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The Shifting Saltmarsh-Mangrove Ecotone in Australasia and the Americas

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... Mangroves are predominantly tropical biomes, where mean air temperatures of the coldest month are higher than 20°C, the seasonal range is not >10°C and sea water temperature is within the 20°C isotherm (Saintilan et al., 2019). Low temperatures and/or the occurrence of frosts are the primary explanation for the latitudinal distribution of mangroves, which constrains mangrove survival and growth to tropical or subtropical coastlines (Sherrod & McMillan, 1985). ...
... australasica is found along the temperate coastlines of Australia and New Zealand where it forms monospecific forests. The distribution of A. marina australasica does not appear to relate to climate factors (Lange & Lange, 1994;Saintilan et al., 2019). The unusually temperate distribution of A. marina australasica in Australia is hypothesized to be either due to ongoing transmission by warm coastal ocean currents (e.g. the Leeuwin current) or that they are relict populations of Quaternary forests that had greater poleward distributions in the past (Duke et al., 1998;Macnae, 1966). ...
... The environmental niche of mangroves tends to be restricted to low-energy coastlines and depositional settings, including channel banks and coral reef islands (Saintilan et al., 2019;Woodroffe et al., 1989) The intertidal landward limit of mangroves is constrained by low soil moisture and high salinity, which prohibits seedling establishment and growth (Saintilan et al., 2019). A. marina is a shade-intolerant species, growing between the mean high-water neap tidal level and mean sea level in open estuaries in Australia (Clarke & Myerscough, 1993). ...
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Aim Mangroves are coastal ecosystems exposed to terrestrial, marine, geomorphic and climatic forcings operating in concert, making the mangrove niche hard to define, as evidenced by extremely poor restoration outcomes. We have developed a set of high‐resolution species distribution models and interpreted the variables that have the largest impact on the niche of Avicennia marina, the most globally widespread mangrove species, to comprehensively detail the forcings driving habitat suitability. Location Australia. Time period 1970–2020. Major taxa studied Avicennia marina (Forssk.) Vierh. Methods We modelled the suitable habitat for A. marina in Australia using the maxent method incorporating 38 environmental variables and the Global Mangrove Watch baseline for the presence records. Using k‐means grouping, we identified subregions where similar suites of environmental variables influence habitat suitability, while also identifying biogeographical commonalities among the subregions. To better understand the low realization of the fundamental niche, we analysed the other land covers occupying the niche. Results Avicennia marina in Australia occupies six different environmental subregions. Maxent distribution models accurately predicted the presence of A. marina in each subpopulation (AUC > 0.9). A. marina's presence in all subregions was strongly determined by its proximity to freshwater. Precipitation and temperature extreme values were more important than average values in predicting the species presence. The species requires low‐energy coastlines with high solar radiation. The suitable areas are primarily shared with salt marshes, seagrass and buildings or cleared land. Main conclusions Our results offer a baseline for the suitable area of A. marina's presence that includes a range of environmental conditions, A. marina currently occupies <50% of its suitable habitat, and there is scope for restoration with significant ecosystem service gains. The six different subregions in Australia map to known phylogenetically distinct populations, indicating genetic plasticity in response to region specific climatic conditions.
... Saltmarshes are being rapidly lost around the world due to sea level rise, coastal development, pollution, invasive species, and mangrove encroachment which displaces saltmarsh habitat in subtropical and temperate regions Adam 2019). The issue of mangroves displacing saltmarsh is a recent phenomenon, where a few cold tolerant mangrove species are extending past their historic poleward limits displacing previously diverse and extensive saltmarsh habitat (Saintilan et al. 2019). Displacement is restricted to cooler climates as warmer latitudes have a higher diversity of mangroves, limited Author contributions: CR, AC designed study and wrote manuscript; MS-N, WG, JM, TG reviewed the manuscript; CR, MS-N, AC implemented fieldwork; AC identified species; CR, JM, AC analyzed the data; WG, MS-N designed restoration project; WG, TG secured funding. 1 diversity, and extent of saltmarsh, and mangroves are in decline (Woodroffe et al. 2016;Saintilan et al. 2019). ...
... The issue of mangroves displacing saltmarsh is a recent phenomenon, where a few cold tolerant mangrove species are extending past their historic poleward limits displacing previously diverse and extensive saltmarsh habitat (Saintilan et al. 2019). Displacement is restricted to cooler climates as warmer latitudes have a higher diversity of mangroves, limited Author contributions: CR, AC designed study and wrote manuscript; MS-N, WG, JM, TG reviewed the manuscript; CR, MS-N, AC implemented fieldwork; AC identified species; CR, JM, AC analyzed the data; WG, MS-N designed restoration project; WG, TG secured funding. 1 diversity, and extent of saltmarsh, and mangroves are in decline (Woodroffe et al. 2016;Saintilan et al. 2019). In many areas an increase in mangrove abundance is encouraged due to their ecosystem services, including carbon sequestration, and vertical accretion counteracting climate change and sea level rise Huxham et al. 2018;Bertolini & da Mosto 2021). ...
Article
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 three 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 favour 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. This article is protected by copyright. All rights reserved.
... At the global scale, variation in mangrove biomass and species composition is strongly influenced by temperature and rainfall, with higher biomass and more diverse mangroves in the moist tropical regions compared to temperate and arid locations (Duke et al. 1998;Simard et al. 2018;Adame et al. 2020). Thus, the distribution of species and structural characteristics of mangroves is expected to be highly sensitive to changing climatic conditions (Krauss et al. 2008;Osland et al. 2017), which are expected to be particularly important at their poleward margins (Cavanaugh et al. 2018;Saintilan et al. 2019). While mangrove distribution and colonisation of saltmarsh habitats at the poleward limits have been observed globally (Saintilan et al. 2019), there is an ongoing need for detailed regional assessments to inform further global modelling of species distributions, forest structure and ecosystem functions (Rogers and Krauss, 2019). ...
... Thus, the distribution of species and structural characteristics of mangroves is expected to be highly sensitive to changing climatic conditions (Krauss et al. 2008;Osland et al. 2017), which are expected to be particularly important at their poleward margins (Cavanaugh et al. 2018;Saintilan et al. 2019). While mangrove distribution and colonisation of saltmarsh habitats at the poleward limits have been observed globally (Saintilan et al. 2019), there is an ongoing need for detailed regional assessments to inform further global modelling of species distributions, forest structure and ecosystem functions (Rogers and Krauss, 2019). Investigations within climatic zones provide valuable baseline data to assess the future impacts of climate change, and develop our understanding of local and regional influences on mangrove forest structure and species composition. ...
Article
Mangroves are important ecosystems of the global subtropical zone providing a wide range of ecosystem services. They are sensitive to human disturbances and climate change. However, regional assessments of species composition, forest structure and soil characteristics, and knowledge of the environmental factors that contribute to regional variation in their characteristics are limited. We surveyed mangrove communities at seven island locations along 450 km of the sandy south-east Queensland coast. We found no latitudinal trends in richness of common species or their abundance, likely due to shared climatic conditions across our study range, however there was considerable variation in forest structure among and within the seven sites. Over all sites, analysis of soil characteristics identified four hydro-edaphic environmental categories differing in soil water content, dry bulk density, organic matter and porewater salinity, which also varied spatially among and within sites. There were few strong relationships among forest structure and hydro-edaphic categories, although relatively small trees and high stem densities tended to be associated with soils having high soil water content, high levels of soil organic matter and low bulk density, while forests with largest trees and lowest stem densities tended to be associated with soils with intermediate characteristics, indicative of regular inundation and drainage. The two most common species (Avicennia marina and Rhizophora stylosa) had very broad distributions over the four hydro-edaphic categories, although A. marina tended to be more strongly associated with soils with high water content where R. stylosa had a lower probability of occurrence. Bruguiera gymnorrhiza had high fidelity to locations with low porewater salinity influenced by groundwater. We conclude that hydrological conditions are key determinants of mangrove soil characteristics, but less important in determining forest structure in the subtropics. Our study provides a baseline from which future change in mangrove soils and forest structure can be detected.
... Coastal wetlands are habitats with global significance, providing vital ecosystem services such as carbon sequestration, coastal protection and biodiversity support. However, climate change has driven a rapid transformation of these ecosystems, which is particularly evident in the encroachment of mangroves into salt marshes (Adams & Rajkaran, 2021;Saintilan et al., 2019;Wang et al., 2022). Mangrove expansion can be influenced by various factors, including sea-level rise pushing mangroves landward, dispersal facilitated by hurricanes and seasonal atmospheric and oceanographic conditions (Rogers & Krauss, 2019;Van der Stocken et al., 2019;Ximenes et al., 2021). ...
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Freeze events govern the distribution and structure of mangrove ecosystems, especially in tropical‐temperate transitional zones. Understanding mangrove responses to freezing is crucial for predicting their poleward expansion under climate change. However, there is a need for field‐based measurements of mangrove freeze resistance and resilience. After an extreme winter storm in December 2022, we measured mangrove post‐freeze damage and recovery (January and November 2023), building on a pre‐freeze baseline assessment conducted in July 2022 across 12 sites along the temperature gradient of Florida's Gulf of Mexico coast (USA). Low‐temperature thresholds for leaf damage to Avicennia germinans, Rhizophora mangle and Laguncularia racemosa were quantified near −6, −4 and − 4°C, respectively. Thresholds for mortality were found to be near −6 to −7°C for A. germinans and −4 to −5°C for R. mangle. A threshold for loss of reproductivity in A. germinans was identified near −6 to −7°C. Resprouting was observed in all three species but limited to just one individual for R. mangle. Surviving A. germinans resprouted vigorously and had the greatest number of resprout branches, which was proportional to leaf damage. Tall A. germinans had a higher resprout percentage than short trees. Strata‐specific differences in freeze damage were most pronounced for R. mangle, with higher damage in tall versus short trees, while no difference was found between A. germinans strata. These results suggest that R. mangle population recovery may depend on the growth of short trees, while A. germinans can recover from all strata. Minimum air temperature was strongly correlated with mangrove height and above‐ground biomass. Projections of future minimum temperature and species‐specific freeze degree days predict warming winters, suggesting further mangrove development and range expansion under climate change. Synthesis: Collectively, our study advances understanding of mangrove responses to freezing and identifies low‐temperature thresholds for each species, aiding predictions of mangrove range expansion.
... australasica, into historical saltmarsh habitat. This has led to a 64% decline in saltmarsh habitat since the early 1950s (Saintilan and Williams 1999, Williamson et al. 2011, Saintilan et al. 2019. As a result, mixed habitat is now common in treatment environments, with intertidal mangroves becoming increasingly prevalent (Griffin et al. 2010, Knight et al. 2012. ...
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The impact of the programmatic use of larvicides for mosquito control on native stingless bees (e.g., Apidae, Meliponini) is a growing concern in Australia due to heightened conservation awareness and the growth of hobbyist stingless bee keeping. In Australia, the two most widely used mosquito larvicides are the bacterium Bacillus thuringiensis var. israelensis (Bti) and the insect hormone mimic methoprene (as S-methoprene). Each has a unique mode of action that could present a risk to stingless bees and other pollinators. Herein, we review the potential impacts of these larvicides on native Australian bees and conclude that their influence is mitigated by their low recommended field rates, poor environmental persistence, and the seasonal and intermittent nature of mosquito control applications. Moreover, evidence suggests that stingless bees may display a high physiological tolerance to Bti similar to that observed in honey bees (Apis mellifera), whose interactions with B. thuringiensis-based biopesticides are widely reported. In summary, neither Bti or methoprene is likely to pose a significant risk to the health of stingless bees or their nests. However, current knowledge is limited by regulatory testing requirements that only require the use of honey bees as toxicological models. To bridge this gap, we suggest that regulatory testing is expanded to include stingless bees and other nontarget insects. This is imperative for improving our understanding of the potential risks that these and other pesticides may pose to native pollinator conservation.
... Among these wetlands, mangroves and saltmarshes are two common species, often found in similar geomorphic settings along low-lying coastlines (Barbier et al., 2011). Globally, mangrove trees and shrubs primarily dominate sheltered tropical shorelines, while grass-like saltmarshes prevail along temperate coastlines at higher latitudes (Saintilan et al., 2019). However, both systems are facing increasing pressure from climate change and human activities, potentially reshaping their habitats across the globe. ...
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Mangrove‐saltmarsh ecotones are experiencing rapid alterations due to climate change and human activities, however, the ecological and morphological implications of these shifts remain largely unknown. This study systematically explores how interspecific interactions and herbivory influence the dominant wetland species, as well as the resultant morphological evolution and landscape configuration. To achieve this, we develop a new eco‐morphodynamic model that integrates hydrodynamics, sediment transport, bed‐level change, and vegetation dynamics. The novelty of the current model lies in newly incorporated modules to simulate biotic interactions between mangroves and saltmarshes, enabling exploration of eco‐morphodynamic feedback in mangrove‐saltmarsh ecotones in response to tidal flows and species interactions. Our results show that vertical growth rates of coexisting vegetation species are dominant factors in determining wetland dominance. When mangroves and saltmarshes exhibit comparable growth rates, mangroves typically become the dominant wetland species. Conversely, if mangroves grow more slowly than saltmarshes, they are unable to outcompete saltmarshes. Additionally, herbivory can fundamentally alter wetland dominance depending on herbivore food preferences. Our simulations further underline that saltmarsh‐dominated wetlands develop channel networks more extensively and rapidly than mangrove‐dominated systems. This pattern is also observed during species invasions, with invading saltmarshes extending channel networks, while invading mangroves inhibit ongoing network expansion. This study highlights the pivotal roles of relative growth properties and herbivory in driving ecotone development in respect to wetland dominance and channel network development at the intertidal scale.
... Because biome shifts are evident through geological time and at continental scales, many models project biome shifts as a major outcome of climate change [6,17,18]. In many places this is already happening, as observed in North American grasslands [19], mangroves [20,21], West African Sahel [22], tropical moist forests and savannas [23], and elsewhere [24]. Despite much interest in biomes, especially as an ecological and biogeographical concept [25], the regions between them have received considerably less attention, a pervasive point we make here. ...
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Tropical forests are incredibly diverse in structure and function. Despite, or perhaps because of, this diversity, tropical biologists often conduct research exclusively in one or perhaps a few forest types. Rarely do we study the ecotone—the interstitial region between forest types. Ecotones are hyper-diverse, dynamic systems that control the flow of energy and organisms between adjacent ecosystems, with their locations determined by species’ physiological limits. In this review, we describe how studying ecotones can provide key indicators for monitoring the state of Neotropical forests from organisms to ecosystems. We first describe how ecotones have been studied in the past and summarize our current understanding of tropical ecotones. Next, we provide three example lines of research focusing on the ecological and evolutionary dynamics of the ecotone between tropical dry forests and desert; between tropical dry and rainforests; and between Cerrado and Atlantic rainforests, with the latter being a particularly well-studied ecotone. Lastly, we outline methods and tools for studying ecotones that combine remote sensing, new statistical techniques, and field-based forest dynamics plot data, among others, for understanding these important systems.
... Mortality and top-kill of mangroves may affect interspecific competition for resources such as light and nutrients which can cause shifts in floral community composition. The encroachment of mangroves into salt marsh ecosystems has been adversely affected by the 2021 freeze event, which may have set back the transition into an earlier stage of succession (Stevens et al. 2006, Saintilan et al. 2019. ...
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In response to warming minimum temperatures, Avicennia germinans is encroaching poleward on the Texas Gulf Coast (TGC) into saline marshes dominated by Spartina alterniflora and Batis maritima. Increased Avicennia cover provides greater protection from soil subsidence and shoreline retreat. However, intense freeze disturbances cause widespread mangrove mortality reversing succession, and increasing the risk of soil subsidence and shoreline retreat due to the loss of below-ground biomass. We conducted a “natural experiment of opportunity” to measure below- and above-ground biomass allocation in Avicennia recovering from catastrophic disturbance caused by the 2021 Winter Storm Uri at sites along a freeze-disturbance gradient across the South and Central TGC. Port O’Connor (28.46°N) was the most severely affected site, Cohn Preserve on Mustang Island (27.71°N) was moderately affected, and Laguna Atascosa National Wildlife Refuge (26.35°N) was minimally affected (min. temp. °C ~ -9.0, ~ -7.4, ~ -5.5 respectively). A second freeze event occurred in December 2022 that severely affected Port O’Connor and moderately affected Cohn Preserve (min. temp. °C ~ -6.6, ~ -5.8 respectively). In an additional methods experiment, we quantified differences in root productivity in in-growth cores containing either peat moss or local substrate at each site. Multiple root ingrowth cores were inserted near the canopy edge of isolated Avicennia shrubs (n=6; 5 at Port O’Connor) at the three sites and collected at 4-month intervals (total cores = 102). Root productivity (g * (m-2 day-1)) assessed in a one-way ANOVA and Tukey multiple comparisons, increased with increasing freeze-disturbance effects (F2,28 = 6.386, p<0.01, Port O’Connor: mean = 0.192, sd = 0.188, Cohn Preserve: mean = 0.065, sd = 0.081, Laguna Atascosa: mean = 0.047, sd = 0.089). Using the below-and above-ground relative growth rates to assess the root:shoot biomass allocation ratio, we found an increasing ratio (greater roots to shoots) with increased freeze disturbance (one-way ANOVA and Tukey HSD tests, F2,12 = 6.049, p<0.05, Port O’Connor: mean = 0.191, sd = 0.118, Cohn Preserve: mean = 0.061, sd = 0.066, Laguna Atascosa: mean = 0.015, sd = 0.01). Further, we found no clear trend in quantity or variability in root productivity between native and peat moss substrate types in root ingrowth cores for root biomass at any site (F2, 53 = 0.021, p=0.8). However, peat moss ingrowth cores did consistently have less root necromass after the Dec. 2022 freeze suggesting better survival or lower turnover. Peat moss ingrowth cores contained lower quantities of live root biomass indicating higher rates of root mortality or a stunting effect on root productivity post-second freeze. This finding suggests that cumulative impacts of two freezes occurring less than two years apart are greater than the effects of individual freezes. Increasing root productivity with greater freeze disturbance suggests that recovering standing root biomass may be important for the recovery of above-ground biomass in freeze-affected Avicennia. Rapid recovery of below-ground biomass will also contribute to ameliorating rates of soil subsidence and shoreline retreat. Lastly, we found that peat moss is a viable substrate type for future root ingrowth studies if the focus of the study is on total root biomass. However, if a second freeze event occurs during the root ingrowth study, there may be unequal effects between peat moss and local substrate.
... Our findings reveal that tidal wetland cover did not display well-segregated zonation patterns but displayed overlapping and mosaic patterns across micro-topographic gradients. Mosaic patterns in the distribution of saltmarshes and mangroves at similar elevations have been described in tropical northern Australia and elsewhere (Osland et al., 2013;Saintilan et al., 2019), and advise that tidal wetland distribution does not respond to strict zonation arrangements that might be commonly applied in management and legislation (e.g., Marine Estate Management Act, 2014; Rogers et al., 2016). Understanding the biological and physical processes that facilitate co-existence compared to the conditions that result in more segregated zonation patterns between saltmarsh, mangroves, and unvegeteted flats would provide valuable insights in our understanding of the zonation of those systems. ...
... Hydroperiod or topographic gradient is considered as a prime factor of eco-zonation in SLC wetlands (Marani et al. 2013;Kirwan et al. 2016;Rosbakh et al. 2020). Coastal engineering events that occur naturally through enhancing accretion determine how an SLC vegetation will evolve in response to climate change (Crosby et al. 2016;Best et al. 2018;Saintilan et al. 2019). RS can be crucial in mapping the tidal range, elevation, and primary production, as well as other factors associated to adaptation to the climate change, where the capacity to trapping sediments by the SLC ecosystem is an indicator, (Morris et al. 2002;Miller et al. 2021). ...
Article
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Saltmarsh land-cover (SLC) ecosystems, composed of unvegetated mudflats, saltmarshes, mangroves, and/or seagrass communities, are vulnerable to climate-induced impacts, such as sea level rise. Extracting a seamless and consistent waterline from satellite imagery is a major challenge because of environmental factors, such as turbidity, water depth and multiple types of underwater vegetation cover that introduce noise in the extraction of information. Hence, a water index, derived from multi-temporal Landsat 8 (OLI) data, acquired under different tides is proposed for mapping land-water across SLC wetlands by tracking waterlines. This provided inundation maps and defined eco-zones to specify south-eastern Bangladesh wetland composition. The NDWI_1 (McFeeters’s water index) applied to 42 OLI images and derived land-water difference maps generated inundation gradient maps with an overall classification accuracy of 87.8%. The simple intersection and union of region-of-interests extracted from the tide heights above the mean low-water springs enabled the mapping of four categories of wetland composition based on hydroperiods: a) irregularly inundated (II), regularly inundated (RI), irregularly exposed (IE; high floodplain), and subtidal (river bed and deep water sea). For all of the three study sites, mangrove, seagrass, non-mangrove and agriculture were all prominent on the IE eco-zone, while only saltmarsh was dominant on the II eco-zone. These maps of SLC wetland will enrich previous concepts of eco-zonation models that include salinity, erosion, accretion and rate of sea level rise as factors, suggesting that inundation extent and tidal phase complexities should be considered in the remote sensing of SLC composition for improved models of SLC vegetation response to climate change.
... Many of these floating plants in fresher upper reaches of estuaries are transported downstream onto salt marsh during high flow conditions causing smothering and die-back. Changes to hydrological regimes (particularly salinity intrusion) and the proliferation of invasive plants are widespread pressures leading to salt marsh deterioration that are being exacerbated by climate change, especially in tropical regions including parts of Australia, Asia and the Americas [64][65][66][67][68]. ...
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Restoration of salt marsh is urgent, as these ecosystems provide natural coastal protection from sea-level rise impacts, contribute towards climate change mitigation, and provide multiple ecosystem services including supporting livelihoods. This study identified potential restoration sites for intervention where agricultural and degraded land could be returned to salt marsh at a national scale in South African estuaries. Overall, successful restoration of salt marsh in some estuaries will require addressing additional pressures such as freshwater inflow reduction and deterioration of water quality. Here, we present, a socio-ecological systems framework for salt marsh restoration that links salt marsh state and the well-being of people to guide meaningful and implementable management and restoration interventions. The framework is applied to a case study at the Swartkops Estuary where the primary restoration intervention intends to route stormwater run-off to abandoned salt works to re-create aquatic habitat for waterbirds, enhance carbon storage, and provide nutrient filtration. As the framework is generalized, while still allowing for site-specific pressures to be captured, there is potential for it to be applied at the national scale, with the largest degraded salt marsh areas set as priorities for such an initiative. It is estimated that ~1970 ha of salt marsh can be restored in this way, and this represents a 14% increase in the habitat cover for the country. Innovative approaches to restoring and improving condition are necessary for conserving salt marshes and the benefits they provide to society.
... Other feedbacks are also at play. Mangrove encroachment exemplifies broader system perturbations, driven by sea level rise and climate factors (Eslami-Andargoli et al. 2009;Saintilan et al. 2019;Whitt et al. 2020), that create positive feedbacks that threaten saltmarsh system collapse. We found that in many cases, runnels cannot persist without the maintenance that holds these feedbacks in check. ...
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Runnelling is a minor and successful form of open marsh water management developed in Australia in the 1980s and integrated into mosquito control programs in the 1990s. While long-term monitoring and investigation of impacts has continued for one site, until recently there has been no assessment of operational runnelling more broadly across mosquito control agencies. This study addresses this issue. Forty-seven runnelled saltmarsh sites were assessed for runnel efficacy, function and condition. Issues impacting on runnel function, including both ecological and geomorphic processes, were noted. Data were mostly acquired from site inspections and discussions with mosquito control staff, with some records as available. Most runnels were constructed between 1990 and 2005, mainly using a dedicated runnelling machine. Almost half (49%) continued to contribute to mosquito control efficacy, either in part or fully, while half did not (51%). Efficacy was attributed to runnels being correctly configured in design, layout and construction and to relatively recent maintenance. Conversely, diminished efficacy was mostly attributed to ineffective hydrologic function, caused by vegetation blockages, erosion and/or deposition. Runnels alter tidal hydrology, affecting other ecological and geomorphic processes. These processes need to be managed to maintain runnel function and it is essential that monitoring and maintenance records are kept. Runnels can control mosquitoes for decades, provided ongoing monitoring and maintenance of the runnel system focusses on responding to early signs of impacts. Restoring degraded runnels should be undertaken using a runnel machine, however, changing environmental conditions may mean that a runnel should be ‘decommissioned’ instead.
... Locally, salt marsh can undergo significant losses in China [6], affecting the sediment budget and even tide [7]. In the context of the global land and sea surface temperature warming, poleward shifts of the mangrove entail established competition with salt marshes in America and Australasia [8]. ...
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The salt marshes, lying at the land-sea temperate interface, furnish a plethora of ecosystems services such as biodiversity niche support, ocean-climate change regulation, ornithology recreo-tourism or plant gathering by hand. They undergo significant worldwide losses due to their conversion into crop fields and to their spatial compression between the rising sea-level and the armoring shoreline. Their monitoring however requires to use a suite of remote sensing sensors to embrace the regional scale while capturing the plant details. This research innovatively adopts a multiscale approach using a cascading spaceborne and airborne process, from the 10-m Sentinel-2, through the 3-m Dove, to the 0.03-m unmanned airborne vehicle (UAV) imageries. The high to very high temporal resolution of the Sentinel-2 and Dove enabled to cover twenties and tens of km2 over five and four years, respectively, in the form of normalized difference vegetation index (NDVI) classes, associated with microphytobenthos, low, medium and high salt marsh vegetation, including the opportunistic Elyma genus. The NDVI was then modelled at the UAV scale (a few km2) using a three-layered NN prediction, providing the final near-infrared (NIR), and the intermediate red, green and blue reflectance imageries, calibrated/validated/tested with the Dove reflectance imageries (R2NIR=0.98, R2red=0.88, R2green=0.84, and R2blue=0.90). The 100fold increase in pixel size allowed to detect the decimeter-scale objects of the tidal flats and salt marshes, to enlarge the NDVI class ranges, and hold great promise to model other spectral bands at the UAV scale for further deeply enhancing the salt marsh mapping.
... By spending time getting better acquainted, I learned to appreciate the ecosystem I was studying. Such knowledge has helped me write review papers and book chapters about mangroves (Mendelssohn and McKee 2000, McKee et al. 2012, Krauss et al. 2014, Woodroffe et al. 2016, Saintilan et al, 2019. ...
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This chapter is included in a book of essays written by leading wetland scientists and that provide different perspectives about a career in wetland science. In this chapter, McKee described her early wetland experiences and some of the factors that influenced her career choices.
... Due to the presence of vegetationmicroclimate feedbacks, the rate of mangrove range expansion has the potential to accelerate during extended freeze-free periods (Osland et al., 2015). To complicate matters, mangrove range expansion will also be influenced by interactions with other aspects of climate change (McKee, Rogers, & Saintilan, 2012;Osland et al., 2018;Saintilan, Rogers, & McKee, 2019;Ward et al., 2016). ...
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Aim Climate change is expected to result in the tropicalization of coastal wetlands in the northern Gulf of Mexico, as warming winters allow tropical mangrove forests to expand their distribution poleward at the expense of temperate salt marshes. Data limitations near mangrove range limits have hindered understanding of the effects of winter temperature extremes on mangrove distribution and structure. Here, we investigated the influence of extreme freeze events on the abundance, height and coverage of black mangroves (Avicennia germinans ) near their northern range limit in Louisiana. Location Coastal Louisiana, USA. Methods We quantified the relationships between the frequency of extreme freeze events and A. germinans abundance, height and coverage using: (a) mangrove observation points recorded via aerial surveys from a fixed‐wing aircraft; (b) 30 years of temperature data; and (c) mangrove mortality and leaf damage temperature thresholds. We used freeze frequency data and mangrove–climate relationships to evaluate and spatially depict the risk of A. germinans freeze damage across Louisiana. Results We identified strong negative relationships between the frequency of extreme freeze events and A. germinans abundance, height and coverage. Avicennia germinans is most abundant, tall and continuous along the south‐eastern outer coast of Louisiana, where the frequency of extreme freeze events is reduced (i.e., lower risk of mangrove freeze damage) by the buffering effects of comparatively warm Gulf of Mexico waters. Conversely, the risk of A. germinans freeze damage has historically been very high across Louisiana's Chenier Plain and within more inland wetlands in the Deltaic Plain. Main conclusions Our analyses advance understanding of how the frequency of extreme freeze events controls the distribution, height and coverage of A. germinans near its northern range limit. In addition to informing climate‐smart coastal restoration efforts, our findings can be used to better anticipate and prepare for the tropicalization of temperate wetlands due to climate change.
... This is an increasing problem as mangrove cover has intensified in recent decades in response to sea-level rise and land-use change. [14][15][16] In some areas, saltmarsh loss has exceeded 70% as a result of mangrove transgression. In response, many operators have opted to use higher density granular Bti products rather than liquid formulations to increase penetration through the canopy and improve control. ...
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BACKGROUND In the Australian southeast, the saltmarsh mosquito Aedes vigilax (Skuse) is the focus of area‐wide larviciding campaigns employing the biological agent Bacillus thuringiensis var. israelensis (Bti). Although generally effective, frequent inundating tides and considerable mangrove cover can make control challenging. Here, we describe the efficacy and persistence of an aqueous Bti suspension (potency: 1200 International Toxic Units; strain AM65‐52) within a mixed saltmarsh–mangrove system and the use of affordable unmanned aerial systems (UAS) to identify and map problematic levels of mangrove canopy cover. RESULTS High mangrove canopy density (>40% cover) reduced product deposition by 75.2% (0.01 ± 0.002 μL cm–2 versus 0.05 ± 0.006 μL cm–2), larval mortality by 27.7% (60.7 ± 4.1% versus 84.0 ± 2.4%), and ground level Bti concentrations by 32.03% (1144 ± 462.6 versus 1683 ± 447.8 spores mL⁻¹) relative to open saltmarsh. Persistence of product post‐application was found to be low (80.6% loss at 6 h) resulting in negligible additional losses to tidal inundation 24 h post‐application. UAS surveys accurately identified areas of high mangrove cover using both standard and multispectral imagery, although derived index values for this vegetation class were only moderately correlated with ground measurements (R² = 0.17–0.38) at their most informative scales. CONCLUSION These findings highlight the complex operational challenges that affect coastal mosquito control in heterogeneous environments. The problem is exacerbated by continued mangrove transgression into saltmarsh habitat in the region. Emerging UAS technology can help operators optimize treatments by accurately identifying and mapping challenging canopy cover using both standard and multispectral imaging. © 2020 Society of Chemical Industry
... However, in global warming scenarios for the future, mangrove species are expected to move to higher latitudes in both hemispheres and establish closed-canopy mangrove forests where Spartina marshes grow today. Those movements have been precisely measured in Florida, [24] Texas, [25] and Australia [26,27] (Figure 11.2). ...
... In general, the field observations in the present study revealed that salt marshes are irregularly inundated by tides. Sesuvium portulacastrum, one of the most common salt marsh species in tropical regions, is widely distributed in India can be attributed to its greater tolerance to extreme conditions (Saintilan et al., 2019). Salt marshes are mostly perennial species in India and have a significant impact on sediment accretion by reducing the hydrodynamics load (Baptist et al., 2019). ...
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Chapter
Geomorphological landforms and processes underpin the valuable ecological functions and ecosystem services performed by river-dominated, tide-dominated and interior mangrove forest systems. At the local scale, mangrove system morphodynamics are characterized by complex interactions between tidal channel flows and shallow-flooded vegetated surfaces, with wave energy dissipation across mangrove margins. Surface elevation change in mangroves reflects the balance between surface sedimentation processes and sub-surface processes and, exceptionally, responses to hurricane and tsunami impacts. Geomorphology has a role to play in assessments of mangrove status under anthropogenic pressures and in providing insights into trajectories for effective mangrove rehabilitation and restoration.
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Anthropogenic global change threatens the Earth’s biodiversity, with the future of plants utilizing carbon-concentrating mechanisms (CCM) being of particular concern. Here, we discuss global change effects on plants utilizing CCMs, relative to plants using the C3 photosynthesis pathway. Terrestrial CCMs include the C4, CAM and C2 photosynthetic pathways, which are collectively utilized by 10% of the world’s plant flora. They are considered at risk because CCMs are adaptations to low CO2 atmospheres which become superfluous at elevated CO2. Rising atmospheric CO2 represents one form of anthropogenic global change, along with climate change, land transformation, over-exploitation of natural species, terrestrial eutrophication, and exotic species invasions. While rising CO2 favors the physiology of C3 over C4 photosynthesis in warmer temperatures, in natural stands where multiple global change drivers are active, outcomes often do not follow what would be predicted from physiological responses. Based on present trends, which already include CO2 enrichment effects, the natural diversity of the C4, CAM and the C2 functional types is declining. A leading cause is aggressive infilling of grassland habitats by woody C3 competitors or invasive species. Woody infilling is the result of a combination of drivers including rising CO2, overgrazing, overhunting of browsers, and land use change.
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Mangroves have shifted in position on the Australian coastline in association with sea-level fluctuations associated with climate change over a range of timescales. Relative sea-level rise following the last glacial maxima has driven mangroves across shallow continental margins punctuated by periods of relative stability and mangrove development on the continental shelf. The slowing of relative sea-level rise following 8000 BP coincided with widespread mangrove development, particularly across the tropical north of the continent. The stabilization of sea level by 7000 BP and probable subsequent decline promoted the infilling of estuaries and the replacement of mangrove with upper intertidal and supratidal habitats in many estuaries. In recent decades, mangrove landward encroachment has reversed some of these longer-term trends and is expected to continue under projected trends in relative sea level. Measures of mangrove vertical accretion utilizing the surface elevation table–marker horizon technique suggest that mangroves are currently keeping pace with sea-level rise and represent an important carbon sink.
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Changes in the distribution and abundance of mangrove species within and outside of their historic geographic range can have profound consequences in the provision of ecosystem goods and services they provide. Mangroves in the conterminous United States (CONUS) are believed to be expanding poleward (north) due to decreases in the frequency and severity of extreme cold events, while sea level rise is a factor often implicated in the landward expansion of mangroves locally. We used ~35 years of satellite imagery and in situ observations for CONUS and report that: (i) poleward expansion of mangrove forest is inconclusive, and may have stalled for now, and (ii) landward expansion is actively occurring within the historical northernmost limit. We revealed that the northernmost latitudinal limit of mangrove forests along the east and west coasts of Florida, in addition to Louisiana and Texas has not systematically expanded toward the pole. Mangrove area, however, expanded by 4.3% from 1980 to 2015 within the historic northernmost boundary, with the highest percentage of change in Texas and southern Florida. Several confounding factors such as sea level rise, absence or presence of sub-freezing temperatures, land use change, impoundment/dredging, changing hydrology, fire, storm, sedimentation and erosion, and mangrove planting are responsible for the change. Besides, sea level rise, relatively milder winters and the absence of sub-freezing temperatures in recent decades may be enabling the expansion locally. The results highlight the complex set of forcings acting on the northerly extent of mangroves and emphasize the need for long-term monitoring as this system increases in importance as a means to adapt to rising oceans and mitigate the effects of increased atmospheric CO2.
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Extensive hydrologic modifications in coastal regions across the world have occurred to support infrastructure development, altering the function of many coastal wetlands. Wetland restoration success is dependent on the existence of hydrologic regimes that support development of appropriate soils and the growth and persistence of wetland vegetation. In Florida, United States, the Comprehensive Everglades Restoration Program (CERP) seeks to restore, protect, and preserve water resources of the greater Everglades region. Herein we describe vegetation dynamics in a mangrove-to-marsh ecotone within the impact area of a CERP hydrologic restoration project currently under development. Vegetation communities are also described for a similar area outside the project area. We found that vegetation shifts within the impact area occurred over a 7-year period; cover of herbaceous species varied by location, and an 88% increase in the total number of mangrove seedlings was documented. We attribute these shifts to the existing modified hydrologic regime, which is characterized by a low volume of freshwater sheet flow compared with historical conditions (i.e. before modification), as well as increased tidal influence. We also identified a significant trend of decreasing soil surface elevation at the impact area. The CERP restoration project is designed to increase freshwater sheet flow to the impact area. Information from our study characterizing existing vegetation dynamics prior to implementation of the restoration project is required to allow documentation of long-term project effects on plant community composition and structure within a framework of background variation, thereby allowing assessment of the project's success in restoring critical ecosystem functions.
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Climate change-driven shifts in species ranges are ongoing and expected to increase. However, life-history traits may interact with climate to influence species ranges, potentially accelerating or slowing range shifts in response to cli- mate change. Tropical mangroves have expanded their ranges poleward in the last three decades. Here, we report on a shift at the range edge in life-history traits related to reproduction and dispersal. With a common garden experiment and field observations, we show that Rhizophora mangle individuals from northern populations reproduce at a younger age than those from southern populations. In a common garden at the northern range limit, 38% of individuals from the northernmost population were reproductive by age 2, but less than 10% of individuals from the southernmost population were reproductive by the same age, with intermediate amounts of reproduction from intermediate latitudes. Field observations show a similar pattern of younger reproductive individuals toward the northern range limit. We also demonstrate a shift toward larger propagule size in populations at the leading range edge, which may aid seedling growth. The substantial increase in precocious reproduction at the leading edge of the R. mangle range could accelerate population growth and hasten the expansion of mangroves into salt marshes.
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Across their range, mangroves are responding to coastal environmental change. However, separating the influence of human activities from natural events and processes (including that associated with climatic fluctuation) is often difficult. In the Gulf of Carpentaria, northern Australia (Leichhardt, Nicholson, Mornington Inlet, and Flinders River catchments), changes in mangroves are assumed to be the result of natural drivers as human impacts are minimal. By comparing classifications from time series of Landsat sensor data for the period 1987-2014, mangroves were observed to have extended seawards by up to 1.9 km (perpendicular to the coastline), with inland intrusion occurring along many of the rivers and rivulets in the tidal reaches. Seaward expansion was particularly evident near the mouth of the Leichhardt River, and was associated with peaks in river discharge with LiDAR data indicating distinct structural zones developing following each large rainfall and discharge event. However, along the Gulf coast, and particularly within the Mornington Inlet catchment, the expansion was more gradual and linked to inundation and regular sediment supply through freshwater input. Landward expansion along the Mornington Inlet catchment was attributed to the combined effects of sea level rise and prolonged periods of tidal and freshwater inundation on coastal lowlands. The study concluded that increased amounts of rainfall and associated flooding and sea level rise were responsible for recent seaward and landward extension of mangroves in this region.
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In subtropical coastal wetlands on multiple continents, climate change-induced reductions in the frequency and intensity of freezing temperatures are expected to lead to the expansion of woody plants (i.e., mangrove forests) at the expense of tidal grasslands (i.e., salt marshes). Since some ecosystem goods and services would be affected by mangrove range expansion, there is a need to better understand mangrove sensitivity to freezing temperatures as well as the implications of changing winter climate extremes for mangrove-salt marsh interactions. In this study, we investigated the following questions: (1) how does plant life stage (i.e., ontogeny) influence the resistance and resilience of black mangrove (Avicennia germinans) forests to freezing temperatures; and (2) how might differential life stage responses to freeze events affect the rate of mangrove expansion and salt marsh displacement due to climate change? To address these questions, we quantified freeze damage and recovery for different life stages (seedling, short tree, and tall tree) following extreme winter air temperature events that occurred near the northern range limit of A. germinans in North America. We found that life stage affects black mangrove forest resistance and resilience to winter climate extremes in a nonlinear fashion. Resistance to winter climate extremes was high for tall A. germinans trees and seedlings, but lowest for short trees. Resilience was highest for tall A. germinans trees. These results suggest the presence of positive feedbacks and indicate that climate-change induced decreases in the frequency and intensity of extreme minimum air temperatures could lead to a nonlinear increase in mangrove forest resistance and resilience. This feedback could accelerate future mangrove expansion and salt marsh loss at rates beyond what would be predicted from climate change alone. In general terms, our study highlights the importance of accounting for differential life stage responses and positive feedbacks when evaluating the ecological effects of changes in the frequency and magnitude of climate extremes.
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Due to their position at the land-sea interface, coastal wetlands are vulnerable to many aspects of climate change. However, climate change vulnerability assessments for coastal wetlands generally focus solely on sea-level rise without considering the effects of other facets of climate change. Across the globe and in all ecosystems, macroclimatic drivers (e.g., temperature and rainfall regimes) greatly influence ecosystem structure and function. Macroclimatic drivers have been the focus of climate-change related threat evaluations for terrestrial ecosystems, but largely ignored for coastal wetlands. In some coastal wetlands, changing macroclimatic conditions are expected to result in foundation plant species replacement, which would affect the supply of certain ecosystem goods and services and could affect ecosystem resilience. As examples, we highlight several ecological transition zones where small changes in macroclimatic conditions would result in comparatively large changes in coastal wetland ecosystem structure and function. Our intent in this communication is not to minimize the importance of sea-level rise. Rather, our overarching aim is to illustrate the need to also consider macroclimatic drivers within vulnerability assessments for coastal wetlands. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
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Mangroves provide a wide range of ecosystem services, including nutrient cycling, soil formation, wood production, fish spawning grounds, ecotourism and carbon (C) storage1. High rates of tree and plant growth, coupled with anaerobic, water-logged soils that slow decomposition, result in large long-term C storage. Given their global significance as large sinks of C, preventing mangrove loss would be an effective climate change adaptation and mitigation strategy. It has been reported that C stocks in the Indo-Pacific region contain on average 1,023 MgC ha−1 (ref. 2). Here, we estimate that Indonesian mangrove C stocks are 1,083 ± 378 MgC ha−1. Scaled up to the country-level mangrove extent of 2.9 Mha (ref. 3), Indonesia’s mangroves contained on average 3.14 PgC. In three decades Indonesia has lost 40% of its mangroves4, mainly as a result of aquaculture development5. This has resulted in annual emissions of 0.07–0.21 Pg CO2e. Annual mangrove deforestation in Indonesia is only 6% of its total forest loss6; however, if this were halted, total emissions would be reduced by an amount equal to 10–31% of estimated annual emissions from land-use sectors at present. Conservation of carbon-rich mangroves in the Indonesian archipelago should be a high-priority component of strategies to mitigate climate change.
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Over the past few decades, many of the world’s mangrove forests have experienced significant change, which can be attributed to human activities and also natural causes. However, a component may also be due to factors that are commonly associated with anthropogenic climate change in- cluding higher air temperatures, variations in rainfall, in- creases in storm frequencies and intensities, and rising sea levels. The expected responses of mangrove to these drivers include changes in extent (latitudinal, seaward and landward), growth rates and productivity, and species composition. This paper reviews such responses and then, using examples from Australia, illustrates how these might appear within and be detected using single-date or time-series of remote sensing data acquired in different modes (e.g., aerial photography, optical and radar). In doing so, it informs countries and orga- nisations of the potential impacts of climate change on man- grove forests and how these may be monitored using remote sensing data.
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The climate change-induced expansion of mangroves into salt marshes could significantly alter the carbon (C) storage capacity of coastal wetlands, which have the highest average C storage per land area among unmanaged terrestrial ecosystems. Mangrove range expansion is occurring globally, but little is known about how these rapid climate-driven shifts may alter ecosystem C storage. Here, we quantify current C stocks in ecotonal wetlands across gradients of marsh- to mangrove-dominance, and use unique chronological maps of vegetation cover to estimate C stock changes from 2003 to 2010 in a 567-km2 wildlife refuge in the mangrove-salt marsh ecotone. We report that over the 7-yr. period, total wetland C stocks increased 22 % due to mangrove encroachment into salt marshes. Newly established mangrove stands stored twice as much C on a per area basis as salt marsh primarily due to differences in aboveground biomass, and mangrove cover increased by 69 % during this short time interval. Wetland C storage within the wildlife refuge increased at a rate of 2.7 Mg C ha−1 yr.−1, more than doubling the naturally high coastal wetland C sequestration rates. Mangrove expansion could account for a globally significant increase of terrestrial C storage, which may exert a considerable negative feedback on warming.
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Landscape-level shifts in plant species distribution and abundance can fundamentally change the ecology of an ecosystem. Such shifts are occurring within mangrove-marsh ecotones, where over the last few decades, relatively mild winters have led to mangrove expansion into areas previously occupied by salt marsh plants. On the Texas (USA) coast of the western Gulf of Mexico, most cases of mangrove expansion have been documented within specific bays or watersheds. Based on this body of relatively small-scale work and broader global patterns of mangrove expansion, we hypothesized that there has been a recent regional-level displacement of salt marshes by mangroves. We classified Landsat-5 Thematic Mapper images using artificial neural networks to quantify black mangrove (Avicennia germinans) expansion and salt marsh (Spartina alterniflora and other grass and forb species) loss over 20 years across the entire Texas coast. Between 1990 and 2010, mangrove area grew by 16.1 km2, a 74% increase. Concurrently, salt marsh area decreased by 77.8 km2, a 24% net loss. Only 6% of that loss was attributable to mangrove expansion; most salt marsh was lost due to conversion to tidal flats or water, likely a result of relative sea level rise. Our research confirmed that mangroves are expanding and, in some instances, displacing salt marshes at certain locations. However, this shift is not widespread when analyzed at a larger, regional level. Rather, local, relative sea level rise was indirectly implicated as another important driver causing regional-level salt marsh loss. Climate change is expected to accelerate both sea level rise and mangrove expansion; these mechanisms are likely to interact synergistically and contribute to salt marsh loss.
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Understanding the influence of future sea-level rise (SLR) on coastal ecosystems is improved by examining response of coastlines during historic periods of SLR. We evaluated stability and movement of the estuarine intertidal zone along eastern Gulf of Mexico, known as the BBig Bend^ of Florida. This relatively undeveloped, low-energy coast is dominated by broad expanses of tidal marsh, providing an opportunity to observe unobstructed response of a coastal ecosystem to SLR. Features from nineteenth century topographic surveys and late twentieth century satellite imagery were compared. Relative change was calculated for intertidal area and lateral migration over 120 years, a period when tidal amplitude increased in addition to SLR. Loss of tidal marsh at the shoreline was −43 km2, representing a 9 % loss to open water. At the same time, 82 km2 of forest converted to marsh and 66 km2 of forest converted to forest-to-marsh transitional habitat. The result was a net regional gain of 105 km2 of intertidal area, an increase of 23 %, constituting a marine transgression of coastal lowlands. Forest retreat was lower at zones of high freshwater input, attributable to salinity moderation and was further complicated by coastal morphology and land-use practices. Shoreline migration may not represent full extent of habitat change resulting from SLR in regions with low coastal gradients. Forest retreat was consistent with what would be predicted by an inundation model; however, shoreline loss was considerably less, resulting in a net increase in intertidal area in this sediment-limited coast.
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Few studies have empirically examined the suite of mechanisms that underlie the distributional shifts displayed by organisms in response to changing climatic condition. Mangrove forests are expected to move inland as sea-level rises, encroaching on saltmarsh plants inhabiting higher elevations. Mangrove propagules are transported by tidal waters and propagule dispersal is likely modified upon encountering the mangrove-saltmarsh ecotone, the implications of which are poorly known. Here, using an experimental approach, we record landward and seaward dispersal and subsequent establishment of mangrove propagules that encounter biotic boundaries composed of two types of saltmarsh taxa: succulents and grasses. Our findings revealed that propagules emplaced within saltmarsh vegetation immediately landward of the extant mangrove fringe boundary frequently dispersed in the seaward direction. However, propagules moved seaward less frequently and over shorter distances upon encountering boundaries composed of saltmarsh grasses versus succulents. We uniquely confirmed that the small subset of propagules dispersing landward displayed proportionately higher establishment success than those transported seaward. Although impacts of ecotones on plant dispersal have rarely been investigated in situ, our experimental results indicate that the interplay between tidal transport and physical attributes of saltmarsh vegetation influence boundary permeability to propagules, thereby directing the initial phase of shifting mangrove distributions. The incorporation of tidal inundation information and detailed data on landscape features, such as the structure of saltmarsh vegetation at mangrove boundaries, should improve the accuracy of models that are being developed to forecast mangrove distributional shifts in response to sea-level rise.
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Seedlings of three mangrove species—Rhizophora mangle L., Avicennia germinans (L.)Stearn., and Laguncularia racemosa (L.) Gaertn. f.—were subjected to different light and nutrient regimes in two separate growth chamber experiments. At higher nutrient or light availability, relative growth rate, leaf production, and branch growth differed significantly among species in the following order: Rhizophora < Avicennia < Laguncularia. At lower nutrient or light levels, however, species’ differences were greatly minimized. Lower nutrient or light levels caused greater investment in root biomass by all species, whereas higher nutrient availability resulted in greater investment in leaf area and maximized species’ differences in total leaf area, number of leaves, and leaf area ratio. Mangrove leaves also differed among species in quantity and composition of secondary compounds that may protect seedlings against herbivores or stress factors such as excessive solar radiation. Relative amounts of condensed tannins, gallotannins, and nitrogen were significantly affected by light and nutrient regimes, but patterns of response differed among species. The results indicate that these sympatric species differ substantially in their potential for growth, acquisition of resources, stress tolerance, and susceptibility to herbivores during the seedling stage, but that these characteristics are significantly modified by availability of resources.
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Allozyme variation in species of the mangrove genus Avicennia was screened in 25 populations collected from 22 locations in the Indo-West Pacific and eastern North America using 11 loci. Several fixed gene differences supported the specific status of Avicennia alba, A. integra, A. marina, and A. rumphiana from the Indo-West Pacific, and A. germinans from the Atlantic-East Pacific. The three varieties of A. marina, var. marina, var. eucalyptifolia, and van australasica, had higher genetic similarities (Nei's I) and no fixed gene differences, confirming their conspecific status. Strong genetic structuring was observed in A. marina, with sharp changes in gene frequencies at the geographical margins of varietal distributions. The occurrence of alleles found otherwise in only one variety, in only immediately adjacent populations of another variety, provided evidence of introgession between varieties. The varieties appear to have diverged recently in the Pleistocene and are apparently not of ancient Cretaceous origin, as suggested earlier. Despite evidence of high degrees of outcrossing, gene flow among populations was relatively low (Ne m < 1-2), except where populations were geographically continuous, questioning assumptions that these widespread mangrove species achieve high levels of long-distance dispersal.
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Rather than attempting to measure sea-level change directly, it is proposed that ecological indicators, such as changes in saltmarsh vegetation, may supply useful additional data and perhaps alternative methods of detecting changes in sea level. -from Author
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Discusses the geographical diversity of saltmarsh vegetation and the ecophysiological mechanisms which permit tolerance of the major consequences of high but variable soil salinity and of characteristically waterlogged substrates. Chapters examine: general features of saltmarshes and their environment; the biota; variation in vegetation; coping with the environment; plant life histories (including pollination, vivipary, dispersal, seed bank, germination and seedling establishment, grazing and reproduction, and clonal growth); saltmarsh ecosystems; and human activity, eg grazing, reclamation, pollution, recreation, conservation, management, marsh creation and rehabilitation. -P.J.Jarvis
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Long-term closure of the Kobonqaba Estuary mouth to the sea resulted in high water levels, inundation of pneumatophores and die-back of the mangroves specifically, Avicennia marina. The recovery of the mangroves was measured by assessing changes in vegetation cover and sediment characteristics along transects and all mangroves present in the estuary were described in terms of height, diameter at breast height and phenology. Measurements were made in 2011, 2012, 2013 and 2015. Surviving adult trees were producing propagules and showed that the forest would be able to regenerate if the seedlings established and were able to survive the environmental conditions. However there may be competition from salt marsh plants which increased in average cover from 0% in 2011 to 18.9% (2013) and 50% in 2015. Sediment characteristics changed over the study period with more oxic conditions in 2013 and 2015 (57.6 and 62.3 mV) compared to 2011 and 2012 (− 189 to − 148 mV) and organic matter decreased from 18.4% in 2011 to 9.8% in 2015. High initial porewater nutrient concentrations in 2013 promoted salt marsh growth. A particular sequence of events was recorded for the establishment of salt marsh in this previous mangrove habitat where stabilization of sediment by microphytobenthos and macroalgae was the key to further colonization by emergent macrophytes.
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Coastal Louisiana is at the latitudinal limits of the black mangrove. Salt marshes dominated by Spartina alterniflora form ecotones with this mangrove species at Bay Champagne. Zonation is distinct with a bay-edge zone dominated by A. germinans, an inland zone dominated by S. alterniflora and an intermediate zone containing both species. A. germinans seedlings were transplanted into randomly located plots in the Avicennia, Spartina high and Spartina low zones. Survival was marginally lower in the Spartina low zone than in the other zones, but almost half of the plants survived there. However, diameter and biomass of the surviving transplants were significantly lower in both Spartina zones than in the Avicennia zone, possibly due to growth inhibition by S. alterniflora in combination with a greater hydroperiod in the Spartina high and low zones. -from Authors
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Black mangrove [Avicennia germinans (L.) L.] occurs at several locations along the Texas gulf coast. A hard freeze in December 1989 severely damaged this species, but the extent of damage has not been determined. Airborne color-infrared (CIR) video imagery was used with global positioning system (GPS) and geographic information system (GIS) technologies for distinguishing and mapping the current distribution of black mangrove. Black mangrove populations could be easily distinguished on CIR video imagery. The integration of a GPS with the video imagery permitted latitude/longitude coordinates of black mangrove populations to be recorded on each image. The GPS coordinates were entered into a GIS to map black mangrove populations along the Texas coast. Major black mangrove concentrations near Port Isabel-South Bay and Port Aransas on the lower and lower-mid Texas coast, respectively, had fully recovered from the freeze. A remnant population of an historical black mangrove concentration on the upper-mid Texas coast near Port O'Connor, that was devastated by a 1983 freeze, was severely damaged and reduced in number by the 1989 freeze. The integration of videography, GPS, and GIS are valuable tools that can enable coastal resource managers to develop regional maps showing the distribution of black mangrove over large areas.
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Shifts in ecosystem structure have been observed over recent decades as woody plants encroach upon grasslands and wetlands globally. The migration of mangrove forests into salt marsh ecosystems is one such shift which could have important implications for global 'blue carbon' stocks. To date, attempts to quantify changes in ecosystem function are essentially constrained to climate-mediated pulses (30 years or less) of encroachment occurring at the thermal limits of mangroves. In this study, we track the continuous, lateral encroachment of mangroves into two south-eastern Australian salt marshes over a period of 70 years and quantify corresponding changes in biomass and belowground C stores. Substantial increases in biomass and belowground C stores have resulted as mangroves replaced salt marsh at both marine and estuarine sites. After 30 years, aboveground biomass was significantly higher than salt marsh, with biomass continuing to increase with mangrove age. Biomass increased at the mesohaline river site by 130 ± 18 Mg biomass km(-2) yr(-1) (mean ± SE), a 2.5 times higher rate than the marine embayment site (52 ± 10 Mg biomass km(-2) yr(-1) ), suggesting local constraints on biomass production. At both sites, and across all vegetation categories, belowground C considerably outweighed aboveground biomass stocks, with belowground C stocks increasing at up to 230 ± 62 Mg C km(-2) yr(-1) (± SE) as mangrove forests developed. Over the past 70 years, we estimate mangrove encroachment may have already enhanced intertidal biomass by up to 283 097 Mg and belowground C stocks by over 500 000 Mg in the state of New South Wales alone. Under changing climatic conditions and rising sea levels, global blue carbon storage may be enhanced as mangrove encroachment becomes more widespread, thereby countering global warming.
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Populations of more tropical origin showed the most chilling injury; those of more temperate origin showed less injury. The range of variation in dysfunction and chilling injury was widest among Avicennia populations, intermediate among Laguncularia populations, and narrowest among Rhizophora populations, but in each of the mangroves, populations from NE Mexico had greater chilling tolerance than did those from southern Mexico, Belize or Panama. Populations from Florida had greater chilling tolerance than did those of Jamaica and St. Croix. The most tolerant to chilling treatments were the Avicennia populations of Texas.-from Authors
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Outlines the climate and geomorphological origin of wetlands in the Northern Territory, and describes the distribution and vegetation of saltmarshes, mangrove swamps, intermittent swamps, seasonally-covered floodplains and lakes. The conservation status of wetlands is reviewed, considering the distribution and ownership of wetland reserves, aboriginal use of wetlands, and threats and management problems (feral animals, pollution, introduced species, mangrove degradation, saltwater intrusion, tourism and recreation, and pastoral activities). Aspects of wetland conservation and maintenance are noted. -P.J.Jarvis
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Discusses a classification of estuaries and stages in estuary evolution: both the physical changes and the accompanying biological changes. A model of evolutionary change is developed.-K.Clayton