Mangrove Encroachment Research Articles

Caught in transition: changes in brachyuran diversity following mangrove encroachment into saltmarshes at a southern distribution limit

Mangroves are expanding polewards due to global change, often encroaching into adjacent temperate saltmarshes. In both vegetated ecosystems, brachyurans are responsible for ecological processes and functions such as nutrient cycling and sediment bioturbation. South African mangroves occur at a latitudinal limit and are establishing further south due to past planting events and global change, making these ideal study systems for the effects of mangrove expansion and encroachment. Here, we investigated the effect of mangrove encroachment on brachyuran community composition at two saltmarsh sites with planted mangrove stands of different ages. Transects were laid perpendicular to each estuary where three habitat types were demarcated (mangrove, ecotone, saltmarsh). Sediment samples were collected for analyses and quadrats were used to measure pneumatophore density, saltmarsh cover, and brachyuran abundance and diversity. We found that brachyuran community structure at each site has significantly changed over seven years, with two mangrove-associated fiddler crab species, Tubuca urvillei and Paraleptuca chlorophthalmus, now recorded at the younger planted site, indicating a new southern distributional limit. Community structure was also significantly different amongst habitat types (p < 0.05) with Parasesarma catenatum dominating saltmarshes while Danielella edwardsii was more prominent in mangroves. However, community composition did not differ significantly between the two (differently aged) sites (p > 0.05). Pneumatophore density had a proportional relationship with crab abundance, diversity and richness, while saltmarsh cover had an inversely proportional relationship with crab abundance, diversity and richness. It is likely that as mangroves continue to expand into saltmarshes, more mangrove-associated species will move into saltmarshes, potentially altering ecosystem processes in this unique habitat.

A bibliometric study on carbon cycling in vegetated blue carbon ecosystems.

Understanding carbon cycling in blue carbon ecosystems is key to sequestrating more carbon in these ecosystems to mitigate climate change. However, limited information is available on the basic characteristics of publications, research hotspots, research frontiers, and the evolution of topics related to carbon cycling in different blue carbon ecosystems. Here, we conducted bibliometric analysis on carbon cycling in salt marsh, mangrove, and seagrass ecosystems. The results showed that interest in this field has dramatically increased with time, particularly for mangroves. The USA has substantially contributed to the research on all ecosystems. Research hotspots for salt marshes were sedimentation process, carbon sequestration, carbon emissions, lateral carbon exchange, litter decomposition, plant carbon fixation, and carbon sources. In addition, biomass estimation by allometric equations was a hotspot for mangroves, and carbonate cycling and ocean acidification were hotspots for seagrasses. Topics involving energy flow, such as productivity, food webs, and decomposition, were the predominant areas a decade ago. Current research frontiers mainly concentrated on climate change and carbon sequestration for all ecosystems, while methane emission was a common frontier for mangroves and salt marshes. Ecosystem-specific research frontiers included mangrove encroachment for salt marshes, ocean acidification for seagrasses, and aboveground biomass estimation and restoration for mangroves. Future research should expand estimates of lateral carbon exchange and carbonate burial and strengthen the exploration of the impacts of climate change and restoration on blue carbon. Overall, this study provides the research status of carbon cycling in vegetated blue carbon ecosystems, which favors knowledge exchanges for future research.

Negative outcomes of novel trophic interactions along mangrove range edges.

Tropicalization is a phenomenon that is changing the structure of ecosystems around the world. Mangrove encroachment is a particular form of tropicalization that may have cascading consequences for resident fauna in subtropical coastal wetlands. There is a knowledge gap regarding the extent of interactions between basal consumers and mangroves along mangrove range edges and the consequences of these novel interactions for consumers. This study focuses on the key coastal wetland consumers, Littoraria irrorata (marsh periwinkle) and Uca rapax (mudflat fiddler crabs), and their interactions with encroaching Avicennia germinans (black mangrove) in the Gulf of Mexico, USA. In food preference assays, Littoraria avoided consuming Avicennia and selectively ingested leaf tissue from a common marsh grass, Spartina alterniflora (smooth cordgrass), a preference that was also previously documented in Uca. The quality of Avicennia as a food source was determined by measuring the energy storage of consumers that had interacted with either Avicennia or marsh plants in the lab and the field. Littoraria and Uca both stored ~10% less energy when interacting with Avicennia, despite their different feeding behaviors and physiologies. The negative consequences of mangrove encroachment for these species at the individual level suggest that there may be negative population-level effects as encroachment continues. Many studies have documented shifts in floral and faunal communities following mangrove replacement of salt marsh vegetation, but this study is the first to identify physiological responses that may be contributing to these shifts.

Innovative tidal control successfully promotes saltmarsh restoration

The reduction of saltmarsh habitat at a global scale has seen a concomitant loss of associated ecosystem services. As such, there is a need and a push for habitat rehabilitation. This study examined an innovative saltmarsh restoration project in Australia which sought to address the threats of mangrove encroachment and sea level rise. The project was implemented in 2017, using automated hydraulic control gates, termed “SmartGates,” to lower the tidal regime over one site, effectively reversing sea level rise at a local level. Measured indicators of saltmarsh cover, number of species, seedling counts, and saltmarsh assemblages all showed significant positive development over time, with trends varying based on saltmarsh zone. The saltmarsh, predominantly Sarcocornia quinqueflora, developed from remnant supralittoral (previously high) marsh which remained at 45% cover to achieve over 15% coverage across the cleared habitat after 3 years. Slower development in the low marsh (<5%) compared to other zones contrasts with other saltmarsh restoration studies which may be due to the unique nature of the restoration method or the nature of Australian saltmarsh species which favor higher elevations and drier conditions. The development of saltmarsh at the treatment site was found to track toward that at comparison sites over time, becoming similar to some comparison sites by the studies end. This study highlights the usefulness of the novel restoration method used and of the measured indicators for assessing saltmarsh development. This innovative tidal control method could play an important role in the future of saltmarsh restoration worldwide.

Nature-based solutions on floodplain restoration with coupled propagule dispersal simulation and stepping-stone approach to predict mangrove encroachment in an estuary

The mangrove ecosystem is significantly affected by human activities, climate change, and rising sea level. The propagules of mangroves dispersal with tide and river currents that extend upstream habitats are why mangroves are the dominant species in the tidal area. Bridging critical knowledge gaps can help to create restoration plans for mangrove extension. However, studies on the hydrodynamic and propagation trajectory model (PTM) simulation of propagule long-distance dispersal (LDD) and mangrove growth potential are scarce. By combining various numerical methods and empirical formulas and verifying them with the data obtained through field surveys, this study established a comprehensive model to assess the dispersal and growth of the propagules of Kandelia ovobata. The stepping-stone approach (SSA) and habitat suitability index (HSI) model were also employed to determine the location of the appropriate new habitats through iterative simulation in propagule dispersal. Dike removal was proposed as a nature-based solution and modeled to evaluate the benefits of ecological conservation and flood prevention. The PTM simulations indicated that the deterministic process of horizontal advection accounted for >80 %, and that the remaining variability in the model could be explained by stochastic processes in predicting mangrove propagules pathways. The integrated model of the PTM and SSA proved that propagules have LDD in an estuary. There were few matches in the regions for mangrove growth when comparing the suitability of habitat distribution and the probability of propagule movement. We suggested that the mangrove spread model incorporating the SSA and HSI models predict the potential for mangrove dispersal into new habitats. In addition, the removal of levees aids floodplain regeneration and allows propagules to disperse across the floodplain at high tide and establishment at low tide. The Guandu floodplain restoration with dike removal supplied a cobenefits on ecological demands and flood risk reduction. Future research could thus utilize the adaptation and mitigation strategies presented in this study by incorporating socioeconomic considerations to enhance practical feasibility.

Coastal Wetland Soil Carbon Storage at Mangrove Range Limits in Apalachicola Bay, FL: Observations and Expectations

Globally, mangrove range limits are expanding, often at the cost of adjacent coastal ecosystems including saltmarshes, potentially leading to a change in ecosystem services such as organic carbon (OC) sequestration. Studies in the southeastern US have focused almost exclusively onAvicennia germinansrange expansion, the most cold-tolerant mangroves in North America. The Apalachicola Bay region of north Florida represents the northern range limit of mangroves in the Gulf of Mexico, and uniquely also includesRhizophora mangle. The objective of this research was to quantify soil OC density beneath both mangrove species and compare results to the soils beneath two contiguous native tidal saltmarsh species:Juncus roemerianusandSpartina alterniflorain a barrier island setting. Dominant plant taxa were not a significant predictor of soil OC density, highlighting the relative importance of site-specific environmental attributes as controls on soil properties. Soil profile δ13C compositions included a range of values reflective of C3and C4plant inputs, suggesting that shifts in plant taxa, both from marsh to mangroves and between marsh species, have been occurring at all sites in this study. These findings support much of the literature on mangrove encroachment, which indicates mangrove soil OC concentrations, densities, or stocks are less than or equal to that of co-located tidal marsh habitats. Through a systematic review, the potential of several proposed explanatory variables (climate, environmental setting, plant physiology and productivity, and duration of encroachment) were identified to evaluate how soil OC density in mangrove habitats might increase over time, which is critical to forecasting how continued mangrove expansion might affect blue C storage as these habitats evolve.

A Late-Holocene palynological record of coastal ecological change and climate variability from Apalachicola, Florida, U.S.A

This study uses radiometric dating, palynological, loss-on-ignition, and X-ray fluorescence analyses to reconstruct the vegetation history and coastal morphological changes at the boreal mangrove range limit along the Gulf of Mexico, based on three sediment cores taken from St. George Island, Apalachicola, Florida, USA. The multi-proxy record indicates that the mangrove stands in the vicinity of St. George Island were formed in the recent decades, and no signs of mangroves were found for the last 1500 years during the Late-Holocene in the sedimentary record. The current mangrove expansion at St. George Island is caused by the recent climate warming instead of a recurring phenomenon tied with cyclical global climate variability. Further analysis based on decadal-scale climatic and environmental records reveal that the accelerated sea-level rise and warmer winters, especially the decrease of winter freeze events in the 21st century, are the most plausible causes for mangrove expansion at their boreal range limit during the recent decades. Under the predicted warming trend and accelerating sea-level rise in the 21st century, it is reasonable to believe that mangrove encroachment into coastal marshes will accelerate at Apalachicola and other areas near their poleward range limits.

Salt Marsh Plant Community Structure Influences Success of Avicennia germinans During Poleward Encroachment.

Along the Florida coast, decreasing freeze events are promoting the range shift of the mangrove species Avicennia germinans northward into temperate salt marsh wetlands. Although plant species’ ranges are tightly linked with their climatic tolerances, there is considerable variability in the magnitude by which biotic factors like competition and facilitation may also influence range shifts. Changes in mangrove and marsh plant abundance can alter both the above and belowground environment, which may in turn influence ecosystem services typically associated with these systems such as storm surge abatement and carbon storage. Therefore, it is key to understand (1) how the above and belowground environment of established salt marshes influences establishment of mangroves, and (2) how above and belowground environments shift in response to mangrove encroachment. Using a semi-natural mangrove planting experiment, we investigated the impact of four distinct marsh plant community structures (Batis maritima, Spartina alterniflora, mixture of B. maritima and S. alterniflora, mudflat) on mangrove survivorship and decomposition rate. In mixed marsh plots, mangrove survivorship was 42 % higher compared to survivorship in mudflat plots, and decomposition rate was 47 % greater in mixed marsh plots compared to mudflat. However, percent cover of vegetation differed across treatments, and was highest in mixed marsh plots. High survivorship in mixed marsh plots is likely due to increased protection from physical stressors by the dense aboveground cover, and belowground plant root-driven effects such as nutrient availability and oxygen delivery. Our findings suggest that above and below ground differences in salt marsh plant community structure can have an impact on the survival of encroaching mangroves, which may have implications for predicting future mangrove encroachment and improving mangrove restoration techniques.

An Australian form of Open Marsh Water Management (runnelling): long term monitoring, ancillary and extended research

The paper outlines the evolution of an Australian form of Open Marsh Water Management (OMWM) called runnelling which was implemented in relatively pristine saltmarshes. Runnelling involves shallow channels and was distinguished from OMWM which involves ditching. Both methods aim to conserve tidal hydrology. The core research covers 28 years of research on the original experimental site (Coomera Island, Queensland), in routine monitoring and impact assessments that were made periodically. It also introduces and incorporates information from associated research. This includes effects of runnelling on non-target aspects—snails and crabs. Emerging issues during the course of the longer-term research included the risk of mobilising acid sulfates and of mangrove encroachment into the saltmarsh. As well, use of the data was extended to retrofit it to address broader questions about saltmarsh processes and how to identify them. That identified progressive changes at the Coomera site, and so a further query was whether the system showed signs of approaching a tipping point. In summary, there were some identifiable effects of runnelling in all parts of the research. These were mostly related to the increased wetness near runnels, as flooding by tides would occur at tides up to 0.30 m lower than the pre-runnelling situation. The increased wetness was related to plants, snails and crabs. From a mosquito management perspective the method was successful. From an environmental perspective the environmental changes were only very close to runnels (within 10 m). The overall changes at the experimental site were consistent with other changes in eastern Australia, especially of mangrove encroachment into saltmarshes. Once the runnelling method appeared to manage mosquitoes without damaging wetland values it was adopted widely throughout south east Queensland, encouraged by the positive attitude of permitting agencies, but longer-term evaluation was hindered by lack of relevant records.

Effects of chronic and acute stressors on transplanted black mangrove (Avicennia germinans) seedlings along an eroding Louisiana shoreline

Coastal wetland restoration can be used to offset past wetland losses and/or reduce future losses due to land‐use changes, rising sea levels, and accelerating climate change. However, there is a need for information regarding the restoration‐relevant performance of foundation species like mangrove and marsh plants, including their responses to acute and chronic stressors that can affect restoration outcomes. Mangrove encroachment and poleward range expansion into marsh, facilitated by warming winters, has provided restoration practitioners in the northern Gulf of Mexico with a new foundation plant species to consider using during restoration. To evaluate the performance of transplanted mangroves and characterize restoration‐relevant marsh–mangrove interactions, we planted nursery‐raised black mangrove (Avicennia germinans) seedlings within different marsh cover treatments along an eroding marsh‐dominated shoreline in Louisiana. Mangrove seedling survival increased with greater densities of marsh cover, indicating that marsh grass (Spartina alterniflora) may facilitate mangrove establishment. However, only 35% of transplanted mangrove seedlings established after 10 weeks, suggesting a low return on resources expended in raising seedlings for 1–3 years in greenhouse conditions. Moreover, a 2018 freeze event killed 100% of transplanted mangrove seedlings, while nearby naturally established mangroves suffered minor damage. Our results, along with those in the mangrove restoration literature, indicate that planting mangroves in the northern Gulf of Mexico may not be the most efficient use of limited resources. Rather, restoration efforts may benefit from focusing initially on the restoration of abiotic conditions (e.g. elevation and hydrologic regimes), followed by using marsh plants (rather than transplanted mangroves) to jump‐start ecosystem development.

Coastal woody encroachment reduces food quality for basal consumers

AbstractWoody plant encroachment (the replacement of grasses and forbs by woody shrubs and trees) is reshaping communities in both terrestrial and coastal environments, but little is known about its effects on basal consumers. We used interactions between fiddler crabs (Uca spp.) and the encroaching mangrove Avicennia germinans in Gulf of Mexico salt marshes to explore trophic interactions between basal consumers and encroaching shrubs. In a series of food preference trials, fiddler crabs consumed over five times more marsh (Spartina alterniflora) plant matter when given a choice between mangrove and marsh diets. In food quality trials, fiddler crabs stored 50% less energy, as measured by hepatopancreas lipid content, when restricted to a mangrove diet. These results demonstrate that mangrove encroachment may have negative consequences for salt marsh basal consumers, which may lead to deleterious population‐level effects and altered ecosystem carbon flows. As fiddler crabs are generalist consumers and mangroves share many physical and chemical characteristics with other encroaching woody species, negative consequences for basal consumers may be widespread in other environments undergoing woody shrub encroachment.

The geomorphic impact of mangrove encroachment in an Australian salt marsh

Mangroves are encroaching into salt marshes throughout the world as a result of environmental change. Previous studies suggest mangroves trap sediment more efficiently than adjacent salt marshes, providing mangroves greater capacity to adapt to sea level rise; this may occur by displacing salt marshes. However, sediment transport in adjacent marsh-mangrove systems and its role in mangrove encroachment upon salt marsh remain poorly understood. Here we directly test the hypothesis that mangroves reduce the ability of adjacent marsh to adjust to sea level rise by measuring sediment transport across salt marsh platforms, with and without 6 m of fringing mangroves at the tidal creek edge. We find that salt marshes and mangroves have equivalent sediment trapping efficiencies along the wetland edge. Suspended sediment concentrations, mass accumulation rates, and long-term accretion rates are not lower in salt marshes landward of mangroves than salt marshes without fringing mangroves. Therefore, our work suggests that a relatively narrow zone of mangroves does not impact salt marsh accretion, and activities that limit mangrove encroachment into salt marsh, such as removal of seedlings, will not improve the capacity of salt marsh to trap sediments.

Predation refuge values of marsh and mangrove vegetation for the marsh periwinkle Littoraria irrorata

Black mangroves Avicennia germinans are becoming increasingly common in coastal wetlands in the Gulf of Mexico (USA). As mangroves displace salt marsh vegetation, there may be consequences for associated wetland fauna. In a series of field studies, we compared prey refuge values between marsh and mangrove vegetation for a vertically migrating gastropod, the marsh periwinkle Littoraria irrorata. Littoraria were tethered to marsh grasses (Spartina alterniflora) or the aerial roots (pneumatophores) of Avicennia in arrays that fully crossed vegetation type (Spartina vs. Avicennia), tether height (base vs. canopy), and wetland location (edge vs. interior marsh platform). After 1 d, acute predation rates were twice as high on Littoraria tethered to the base of Spartina stems than on those tethered to pneumatophores, suggesting that mangroves provided superior refuge from benthic predators like blue crabs. In the canopy, Spartina reduced acute predation rates by 75%, indicating that marsh grasses may provide superior refuge from aerial predators (possibly wetland birds). After 7 d, the effect of vegetation type diminished, but Littoraria mortality was 2 times higher on the benthos than in the canopy. Links between vegetation type and predation intensity on this important basal consumer may have broader consequences for trophic dynamics in coastal wetlands that are experiencing mangrove encroachment.

Changes in Ecosystem Nitrogen and Carbon Allocation with Black Mangrove (Avicennia germinans) Encroachment into Spartina alterniflora Salt Marsh

Increases in temperature are expected to facilitate encroachment of tropical mangrove forests into temperate salt marshes, yet the effects on ecosystem services are understudied. Our work was conducted along a mangrove expansion front in Louisiana (USA), an area where coastal wetlands are in rapid decline due to compounding factors, including reduced sediment supply, rising sea level, and subsidence. Marsh and mangrove ecosystems are each known for their ability to adjust to sea-level rise and support numerous ecosystem services, but there are some differences in the societal benefits they provide. Here, we compare carbon and nitrogen stocks and relate these findings to the expected effects of mangrove encroachment on nitrogen filtration and carbon sequestration in coastal wetlands. We specifically evaluate the implications of black mangrove (Avicennia germinans) encroachment into Spartina alterniflora-dominated salt marsh. Our results indicate that black mangrove encroachment will lead to increased aboveground carbon and nitrogen stocks. However, we found no differences in belowground (that is, root and sediment) nitrogen or carbon stocks between marshes and mangroves. Thus, the shift from marsh to mangrove may provide decadal-scale increases in aboveground nitrogen and carbon sequestration, but belowground nitrogen and carbon sequestration (that is, carbon burial) may not be affected. We measured lower pore water nitrogen content beneath growing mangroves, which we postulate may be due to greater nitrogen uptake and storage in mangrove aboveground compartments compared to marshes. However, further studies are needed to better characterize the implications of mangrove encroachment on nitrogen cycling, storage, and export to the coastal ocean.

Mangrove Encroachment Alters Decomposition Rate in Saltmarsh Through Changes in Litter Quality

Global climate change is driving the expansion of mangroves into saltmarsh habitat, which may alter the rate and magnitude of organic matter decomposition and nutrient cycling due to differences in the structural complexity, litter quality, and other ecophysiological traits of foundation species. This work quantified and compared aboveground litter decomposition of the range-expanding mangrove, Avicennia germinans, and resident saltmarsh cordgrass, Spartina alterniflora, and decomposition of a standard substrate belowground, in the saltmarsh and saltmarsh-mangrove ecotone habitat along the Atlantic coast of Florida, USA. Plant and soil fractions were tested for natural abundances of δ13C and δ15N stable isotopes to elucidate soil nutrient sources. Although aboveground decomposition rates differed between marsh and mangrove species due to differences in litter quality, decomposition rates did not vary between saltmarsh and ecotonal habitats. Decay rates were higher for A. germinans leaf litter (0.007 ± 0.0003 k day−1) than for S. alterniflora (0.004 ± 0.0003 k day−1) regardless of habitat, which suggests that increasing inputs of A. germinans litter with encroachment may increase nutrient availability through rapid turnover. Furthermore, belowground decomposition was similar between habitats (0.015 ± 0.0008 k day−1), whereas soil δ13C and δ15N stable isotopes differed significantly. Collectively, these results suggest that mangrove encroachment may not modify the environmental factors driving decomposition, but alterations in foundation plant species may ultimately alter nutrient cycling within habitats through shifts in litter quality.

Integrating outcomes of IUCN red list of ecosystems assessments for connected coastal wetlands

Abstract Human-induced habitat clearing and pollution are leading drivers of biodiversity loss. Ecosystem assessments are required to identify ecosystems at risk of collapse, but they should account for cross-system linkages and dynamics where necessary. This is particularly true for coastal wetlands (e.g. seagrass, mangroves and saltmarsh), which exhibit high ecological connectivity and have individually suffered global declines over the last century. We use the coastal wetlands of Moreton Bay, Queensland, Australia, as a model system to examine how integrating outcomes of multiple, simultaneously conducted, ecosystem assessments can assist in identifying appropriate management and conservation strategies. We simultaneously conducted separate assessments of seagrass, mangrove and saltmarsh ecosystems against the IUCN Red List of Ecosystems criteria. Despite substantial human population growth in the region, seagrass and mangroves were assessed as Least Concern. Mangroves were found to be rapidly encroaching on saltmarsh. This process, together with past clearing, were the major drivers behind saltmarsh being assessed as Endangered. Given the importance of connectivity among these connected ecosystems, collapse in any one ecosystem can have seascape-wide consequences, highlighting the benefit of conducting multi-ecosystem assessments. Consequently, a fully integrated assessment of the coastal wetlands as a single entity would miss key processes, such as mangrove encroachment, potentially underestimating overall risks. Our study highlights the plight of saltmarsh and the value of conducting simultaneous RLE assessments for multiple ecosystems comprising seascapes. We recommend that connectivity be accounted for explicitly in assessments of other connected, high-risk ecosystems.

March of the mangroves: Drivers of encroachment into southern temperate saltmarsh

Tidal wetlands are experiencing climate change driven shifts in their community composition, distribution, and functions. Mangroves have expanded into saltmarshes where their ranges overlap on five continents. Yet questions remain about the spatial variability of climate change drivers and subsequent local ecological feedbacks on these distributional shifts. Along the southernmost range of mangroves, we studied shifts between Avicennia marina and threatened saltmarsh communities. We used semi-automated object based image analysis to classify wetlands in aerial imagery from 1970 to 2017. Saltmarsh coverage decreased at the expense of mangrove encroachment. Landward migration of saltmarsh was negligible. Unlike other mangrove-saltmarsh ecotone studies, neither changes in seasonal extreme temperatures nor sea level rise were associated with the observed transitions. Rather, mangrove encroachment correlated with increasing annual maximum temperature and was more extensive at sites with lower mean elevations, shorter mean distances to earliest known mangrove-saltmarsh boundary, and higher densities of tidal creeks. Accounting for local variation in mangrove encroachment is informative for managing the provision of desirable ecosystem services. As climate change impacts the southernmost mangrove-saltmarsh ecotone, areas with higher mean elevations and fewer tidal creeks could be prioritized to mitigate the loss of saltmarsh.

Effects of mangrove encroachment on tidal wetland plant, nekton, and bird communities in the Western Gulf of Mexico

Woody encroachment is occurring in many marsh-mangrove ecotones across the globe, with multiple drivers contributing to an increase in mangrove cover. As a result, marsh plant species are often displaced, resulting in a striking regime shift from grass and forb-dominated habitats to taller, woody vegetation. Our goal was to quantify the bottom-up effects of mangrove woody encroachment into coastal wetlands on associated plant, nekton, and bird assemblages. In 2012, we established several large (>20 ha) survey areas at tidal wetland sites with or without black mangroves (Avicennia germinans) on the Texas (USA) coast in the Gulf of Mexico, an area highly susceptible to mangrove encroachment. From 2012 to 2015, we recorded vascular plant cover and diversity, sampled nekton at the water-vegetation interface using seine nets, and compared wintering shorebird and wader use among sites using the citizen science database eBird. Marsh plant species richness was 50% lower at sites with mangroves, and some species, such as Distichlis spicata, were absent from these sites entirely. The relative abundance and species richness of nektonic fish and invertebrates were similar between sites, but certain species (e.g., bay anchovies Anchoa mitchilli, grass shrimp Palaemonetes spp.) were more common at sites without mangroves. Citizen science data indicated that shorebird and wader species richness was 20% lower at sites with mangroves. Shorebird assemblages were markedly different between site types; six shorebird and three wader species were only detected in marsh sites. Our results indicate that coastal wetlands dominated by mangroves support different plant and animal assemblages than marsh-dominated areas. These results were largely consistent with the results of a previous manipulative experiment in the same area, lending considerable confidence to the conclusion that mangroves influence the associated plant and animal communities. Therefore, as woody encroachment continues and mangrove cover gradually increases, this change may lead to complex bottom-up effects on a range of ecosystem processes and services.

Soil surface elevation dynamics in a mangrove-to-marsh ecotone characterized by vegetation shifts

Mangrove forest encroachment into coastal marsh habitats has been described in subtropical regions worldwide in recent decades. To better understand how soil processes may influence vegetation change, we studied soil surface elevation change, accretion rates, and soil subsurface change across a coastal salinity gradient in Florida, USA, an area with documented mangrove encroachment into saline marshes. Our aim was to identify if variations in the soil variables studied exist and to document any associated vegetation shifts. We used surface elevation tables and marker horizons to document the soil variables over 5 years in a mangrove-to-marsh transition zone or ecotone. Study sites were located in three marsh types (brackish, salt, and transition) and in riverine mangrove forests. Mangrove forest sites had significantly higher accretion rates than marsh sites and were the only locations where elevation gain occurred. Significant loss in surface elevation occurred at transition and salt marsh sites. Transition marshes, which had a significantly higher rate of shallow subsidence compared to other wetland types, appear to be most vulnerable to submergence or to a shift to mangrove forest. Submergence can result in herbaceous vegetation mortality and conversion to open water, with severe implications to the quantity and quality of wetland services provided.

Elevated carbon dioxide and reduced salinity enhance mangrove seedling establishment in an artificial saltmarsh community.

The global phenomenon of mangrove encroachment into saltmarshes has been observed across five continents. It has been proposed that this encroachment is driven in part by rising atmospheric CO2 concentration and reduced salinity in saltmarshes resulting from rising sea levels enhancing the establishment success of mangrove seedlings. However, this theory is yet to be empirically tested at the community-level. In this study, we examined the effect of CO2 and salinity on seedling growth of two mangrove species, Aegiceras corniculatum and Avicennia marina, grown individually and in a model saltmarsh community in a glasshouse experiment. We found that the shoot (210%) and root (91%) biomass of the saltmarsh species was significantly greater under elevated CO2. As a result, both mangrove species experienced a stronger competitive effect from the saltmarsh species under elevated CO2. Nevertheless, A. marina seedlings produced on average 48% more biomass under elevated CO2 when grown in competition with the saltmarsh species. The seedlings tended to allocate this additional biomass to growing taller suggesting they were light limited. In contrast, A. corniculatum growth did not significantly differ between CO2 treatments. However, it had on average 36% greater growth under seawater salinity compared to hypersaline conditions. Avicennia marina seedlings were not affected by salinity. From these results, we suggest that although CO2 and salinity are not universal drivers determining saltmarsh-mangrove boundaries, it is likely that rising atmospheric CO2 concentration and reduced salinity associated with sea level rise will enhance the establishment success of mangrove seedlings in saltmarshes, which may facilitate mangrove encroachment in the future.