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Spatial variation and drivers of vegetation structure and composition in coastal freshwater wetlands of subtropical Australia

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Abstract

Coastal freshwater wetlands (CFWs) are among the most understudied wetlands globally and are highly vulnerable to projected climate changes. To address CFW knowledge gaps in south-east Queensland, Australia, we surveyed the floristic composition and structure of wooded CFWs and explored variation in vegetation patterns in relation to selected environmental drivers. Understorey and shrub assemblages were surveyed using a cover-class scale and stem counts for tree species abundance. Vegetation structure attributes (stem density, basal area) were calculated from survey data. Redundancy analysis was used to investigate drivers of vegetation structure and the species composition of each stratum. Vegetation structure patterns were associated with gradients of rainfall, soil moisture, salinity and pH. Understorey species composition was associated with wallum wetland species, native perennial grass and herb species, and vegetation patterns of the canopy. Common CFW species, namely Melaleuca quinquenervia and Eucalyptus tereticornis, dominated tree assemblage variation. Overall, CFW vegetation exhibited strong associations with gradients of salinity, rainfall, groundwater dependence and disturbance. Alterations to key drivers of vegetation pattern with future climate changes are likely to markedly influence the composition, structure and function of CFW vegetation communities. Action is therefore required to maintain CFW vegetation communities and ecological function in these diverse and unique wetland systems.

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... Coastal wetlands, found within continental margins, are low-lying areas of extraordinarily high biodiversity and with high human accessibility [1][2][3]. Many types of wetlands, ranging from saline wetlands (such as mangroves and saltmarshes) that are regularly inundated with sea water to freshwater swamps upstream of the tidal limit in estuaries where salinity is rarely above 0.5 ppt [4], occur along the gradients of tidal influence. ...
... Many regional and global monitoring and assessment frameworks, such as the high-precision rod surface-elevation table-marker horizon (RSET-MH) [14][15][16][17], mainly target mangroves and saltmarshes. Other wetlands, such as coastal floodplain forests, swamps, and lagoons, have received much less attention [2,13]. Globally, there is currently a lack of information on the distribution of these wetlands as well as basic information on the physiological ecology of major coastal freshwater wetland species under natural settings; the structure and dynamics of pure and mixed species communities, soil-plant interactions, biogeochemistry, hydrology, soils, wildlife habitat, primary biotic and abiotic functions; and the response of these systems to natural and human-caused disruptions. ...
... Additionally, in North America, there has been extensive mortality of tree stands in the Atlantic coastal landscapes due to storm surges, which create windows for saline wetland transgression [23]. Similarly, the widespread dieback of native Melaleuca forest in the coastal floodplains of northern Australia due to saltwater intrusion [24] has led to the expansion of mangrove forests and saltmarshes [2]. These studies documented the retreat of coastal forests and the subsequent replacement with mangroves and/or saltmarshes as the sea level rises. ...
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An amendment to this paper has been published and can be accessed via a link at the top of the paper.
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The standard textbook of Vegetation Ecology. A reprint (2002) is available from The Blackburn Press, Caldwell, New Jersey.
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Environmental perturbations in wetlands affect the integrated plant-microbial-soil system, causing biogeochemical responses that can manifest at local to global scales. The objective of this study was to determine how saltwater intrusion affects carbon mineralization and greenhouse gas production in coastal wetland soils. Working with tidal freshwater marsh soils that had experienced roughly 3.5 years of experimental in situ saltwater intrusion, we quantified changes in soil properties, measured extracellular enzyme activity associated with the initial stages of organic matter breakdown, and determined potential rates of anaerobic carbon dioxide (CO2) and methane (CH4) production. Rates of CO2 and CH4 production were also measured after soils were exposed to brackish water for several days in the laboratory. Soils from the field plots treated with brackish water had lower carbon content and higher C:N ratios than soils from freshwater plots, indicating that saltwater intrusion reduces carbon availability and increases its recalcitrance. This was reflected in reduced activities of enzymes associated with the hydrolysis of cellulose and the oxidation of lignin. Long-term saltwater intrusion reduced rates of soil CO2 and CH4 production. This contrasts with the effects of short-term exposure to brackish water, which increased rates of CO2 production but lowered rates of CH4 production. Our analysis suggests that the long-term effect of saltwater intrusion on soil CO2 production is indirect, mediated through the effects of elevated salinity on the quantity and quality of autochthonous organic matter inputs to the soil. In contrast, salinity, organic matter content, and enzyme activities directly influence CH4 production. Collectively, our data indicate that saltwater intrusion into tidal freshwater marshes affects the entire carbon mineralization process, from the availability of organic carbon through its terminal metabolism to CO2 and/or CH4, and illustrate that long-term shifts in biogeochemical functioning are not necessarily consistent with short-term disturbance-type responses.
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We investigated patterns, rates, and mechanisms of forest replacement by salt marsh in relation to sea-level rise on the west coast of Florida, USA. The geomorphology of this region typifies that of low-lying, limestone coastlines considered highly susceptible to sea-level rise (e.g., much of the eastern Gulf of Mexico, the Yucatan Peninsula, and low-lying limestone islands throughout the world). This coast is microtidal, shallowly sloping, and has a rate of relative sea-level rise similar to that of eustatic rise. To determine patterns of forest change in relation to sea-level rise, we examined patterns of tree species zonation, tree recruitment, and tree mortality in relation to site elevation and tidal-flooding frequency. To reconstruct histories of forest change in relation to sea-level rise, we estimated age distributions of Sabal palmetto, the most widely distributed tree species at our site, relating age structures of stands to reconstructed histories of tidal flooding in the stands. Finally, to assess the relative roles of flooding stress (hypoxia), salt exposure, and competition from encroaching salt-marsh vegetation in the decline of forest stands, we examined patterns of soil redox potential, groundwater salinity, and density of halophytic vegetation among stands in different stages of decline. Zonation among tree species was related to tidal-flooding frequency. For most trees, seedlings were absent from the most frequently flooded stands in which the species occurred. Reconstructed flooding histories of stands and age estimates for S. palmetto suggest that many decades elapse between cessation of regeneration and local elimination of a tree species. Even during the relatively short duration of the study (4 yr), however, composition of some stands changed in the direction predicted from species zonation and sea-level rise. Forest understory replacement by halophytic vegetation appeared to follow, rather than cause, failure of tree regeneration. Tidal flooding rarely produced severe reducing conditions in soil, but groundwater salinity was correlated with tidal-flooding frequency. Forest retreat in this system, therefore, involves the development of relict (non-regenerating) stands of different tree species at different flooding frequencies. Exposure to salt appears to be the major cause of tree regeneration failure, with flooding stress and interference from marsh vegetation playing minor or negligible roles. These interactions differ somewhat from those on deltaic coasts or coasts with high freshwater outflows, where flooding stress may play a larger role in regeneration failure, and from sandy coasts, where erosion may play a larger role in forest retreat. Regardless of the cause of tree regeneration failure, the development of relict stands may be a general forest response to sea-level rise.
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The pollen and sedimentary record from a coastal backdune swamp on the island of Mua, Torres Strait, Australia, is presented. A 4.55 m core collected from the swamp centre provides a record of vegetation and landscape change spanning the postglacial marine transgression to present. Prior to 6000 radiocarbon years before present (yr BP) results show mangrove vegetation encroaching on the core site, periodically displacing non-mangrove taxa until the establishment of an extensive mangrove forest between 6000 yr BP and 3000 yr BP. Within the mangrove community a transition from lower-tidal Rhizophora forest to an upper-intertidal Ceriops community is evident. This is followed by the development of the current herbaceous freshwater swamp in the late Holocene. The dryland vegetation record is dominated by sclerophyll and rainforest elements with strongest forest representation occurring around the mid Holocene before a decline in tree density and the establishment of open woodlands in the late Holocene. The data suggest vegetation change accompanied marine transgression and a humid mid-Holocene climate, before stabilization of sea levels and the initiation of dominant on-shore catchment processes, signalling drier climatic conditions and possible human activity.
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This study presents three records of environmental change during the late-Holocene from wetlands across Bentinck Island in the South Wellesley Islands, northern Australia. Radiometric dating provided ages for sediment cores with the longest chronology spanning the last 1250 cal. yr BP. Palynological results show the diverse mangrove community transitioned to woodland- and wetland-dominated vegetation over the last 850 years on the southeast coast. The key driver of this landscape change was likely late-Holocene sea level regression and coastal progradation in the Gulf of Carpentaria. This study found freshwater wetlands expanded across Bentick Island over the last 500 years, with sedges and rushes peaking in the last 350 years. Macroscopic and microscopic charcoal records, coupled with archaeological evidence, highlights the spatial and temporal variation in fire regimes across the island, reflecting the traditional fire management practices of the Kaiadilt people during the late-Holocene. This study finds a significant increase in charcoal accumulation in the 1900s when Kaiadilt fire practices were disrupted and the South Wellesley Islands were abandoned. The pollen record reflects little change in the vegetation despite the shifting fire regime, highlighting the importance of multi-proxy approaches to reconstructing past environments in tropical northern Australia where vegetation is adapted to fire.
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Background & aims: The capacity of dispersal to promote or hinder species' responses to global change remains a major question in ecology. One ecosystem experiencing rapid change is the tidal marsh, which is migrating inland in response to accelerated sea level rise. Few studies to date have investigated the ecological dynamics that impact this large-scale migration. Seed dispersal and persistence in the soil seed bank is a component that can be strongly indicative of community trajectories. With this in mind, the aim of our study was to characterize the germinable seed bank across a marsh-forest ecotone in the Chesapeake Bay. Methods: Soil samples were collected across transects that ran from the high marsh to the coastal loblolly pine forest in a brackish marsh in Blackwater National Wildlife Refuge, Maryland, USA. Samples were grown in a greenhouse and watered with either freshwater or 3 ppt seawater solution. We compared community composition across transects and between salinity treatments. Additionally, we compared the seed bank to standing vegetation and used seed trait data from the TRY Database to investigate changes in functional traits along this ecotone. Key results: We found halophytic species dispersing up to 15 m into the forest and a general lack of obligate upland species, including near absence of Pinus taeda, the dominant species in the forest canopy. A majority of species detected in the seed bank were wetland species of various types, with species with wide salinity tolerance arising most frequently. Salinity addition had a significant negative influence on seed bank diversity. Conclusion: Overall, our seed bank results suggest dispersal and germination under the conditions of saltwater intrusion will limit forest regeneration and favor marsh plant dispersal. This indicates that the ecological processes that determine the soil seed bank community will support continued migration of marsh species into uplands.
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This introductory chapter defines the key subjects for this volume of ecological studies and briefly presents the common context and the cohesion of the contents of the various chapters. It gives a definition of wetland ecosystem services and outlines briefly the 17 services that have been identified in ecosystems globally and are generally considered as being of major importance. Provisioning wetland ecosystem services such as food chain support, regulating services such as climate cooling, and the enhancement of biodiversity are prime examples. Definitions are also given for wetland character and wise use, as adopted by the Ramsar Convention on Wetlands. New developments in wetland restoration to enhance wetland ecosystem services, as they are described in this volume, are outlined briefly. The overview also pays attention to the chapters on the latest developments of our understanding of water quality enhancement services and climate regulation services of wetlands; on threats and resilience to disturbances such as climate change and invasion of exotic species; on new initiatives for wise use of large, internationally divided wetlands in the Yellow Sea; and on restoration and creation of wetlands in urban environments, in particular in China.
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Online enhancements: appendix, zip file. abstract: Patterns in species incidence and compositional turnover are central to understanding what drives biodiversity. Here we propose zeta (z) diversity, the number of species shared by multiple assemblages, as a concept and metric that unifies incidence-based diversity measures, patterns, and relationships. Unlike other measures of species compositional turnover, zeta diversity partitioning quantifies the complete set of diversity components for multiple assemblages, comprehensively representing the spatial structure of multispecies distributions. To illustrate the application and ecological value of zeta diversity, we show how it scales with sample number, grain, and distance. Zeta diversity reconciles several different biodiversity patterns, including the species accumulation curve, the species area relationship, multispecies occupancy patterns, and scaling of species endemism. Exponential and power-law forms of zeta diversity are associated with stochastic versus niche assembly processes. Zeta diversity may provide new insights on biodiversity patterns, the processes driving them, and their response to environmental change.
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Involvement of Indigenous people and knowledge in conservation science has become a clear directive in international covenants. Currently, approximately one-third of Australia is owned and managed by Indigenous people, including 84% of the Northern Territory coastline, making Indigenous-led and cross-cultural research highly relevant. Recently, the Yolu Senior Knowledge Custodians of the Laynhapuy Indigenous Protected Area in northern Australia expressed concern about the dieback of culturally significant coastal Melaleuca (paperbark) stands. A partnership between Senior Knowledge Custodians and Western scientists was used to develop an ecocultural research framework to investigate the dieback. Semistructured interviews about the likely causes were conducted with Senior Knowledge Custodians of five coastal flood plain sites where dieback occurred. At these sites, comparative ecological assessments of paired dieback and healthy Melaleuca stands were conducted to explore relationships between Melaleuca stand health, salt water intrusion, acid sulfate soils and feral ungulate damage. Melaleuca dieback was observed in three species: nämbarra (M. viridiflora), raan (M. cajuputi) and gulun'kulun (M. acacioides). The sociocultural and ecological research approaches similarly suggested that ∼70% of Melaleuca spp. dieback was attributed to combinations of salinity and feral ungulate damage. An ecocultural approach heightened understanding of Melaleuca dieback because we detected similarities and differences in likely causal factors.
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Wallum is the regionally distinct vegetation on coastal dunefields, beach ridge plains and sandy backbarrier flats in subtropical northern NSW and southern Queensland (22°S to 33°S). This study examined floristic patterns in the wallum and allied vegetation along 400 km of coastline in northeastern NSW. Floristic and environmental data were compiled for 494 quadrats allocated on the basis of air photo pattern and latitude. A phytosociological classification displayed strong congruence with an initial classification based upon photo pattern, especially for single stratum vegetation, thereby suggesting that API (air photo interpretation) is a valuable technique for the recognition of floristic assemblages. The utility of API for depicting the spatial distribution of tallest stratum species in multi-stratum vegetation was also confirmed. Nonetheless, photo signatures of the tallest stratum are less satisfactory as surrogates for identifying noda for the full complement of species in multi-stratum vegetation. Ordination supported the numerical classification, and reinforced the value of API for capturing meaningful biological and environmental data. Plant-environment relationships were examined for a range of variables. The consistent trend to emerge was a comparatively strong correlation between floristic composition and topographic position, and in some instances also between floristic composition and geology. Mean species richness at the 25 m 2 scale was lower in wetter habitats, although differences were not consistently significant. Cunninghamia (2003) 8(2): 202-252
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Carbon (C) standing stocks, C mass balance, and soil C burial in tidal freshwater forested wetlands (TFFW) and TFFW transitioning to low-salinity marshes along the upper estuary are not typically included in "blue carbon" accounting, but may represent a significant C sink. Results from two salinity transects along the tidal Waccamaw and Savannah rivers of the U.S. Atlantic Coast show that total C standing stocks were 322-1,264 Mg C/ha among all sites, generally shifting to greater soil storage as salinity increased. Carbon mass balance inputs (litterfall, woody growth, herbaceous growth, root growth, and surface accumulation) minus C outputs (surface litter and root decomposition, gaseous C) over a period of up to 11 years were 340-900 g C · m⁻² · year⁻¹. Soil C burial was variable (7-337 g C · m⁻² · year⁻¹), and lateral C export was estimated as C mass balance minus soil C burial as 267-849 g C · m⁻² · year⁻¹. This represents a large amount of C export to support aquatic biogeochemical transformations. Despite reduced C persistence within emergent vegetation, decomposition of organic matter, and higher lateral C export, total C storage increased as forests converted to marsh with salinization. These tidal river wetlands exhibited high N mineralization in salinity-stressed forested sites and considerable P mineralization in low-salinity marshes. Large C standing stocks and rates of C sequestration suggest that TFFW and oligohaline marshes are considerably important globally to coastal C dynamics and in facilitating energy transformations in areas of the world in which they occur.
Article
Land-use change is a principal factor affecting riparian vegetation and river biodiversity. In Chile, land-use change has drastically intensified over the last decade, with native forests converted to exotic forest plantations and agricultural land. However, the effects thereof on aquatic ecosystems are not well understood. Closing this knowledge gap first requires understanding how human perturbations affect riparian and stream biota. Identified biological indicators could then be applied to determine the health of fluvial ecosystems. Therefore, this study investigated the effects of land-use change on the health of riparian and aquatic ecosystems by assessing riparian vegetation, water quality, benthic macroinvertebrate assemblages, and functional feeding groups. Twenty-one sites in catchment areas with different land-uses (i.e. pristine forests, native forests, exotic forest plantations, and agricultural land) were selected and sampled during the 2010 to 2012 dry seasons. Riparian vegetation quality was highest in pristine forests. Per the modified Macroinvertebrate Family Biotic Index for Chilean species, the best conditions existed in native forests and the worst in agricultural catchments. Water quality and macroinvertebrate assemblages significantly varied across land-use areas, with forest plantations and agricultural land having high nutrient concentrations, conductivity, suspended solids, and apparent color. Macroinvertebrate assemblage diversity was lowest for agricultural and exotic forest plantation catchments, with notable non-insect representation. Collector-gatherers were the most abundant functional feeding group, suggesting importance independent of land-use. Land-use areas showed no significant differences in functional feeding groups. In conclusion, anthropogenic land-use changes were detectable through riparian quality, water quality, and macroinvertebrate assemblages, but not through functional feeding groups. These data, particularly the riparian vegetation and macroinvertebrate assemblage parameters, could be applied towards the conservation and management of riparian ecosystems through land-use change studies.
Article
Sea level rise elicits short- and long-term changes in coastal plant communities by altering the physical conditions that affect ecosystem processes and species distributions. While the effects of sea level rise on salt marshes and mangroves are well studied, we focus on its effects on coastal islands of freshwater forest in Florida's Big Bend region, extending a dataset initiated in 1992. In 2014-2015, we evaluated tree survival, regeneration, and understory composition in 13 previously established plots located along a tidal creek; ten plots are on forest islands surrounded by salt marsh, and three are in continuous forest. Earlier studies found that salt stress from increased tidal flooding prevented tree regeneration in frequently flooded forest islands. Between 1992 and 2014, tidal flooding of forest islands increased by 22-117%, corresponding with substantial declines in tree species richness, regeneration, and survival of the dominant tree species, Sabal palmetto (cabbage palm) and Juniperus virginiana (southern red cedar). Rates of S. palmetto and J. virginiana mortality increased nonlinearly over time on the six most frequently flooded islands, while salt marsh herbs and shrubs replaced forest understory vegetation along a tidal flooding gradient. Frequencies of tidal flooding, rates of tree mortality, and understory composition in continuous forest stands remained relatively stable, but tree regeneration substantially declined. Long-term trends identified in this study demonstrate the effect of sea level rise on spatial and temporal community reassembly trajectories that are dynamically re-shaping the unique coastal landscape of the Big Bend. This article is protected by copyright. All rights reserved.
Article
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, 4.6 PSU). Salinity was continuously monitored, litterfall was measured monthly, and growth of overstory trees ⩾10 cm 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.
Article
Coastal wetland systems experience both short-term changes in salinity, such as those caused by wind-driven tides and storm surge, and long-term shifts caused by sea level rise. Salinity increases associated with storm surge are known to have significant effects on soil porewater chemistry, but there is little research on the effect of flooding length on salt penetration depth into coastal marsh soils. A simulated storm surge was imposed on intact soil columns collected from a non-vegetated mudflat and a vegetated marsh site in the Wax Lake Delta, LA. Triplicate intact cores were continuously exposed to a 35 salinity water column (practical salinity scale) for 1, 2, and 4 weeks and destructively sampled in order to measure porewater salinity and extractable NH4-N at two cm depth intervals. Salinity was significantly higher in the top 8 cm for both the marsh and mudflat cores after one week of flooding. After four weeks of flooding, salinity was significantly higher in marsh and mudflat cores compared to the control (no salinity) cores throughout the profile for both sites. Extractable ammonium levels increased significantly in the marsh cores throughout the experiment, but there was only a marginally (p < 0.1) significant increase seen in the mudflat cores. Results indicate that porewater salinity levels can become significantly elevated within coastal marsh soil in just one week. This vertical intrusion of salt can potentially negatively impact macrophytes and associated microbial communities for significantly longer term post-storm surge.
Article
Melaleuca forest is one of the unique ecosystems in Australia which plays an important role to provide carbon storage helping mitigation to the global climate change, thus understanding how much carbon can be stored in the types of forests is necessary. In this study, data was collected and analyzed from four typical sorts of Melaleuca forests in Australia including: primary Melaleuca forests subject to continuous water inundation; primary Melaleuca forests not inundated by water; degraded Melaleuca forests subject to continuous water inundation; and regenerating Melaleuca forests subject to continuous water inundation. The carbon stocks of these typical Melaleuca forests were 381; 278; 210; and 241 t ha−1 of carbon, respectively. Averagely, carbon stocks were 169 (±26) t ha−1 of carbon in the above-ground biomass and 104 (±16) t ha−1 of carbon in soil and roots. The results provide important information for the future sustainable management of Melaleuca forests at both the national and regional scales, particularly in regards to forest carbon conservation and carbon farming initiatives. The results establish that Melaleuca forests in Australia hold globally significant stores of carbon which are likely to be much higher than previously estimated and used in national emissions reporting.
Article
Coastal forested hummocks support clusters of trees in the saltwater-freshwater transition zone. To examine how hummocks support trees in mesohaline sites that are beyond physiological limits of the trees, we used salinity and stable isotopes (2H and 18O) of water as tracers to understand water fluxes in hummocks and uptake by baldcypress (Taxodium distichum (L.) Rich.), which is the most abundant tree species in coastal freshwater forests of the southeastern U.S. Hummocks were always partially submerged and were completely submerged 1 to 8% of the time during the two studied growing seasons, in association with high water in the estuary. Salinity, δ18O, and δ2H varied more in the shallow open water than in groundwater. Surface water and shallow groundwater were similar to throughfall in isotopic composition, which suggested dominance by rainfall. Salinity of groundwater in hummocks increased with depth, was higher than in swales, and fluctuated little over time. Isotopic composition of xylem water in baldcypress was similar to the vadose zone and unlike other measured sources, indicating that trees preferentially use unsaturated hummock tops as refugia from higher-salinity, saturated soil in swales and the lower portions of hummocks. Sustained upward gradients of salinity from groundwater to surface water and vadose water, and low variation in groundwater salinity and isotopic composition, suggested long residence time, limited exchange with surface water, and that the shallow subsurface of hummocks is characterized by episodic salinization and slow dilution.
Article
Coastal floodplains are among the most modified landscapes in southeastern Australia. We used available vegetation survey data for coastal alluvium and other unconsolidated Quarternary sediments to construct a diagnosis of the major plant communities and document their flora. We used soil landscape maps and historical portion plans to gain an understanding of the distribution and environmental relationships of the communities. The flora of coastal floodplains includes more than 1000 native vascular plant taxa and more than 200 introduced taxa. The introduced flora is likely to be considerably larger, given that sampling was biased toward the least disturbed sites. Six major plant communities were diagnosed including a rainforest found north from the Shoalhaven floodplain, a mixed forest of eucalypts and melaleucas found north from Jervis Bay, a casuarina forest (sometimes with melaleuca) found throughout the coast, one open eucalypt forest found principally south from the Hunter region, another open eucalypt forest found north of the Hunter region and a complex of treeless wetland assemblages scattered throughout the coast. The extent and spatial arrangement of these communities varies between floodplains, with landform, rainfall, water regime and soil properties including moisture, fertility and salinity thought to be important factors mediating their distribution patterns. All six assemblages are listed as Endangered Ecological Communities under Threatened Species legislation. The coastal floodplain communities continue to be threatened by land clearing and crop conversion, fragmentation, changes to water flows, flooding and drainage, input of polluted runoff, weed invasion, activation of acid sulphate soils, climate change and degradation through rubbish dumping and other physical disturbances.
Article
Sea-level rise, changing intensities of tropical storms, and changing rainfall patterns are all components of global change that are predicted to affect coastal systems. These factors may interact in shaping coastal ecosystems. The occurrence of a violent storm and a historic drought during an 8-year study of sea-level rise effects on coastal forest in west central Florida presented an opportunity to study these interactions. The system studied was a marshy coastline, on a tectonically stable, karstic limestone platform, where coastal hydric hammock (a wetland hardwood forest) abutted salt marsh. Both the storm and the drought that occurred during the study were associated with pulses of tree mortality that selectively removed Juniperus virginiana var. silicicola (southern red cedar) from stands. Stable isotope data suggested that these trees used less fresh ground water and more sea water as these stands declined in the face of rising sea level. Drought-associated tree death only occurred in a stand in very late stages of sea-level-induced decline, where ground water became hypersaline during the drought. A storm that occurred in 1993 also selectively removed Juniperus from stands, damaging stands primarily in areas where tree reproduction had already ceased or declined due to sea-level rise. Thus, although these episodic events (drought and storm) caused notable tree death, the projected inability of forest stands to recover from these events was due to prior effects of sea-level rise.
Article
Denitrification in tidal freshwater river channels and their adjoining freshwater wetlands greatly affects nitrogen export from river networks, yet the relative importance of these two habitats to nitrogen export has not been examined. Knowledge of how these habitats contribute to denitrification of the river nitrogen load is critical for improving models of nitrogen transport. Denitrification rates were measured in sediments from the channel, bank, and floodplain at upstream and downstream sites of two forested tidal freshwater zones (TFZs) in North Carolina, the New River and Newport River, using membrane inlet mass spectrometry to measure N2 production. Denitrification rates did not usually differ statistically between the channel, bank, and floodplain, although denitrification was highest on the floodplain at the upstream site in the Newport River. When these rates were extrapolated across the entire area of the TFZ, the channel contributed more to the N2 flux than the riparian zone. These results indicate that denitrification rates are comparable between the channel and riparian zone in forested TFZs, and that the importance of the channel versus the riparian zone depends on channel and floodplain morphology.
Article
The coastal freshwater wetlands of western Arnhem Land in the monsoon tropics of the Northern Territory of Australia are subject to new and increasing pressures from a range of land uses. Wetland history is needed as a bais for management decisions. Radiocarbon dates and pollen analyses of samples from fifty surveyed sites on the Magela floodplain show mangrove vegetation encroaching as sea level rose from about 8000 BP to 6000 BP. The extensive Rhizophora forest established at that time lasted until about 3000 BP, when the sediments built up above the upper tidal limit for these mangroves. Avicennia and other mangrove genera became more abundant in the subsequent transition phase. The floodplain has been a freshwater wetland since about 1300 BP. High resolution pollen analyses of contiguous 1 cm samples through the transition at two sites show parallel sequences of vegetation changes. Large-scale spatial and temporal stability of the two mutually exclusive ecosystems, mangrove forest and freshwater wetland, contains considerable small-scale variation. Maximum diveristy and variability occureed during the time of least environmental stability in the transition phase. Vegetation changes was discontinuos, with each major shift followed by variation aorund a new man. A rise in sea level of 0.5-1.0 m could destroy the present freshwater wetland and allow some mangroves to return to the Magela.
Article
The effective management and conservation of coastal wetlands requires an appropriate typology to underpin classification and mapping, adequate inventory information, and a robust assessment of ecological condition and threats. Extensive and floristically diverse coastal wetlands occur along much of the coast of Victoria (south-eastern Australia), but there are serious deficiencies in all these information requirements. Previously unanalysed data from the Victorian Biodiversity Atlas were used to revise the typology currently applied to coastal saltmarsh in Victoria. To supplement the single unit currently used for State-endorsed mapping and inventory (EVC 9 Coastal Saltmarsh Aggregate), seven new Ecological Vegetation Classes are proposed to better reflect the floristic and structural diversity of coastal saltmarsh in south-eastern Australia. Coastal saltmarsh is currently allocated the lowest conservation status ('least concern') across much of Victoria, and it is recommended that this be upgraded variously to the higher categories of 'endangered', 'vulnerable', or 'rare'. A State-wide inventory using the new typology, prepared using recently flown, high-resolution aerial photographs and extensive ground-truthing (212 person-days), indicated that there were 19212ha of coastal saltmarsh of all types, 5177ha of mangroves, and 3227ha of EVC 10 Estuarine Wetland (a wetland type dominated by Juncus kraussii) in Victoria. On-ground assessments undertaken across 30 geographic sectors of the coast indicated that coastal wetlands were confronted by a wide range of anthropogenic threats, which in many cases were quite different from those outlined in prior reviews of Australian wetland systems. Weed invasions were especially problematic, not so much of the exotic and highly publicized Spartina in the lower levels of tidal wetlands but from a wide range of exotic taxa in more elevated saltmarshes (e.g. tall wheat grass Lophopyrum ponticum).
Article
We investigated whether within wetland environmental conditions or surrounding land cover measured at multiple scales were more influential in structuring regional vegetation patterns in estuarine tidal wetlands in the Pacific Northwest, USA. Surrounding land cover was characterized at the 100, 250, and 1,000 m, and watershed buffer scales. Vegetation communities were characterized by high species richness, lack of monotypic zonation, and paucity of invasive species. The number of species per site ranged between 4 and 20 (mean ± standard deviation = 10.2 ± 3.1). Sites supported a high richness (mean richness of native species 8.7 ± 2.8) and abundance of native macrophytes (mean relative abundance 85 % ± 19 %). Vegetation assemblages were dominated by a mix of grasses, sedges, and herbs with Sarcocornia pacifica and Distichlis spicata being common at sites in the oceanic zone of the estuary and Carex lyngbyei and Agrostis stolonifera being common at the fresher sites throughout the study area. The vegetation community was most strongly correlated with salinity and land cover within close proximity to the study site and less so with land cover variables at the watershed scale. Total species richness and richness of native species were negatively correlated with the amount of wetland in the buffer at all scales, while abundance of invasive species was significantly correlated to within wetland factors, including salinity and dissolved phosphorus concentrations. Landscape factors related to anthropogenic disturbances were only important at the 100-m buffer scale, with anthropogenic disturbances further from the wetland not being influential in shaping the vegetation assemblage. Our research suggests that the traditional paradigms of tidal wetland vegetation structure and environmental determinants developed in east coast US tidal wetlands might not hold true for Pacific Northwest wetlands due to their unique chemical and physical factors, necessitating further detailed study of these systems.
Article
Questions:What is the relative importance of topographic (elevation), edaphic (soil salinity, nitrogen and particle size) and hydrologic (estuarine river flow) gradients for variation in tidal wetland plant composition and diversity? Location: Four Oregon estuaries: a marine-dominated lagoon, two tidal-driven bays, and a river-dominated site. Methods: We surveyed species presence, cover and richness; and environmental factors (soil salinity, grain size, soil nitrogen and elevation) in plots in marsh and swamp. We assessed patterns of community structure and the relative importance of environmental gradients with hierarchical partitioning, ordination, species accumulation curves and path analysis. Results: The relative importance ofmeasured environmental gradients on plant occurrence differed by species. Soil salinity or elevation explained the most variation in the majority of common species. Estuarine hydrology, soil nitrogen and soil clay content were usually of secondary or minor importance. Assemblage composition and species richness variedmost strongly with tidal elevation. Local soil salinity also affected composition, but differences in estuarine hydrology had comparatively less effect on composition and richness. Higher-elevation wetlands supported larger species pools and higher plot-level richness; fresher wetlands had larger species pools than saltmarsh but plot-level richness was relatively invariant to differences in soil salinity. Conclusions: Elevation and salinity tended to exertmore influence on the vegetation structure of tidal wetlands than estuarine hydrology or other edaphic variables. With relative sea-level rise expected to increase both flooding intensity and salinity exposure in future wetlands, global climate change may lead to changes in species distributions, altered floristic composition and reduced plant species richness.
Article
The Alligator River Region (ARR) of the Northern Territory, Australia, has been identified as being particularly susceptible to saltwater intrusion. The Parks and Wildlife Commission of the Northern Territory (PWCNT) have attempted to restrict the intrusion and preserve the natural environment by creating physical earth barrages. A consequence of the intrusion is the conversion of freshwater paperbark (Melaleuca spp.) swamps into extensions of the mangrove margin. During this process stands of dead Melaleuca result and juvenile mangroves (primarily Avicennia marina) invade and establish in the affected areas. The use of Airborne Polarimetric Synthetic Aperture Radar (AirSAR) fused with Thematic Mapper (TM) is investigated as a means for mapping Melaleuca dieback as a measure of the extent of the saltwater intrusion. Individually the sensors cannot map areas of salt affected Melalecua as there is confusion with other cover types in the study area. The combined data set were evaluated for the most useful components of the optical and AirSAR data in resolving the overlap of land cover types and a map of Melalecua dieback was produced. A total of 0.4% of the Study Area or 2.25 square km was identified as being affected by recent saltwater intrusion with an aerial assessment yielding 100% accuracy. The individual areas range from 100 to 3100 square metres and are typically located along tidal creek lines or at the edge between floodplain and woodland. The high identification accuracy using this methodology may allow the use of Melalecua dieback as a bio-indicator of environmental change in tropical floodplain systems.