This project evaluated the long-term (24-years after harvest) effects of three disturbance treatments on the carbon budget of a tupelo-baldcypress swamp in southwestern Alabama. The site was clearcut in 1986 and timber removed by helicopter. Disturbance treatments were: 1. helicopter (HELI), 2. skidder transportation simulation (SKID), and 3. glyphosate herbicide removal of vegetation following harvesting during years one and two (GLYPH). An adjacent portion of the original stand provides a non-harvested reference (REF) condition. Measurements included aboveground biomass, belowground biomass, soil carbon, soil CO2 efflux and estimates of buried coarse woody debris on the site. SKID treatments had the highest aboveground biomass (73.9 Mg ha−1) and total carbon storage (206.1 Mg ha−1) of the three treatments. GLYPH treatments had the lowest carbon storage (144.2 Mg ha−1) and lower aboveground woody biomass (26.4 Mg ha−1). HELI and SKID treatments were similar, but have slightly less in total forest carbon (168.7 Mg ha−1) than the REF condition. Belowground coarse woody debris storage increased carbon storage on these sites with an additional 8–13 Mg ha−1 being stored. GLYPH treatments had the highest total soil CO2 efflux followed by HELI and SKID treatments, respectively. Higher biomass and carbon storage of SKID treatments is explained by the flood tolerance of tupelo, diverse microtopography created by skidder traffic, and decreased soil aeration of skidder-rutted areas immediately after harvest which favored tupelo coppice.

Tree-ring analysis of ancient baldcypress trees and subfossil wood

Ancient baldcypress trees found in wetland and riverine environments have been used to develop a network of exactly dated annual ring-width chronologies extending from the southeastern United States, across Mexico, and into western Guatemala. These chronologies are sensitive to growing season precipitation in spite of frequently flooded site conditions, and have been used to reconstruct moisture levels the southeastern United States and Mexico for over 1000 years. The El Nino/Southern Oscillation (ENSO) is a major influence on the climate reconstructions derived from these baldcypress chronologies, especially in Mexico where some of the most extreme reconstructed droughts occurred during El Nino events. In the Southeast, the ENSO influence on climate and tree growth changes sign from spring to summer, and this change in dynamical forcing is recorded by sub-seasonal chronologies of earlywood and latewood width. Most existing baldcypress chronologies have been extended with tree-ring data from “subfossil” wood recovered from surface and submerged deposits. Well-preserved subfossil logs have also been recovered in quantity from buried deposits of great age, and may permit development of long continuously dated Holocene chronologies and discontinuous “floating” Pleistocene chronologies. The extensive subfossil baldcypress swamp discovered 6 m below the streets of Washington D.C. was overrun by a transgression of the Potomac estuary, possibly during the previous super interglacial (marine OIS 5e), and provides direct evidence for one potential impact of unmitigated anthropogenic warming and sea level rise.

Comparison of 15N2 flux and acetylene inhibition denitrification methods in Atchafalaya River basin sediments

Flooded swamps in the Atchafalaya River basin, Louisiana, USA, are thought to be a major mechanism for removing excess riverine nitrate that may be causing hypoxia in coastal Louisiana. Two denitrification methods, the 15N2 flux and acetylene inhibition technique, were compared and evaluated. Sediment samples were collected from three baldcypress swamp sites located within the Atchafalaya River basin. Highly 15N-labeled (61.7 at.%) and unlabeled nitrate (100 mg NO3–N L−1) were applied to the floodwater of separate sets of sediment cores. Denitrification potential was measured 2, 6, 10, and 16 days after nitrate addition. Two days after nitrate addition, no significant differences in denitrification rates were observed between methods. On days 6, 10, and 16, denitrification measured by the 15N2 method was higher compared to the acetylene inhibition procedure. When significance was observed, 15N2 denitrification was approximately 5.3 times greater.

Soil Characteristics of Sediment-Amended Baldcypress (Taxodium distichum) Swamps of Coastal Louisiana

Amendments of sediment from dredging activities have played an important role in raising the elevation of sinking coastal wetlands. This study compared the soil characteristics of sediment- amended coastal swamps in the Barataria Preserve unit of Jean Lafitte National Historical Park and Preserve with natural swamps along Bayou des Familles. The sandy sediment amendments used in the coastal forests had different soil texture and characteristics than the more organic soils of the natural swamps. Three years after the application of these sediments on the sediment-amended swamps, dewatering and compaction of the sediment had occurred but the sediment still had high salinity and bulk density, and low organic matter content. The two sediment-amended swamps differed from each other in that Site 1 had a higher elevation (mean = 25 cm higher) and drier soil than Site 2. The effects of sediment in coastal forested wetlands require separate consideration from studies of salt marshes, e.g., the weight of the sediment might damage tree roots, or the amendments might influence soil stability during storms in a different way. Generally, this study suggests that shallower depths of sediment are more likely to yield environments beneficial to these sinking baldcypress swamps in coastal Louisiana.

Comparison of denitrification characteristics among three habitat types of a large river floodplain: Atchafalaya River Basin, Louisiana

Mobile forms of nitrogen leach from upland environments into aquatic systems, often discharging to coastal zones. Addition of nitrogen to once N-limited systems results in a host of changes ranging from eutrophication to loss of biodiversity. Floodplains can ameliorate these changes by removing and sequestering nitrogen. In many coastal floodplains, sedimentation causes lakes to transition to baldcypress swamps, and ultimately to bottomland hardwood (BLHW) forests. These habitats differ in their contact with floodwater, which directly and indirectly affects their ability to process nutrients, but the effects of habitat change on denitrification at the floodplain scale cannot be predicted because of lack of suitable data. This study compared denitrification characteristics among the aforementioned habitats within the Atchafalaya River Basin (ARB). Microcosms were established in the laboratory, and the acetylene block technique was used to estimate four denitrification characteristics: background denitrification rates, maximum rates, time to reach maximum rates, and the linear response of denitrification to nitrate concentration. There were significant differences in denitrification characteristics among the three habitat types; specifically, all habitats differed in the time required for denitrification to respond to nitrate in the overlying water, and denitrification in lake habitats differed from both BLHWs and baldcypress swamps. Landscape-scale models should account for different linear relationships between denitrification and nitrate concentrations, and different response times to nitrate concentrations for different habitats. Because denitrification characteristics differ across habitats within the ARB, continued habitat change within the floodplain will alter nutrient discharge to coastal waters.

Vegetation and climate in the latest Oligocene–earliest Miocene in Jylland, Denmark

Two exposures in Jylland, Denmark, encompassing beds of latest Oligocene to earliest Miocene age (latest Chattian–early Aquitanian) yielded well-preserved palynofloras. The assemblages indicate that Jylland was covered by extensive Taxodiaceae swamp forests in the mid-Cenozoic. Besides a Taxodiaceae–Cupressaceae association, which was overwhelmingly dominant, other common plants in this habitat were Alnus, Nyssa, Betula, Salix, Cyrilla and Myrica. Most of the trees and shrubs are well adapted to swamps and thrive under more or less flooded conditions in modern bald cypress swamps of the southeastern North America. Vegetation composition indicates that a warm–temperate climate prevailed in Denmark during the Oligocene–Miocene transition. According to calculations using the Coexistence Approach, the mean annual temperature during this time span ranged from 15.6 to 16.6 °C. An increase to 16.5–21.1 °C is inferred from the palynoflora in the upper part of the section. The earlier, cooler period possibly reflects global cooling associated with the Mi-1 glaciation event at the Oligocene–Miocene boundary. No data from the very coldest part of the Mi-1 event has been recorded, as this is represented by a gravel layer (representing a hiatus) in the lowermost part of the studied succession. The length of the missing time is not known precisely, but is probably in the order of some hundred thousand years. Correlation with the well-established chronostratigraphic and sequence stratigraphic framework for the studied succession reveals that the most distinctive change in palynoflora probably reflects a shift in depositional facies (due to an increase in sea level) rather than direct climatic change. The sea-level rise is herein interpreted to be eustatic and related to melting of Antarctic ice caps at the end of the Mi-1 glaciation event.

The Wetlands of Lakes Pontchartrain and Maurepas: Past, Present and Future

One of the largest wetlands along the Gulf Coast of North America (ca. 150 000 ha) occurs around the shorelines of Lake Pontchartrain and Lake Maurepas in southeastern Louisiana, just north and west of New Orleans. We provide an introduction to the environmental history of the marshes and swamps in the upper Lake Pontchartrain basin, a review of the existing vegetation patterns and their possible causes, and a discussion of restoration targets and priorities. The Mississippi River produced the St. Bernard Delta 3000–4000 years ago, trapping fresh water to produce both Lake Maurepas and Lake Pontchartrain (Frazier 1967). The natural vegetation of much of the region remains fresh or brackish marshes, mixed with swamps dominated by bald cypress ( Taxodium distichum ) and tupelo ( Nyssa aquatica ). Yearly flooding by the Mississippi River was once a major factor controlling vegetation patterns, but these processes have been greatly impaired by the construction of artificial levees for flood control. Humans also removed most of the cypress swamps in a pulse of logging between 1876 and 1956. Continued subsidence of the land, slowly rising sea levels, salinity pulses from hurricanes, and canals from the Gulf of Mexico, add further stress to these wetlands. Over the past century there has been a steady loss of wetland area, and a gradual conversion of fresh water to salt water vegetation types. Biotic processes are also important. An exotic species of mammal, nutria ( Myocastor coypus ), consumes both aboveground and belowground parts of wetland plants. Reforestation is strictly limited by the combination of salt pulses, competition, and nutria. Alligators are the top predator in this system, but their potential for reducing the impacts of nutria has received minimal attention from biologists. There are many potential future states for this ecosystem. In the extreme case of rising sea level and warmer climate, the area may become a salt and brackish embayment fringed with mangroves. The state closest to historical conditions would be large areas of bald cypress swamp. Two important priorities are to increase flow of freshwater into the system from multiple pulsed fresh water diversions, and to decrease saltwater intrusions by closing canals such as the Mississippi River Gulf Outlet.

Primary production in an impounded baldcypress swamp (Taxodium distichum) at the northern limit of the range

The ability of baldcypress (Taxodium distichum)swamps to maintain themselves near the northern limit of their range depends on their levels of production, which is not only are response to climate but also to local environmental factors(e.g., impoundment). We asked if primary production was reduced under impounded conditions and if species’ responses to impoundment were individualistic or more generalized. To examine long-term production trends in a permanently impounded baldcypress swamp, a 6-year study of leaf litterfall was conducted in Buttonland Swamp, Illinois, which had been impounded for 10 years before the beginning of the study. Buttonland Swamp is at the northern boundary of the baldcypress swamp region along the Cache River, Illinois, in the Mississippi River Alluvial Valley of the United States. When the litter production of impounded sites was compared to those with natural hydrology in the same region, impounded sites had about half of the total litterfall of natural sites. Overall, leaf litterfall rates declined during the study(201 vs. 113 gm−2 yr−1), but the pattern was negatively correlated with water depth, which explained 97% of the variation in the data. Along the transect with the lowest mean minimum water depth(<0.5 cm), leaf litterfall decreased linearly over 6 years from 377 to 154gm−2 yr−1. Total leaf litterfall rates were lower at the other three depths(5, 43, and 49 cm mean minimum water depths)and remained below 200 gm−2 yr−1 throughout the study. Acer saccharinum, Nyssa aquatica, and Salix nigra were most responsible for the decline in total leaf litterfall. Amounts of leaf litterfall of T. distichum and Liquidambar styraciflua also generally decreased, while that of Cephalanthus occidentalis increased overtime. Because species’ responses to environmental factors such as impoundment are individualistic, models should be based on the responses of individual species, rather than on communities. Our study further suggests that the effects of climate change on impounded swamps may differ from those with natural hydrology, since impounded swamps already have reduced production levels. This finding is particularly relevant since the hydrology of the majority of the world’s riverine ecosystems has been altered.

Use of a latitudinal gradient in bald cypress (Taxodium distichum) production to examine physiological controls of biotic boundaries and potential responses to environmental change

ABSTRACTAim Predictions of vegetation change with global warming require models that accurately reflect physiological processes underlying growth limitations and species distributions. However, information about environmental controls on physiology and consequent effects on species boundaries and ecosystem functions such as production is limited, especially for forested wetlands that are potentially important carbon sinks.Location The bald cypress (Taxodium distichum) region of the south‐eastern United States was studied to examine how production of an important forested wetland varies with latitude and temperature as well as local hydrology.Methods We used published data to analyse litter production across a latitudinal gradient from 26.2 to 37.8° N to determine how bald cypress swamps might respond to alternate climate conditions and what changes might occur throughout the distributional range.Results Litterfall rates followed a bell shaped curve, indicating that production was more limited at the distributional boundaries (c. 225 g/m2 year−1) compared to the mid‐range (795–1126 g/m2 year−1). This pattern suggests that conditions are suboptimal near both boundaries and that the absence of populations outside this latitudinal range may be largely due to physiological constraints on the carbon balance of dominant species. While dispersal limitations cannot be totally discounted, competition with other wetland types at the extremes of the range does not seem likely to be important because the relative basal area of bald cypress does not decrease near the edges of the range. Impaired hydrology depressed production across the entire range, but more in the south than the north.Main conclusions Our findings suggest that (1) physiological limitations constrain biotic boundaries of bald cypress swamps; (2) future changes in global temperature would affect litter production in a nonlinear manner across the distributional range; (3) local changes in hydrology may interact with climate to further reduce litter production, particularly at lower latitudes; and (4) southernmost forests could be extirpated if environmental conditions compromise carbon balance and water‐use efficiency of trees.

Soil seed banks and the potential restoration of forested wetlands after farming

SummaryChanges in farming practice provide an opportunity to restore once extensive forested wetlands on agricultural land. In some parts of the world, however, it has proved difficult to restore the full complement of plant species through natural regeneration. Similarly, the restoration of forested wetlands by replanting has often resulted in ecosystems of low diversity. Better methods of restoring these important ecosystems are now required and baldcypress swamps provide an opportunity to investigate alternative approaches to the restoration of forested wetlands. This study examined the composition of seed banks of farmed fields to determine their value in restoring swamps in the south‐eastern United States.A seed bank assay of soils from baldcypress swamps was conducted to determine the extent to which seeds are maintained during farming for various lengths of time. Soils from swamps that were farmed for 0–50 years were collected near the northern boundary of the Mississippi Alluvial Valley along the Cache River, Illinois. Soils were placed in a glasshouse setting in flooded and freely drained conditions, and the numbers and species of seeds germinating were recorded.Woody species including trees, shrubs, and vines were poorly represented in seed banks of both farmed and intact sites (51 and 9 sites, respectively). Missing dominants in the seed banks included tree species with short‐lived seeds such asTaxodium distichumandNyssa aquatica.Cephalanthus occidentalisconstituted the most abundantly dispersed seed of all woody species.Herbaceous species were well represented in the seed banks of both farmed and intact swamps (species richness of 207 vs. 173 species, respectively) suggesting that herbaceous species may live longer than woody species in seed banks. Few of the herbaceous species decreased in seed density in seed banks with time under cultivation, although seed density was lower at sites that had not been farmed. Species that relied on vegetative organs for dispersal were absent in the seed banks of farmed sites includingHeteranthera dubia,Hottonia inflata,Lemna minor,Lemna trisulcaandWolffia columbiana. These species may require active reintroduction during restoration.Synthesis and applications.Both restoration ecologists and managers of nature conservation areas need to be cognisant of seed bank and dispersal characteristics of species to effectively restore and manage forested wetlands. In the case of baldcypress swamps, critical components of the vegetation are not maintained in seed banks, which may make these floodplain wetlands difficult to restore via natural recolonization. Ultimately, the successful restoration of abandoned farm fields to forested wetlands may depend on the re‐engineering of flood pulsing across landscapes to reconnect dispersal pathways.