Mangrove Colonization Research Articles

Monitoring the Mangrove Colonization on the Uplifted Reef Beds in the North Andaman Island, India

Monitoring the Mangrove Colonization on the Uplifted Reef Beds in the North Andaman Island, India

Ecological and archaeological consequences of ongoing, largely human-mediated colonisation of endemic soft, shelly habitats by mangroves in a northern New Zealand embayment

ABSTRACT Areal extent of native mangroves, Avicennia marina, has doubled in Bay of Islands since 1951. This expansion is widely attributed to deforestation and poor land-use practices beginning in the late 1800s to early 1900s, and led to greatly transformed intertidal and supratidal habitats. One example has been the ecological isolation and crowding (sometimes, colonisation) by mangroves of shelly beaches, together with their associated ‘chenier-like’ spits – themselves now among the rarest of New Zealand’s inshore marine habitats, with only six still relatively intact ecologically out of at least 44 in the Bay of Islands. These shores were also where pre Contact Māori often gathered to harvest intertidal cockles Austrovenus stutchburyi and to fish, archaeological signatures typically becoming obscured once mangroves colonised upper shores. Although recent seaward intrusion by mangroves into the cockle habitat of the Parekura Bay study site appears to have been proportionately small, there may be potential for more as elevated levels of sedimentation persist. With wholesale mangrove removals demonstrably ineffective in re-establishing original communities, the ongoing ecological and archaeological disruption of endemic soft, shelly contexts through largely human-mediated mangrove colonisation might be curbed in certain situations through strategic, proactive removal of new mangrove recruits, particularly on expanding edges.

Distribution of Avicennia spp. in the Andaman and Nicobar Islands with special reference to new distributional reports and post-tsunami colonization patterns

Abstract Post 2004 tsunami and earthquake, the landmass of Andaman and Nicobar Islands (ANI's) experienced uplift (North Andaman and Mayabunder) and subsidence (South Andaman & Nicobar Islands). The altered geomorphology modified the tidal regime, which resulted in mangrove degradation in their current locations and the formation of new intertidal zones potential for mangrove colonization. Avicennia species; a pioneer in mangrove succession was expected to colonize such new intertidal zones. Therefore, to understand the colonization pattern of Avicennia species in these new intertidal zones and their distribution in the old forests, we surveyed 79 sites across ANI's (55 Andaman and 24 Nicobar Islands). Our survey confirms the presence of three Avicennia species namely A. marina, A. officinalis, and A. alba – a new distribution record to the ANI's. Further, A. marina was found to be the most widely distributed, and abundant among three Avicennia species (Relative Abundance (RA) – 97.92%; Relative Frequency (RF) – 68.75%). In contrast, A. officinalis (RA – 1.93%; RF – 26.25%) and A. alba (RA – 0.16%; RF – 5%) were found limited in their distribution and abundance. As per the IUCN Red List, the Avicennia population is decreasing globally, whereas, its population may increase significantly across ANI's due to the availability of vast new intertidal zones. Hence, Avicennia spp. can be utilized in the plantation programs to facilitate rapid colonization in the unvegetated potential mangrove habitats across ANI's. Such an effort will improve the mangrove ecosystem services that were hampered due to mangrove degradation by the 2004 tsunami.

Optimising the wave attenuation of bamboo fences using the numerical wave model SWASH

Mangroves protect tropical coastlines from flooding and erosion, reducing flood risks along coastal communities worldwide. Nevertheless, mangrove forests have experienced considerable losses due to activities like urbanization and aquaculture, exposing coastal areas to wave attack. To restore mangroves at eroding coastlines, structures formed by bamboo poles and a brushwood filling are used to shelter the coast from waves, and create a favourable habitat for mangrove colonization. However, these structures often lose their brushwood filling during storms, resulting in high maintenance costs. This study proposes a new type of design to reduce maintenance expenses, consisting of only vertical bamboo poles without a filling of brushwood. Structure designs are evaluated for a case study in Demak, Indonesia, using the numerical wave model SWASH to predict wave attenuation through the structures. Field measurements and WaveWatch III data are analyzed to obtain the design conditions for the structures in Demak. SWASH is validated against laboratory experiments, and applied to investigate the optimum number of rows and their optimum spacing (in the direction of wave propagation). The model shows that for a structure consisting of two rows of bamboo poles, the transmission rate Et/Ei decreases from 75% to 55% when the row spacing in the wave direction is increased from sx = 0.42 m to sx = 5.8 m. Larger spacings do not result in less transmission, and at least three rows are needed to have a transmission rate lower than 50 % - a common wave reduction target used in restoration efforts with structures. This study thus identifies potential strategies to maximize wave attenuation by bamboo structures, which can be used to reduce wave attack along muddy coasts without the need of a brushwood filling. Hereby, it provides an economically and user friendly alternative with respect to the previous brushwood structure designs, as it requires less material costs and maintenance. In addition, this study presents a new method to schematize these kind of structures in SWASH in an efficient way.

Late Quaternary mangrove biogeography and paleoenvironments in the reef area of the South China Sea based on analysis of palynomorph assemblages

In this study, we analyse the development of mangroves around the South China Sea (SCS) over the last 40 kyrs BP. Findings are based on 208 palynological samples obtained from a sediment core (NS07–25) in the reef area of the SCS. These palaeo-records are interpreted in the context of 161 surface sediment palynological samples obtained from across the SCS that serve as a modern reference set. Employing the Near Analysis function in ArcGIS, mangrove pollen in modern surface sediment comprises two major assemblages: dominant Rhizophoraceae (subtropical, wind- and animal-pollinated taxa) in the northern SCS and diverse Sonneratia (tropical, animal-pollinated taxa) in the southern SCS. The distribution field of these modern mangrove pollen assemblages is bounded by the Nansha Islands and reefs, which may act to obstruct pollen transportation between the northern and southern parts of the SCS. In our late Quaternary core, concentrations of Rhizophoraceae and Sonneratia pollen assemblages gradually increase in the late stage of Marine Isotope Stage 3 and during the Last Glacial Maximum, as a result of falling sea level and mangrove colonization of the exposed marine shelf, whereas they rapidly decrease in the Deglacial period, coincident with an increase in sea level and coastal retreat. The occurrence of Rhizophoraceae pollen assemblages in the southern SCS during the Last Glacial Maximum indicates that these communities disperse southward in response to climate change. Our findings highlight the complex effects of growth conditions, varied pollinators, and transportation patterns on the preservation of Rhizophoraceae and Sonneratia pollen assemblages with implications for understanding change in climate and sea level since 40 kyr BP.

Coastal wave-energy attenuation by artificial wooden fences deployed for mangrove restoration: an experimental study

By offering natural protection along offshore wetlands, mangroves play a crucial role in providing great ecological and economic benefits to local communities. However, mangroves are currently facing an increasing threat of decline worldwide due to widespread human activities and climate change. Recently, eco-friendly wooden fences have been deployed along eroded coasts for mangrove restoration projects, and these fences have the capability to attenuate incoming waves and strengthen sediment deposition in new habitats for mangrove colonization and persistence. However, the design and performance of the fences used can differ substantially among different projects; therefore, it is necessary to study the major factors affecting the wave dissipation performance of these fences and find out a more effective structural design. Thus, we focus on two distinct types of fences with and without porous infill to study the function of infill porosity and frame density, and physical experiments of waves transmission through the fences were carried out in a wave flume, in which nine wooden fences with varied infill porosities (0.60–0.90) and frame densities (0.40 and 0.70) were predetermined to measure the wave transmission, reflection, and wave dissipation. In total, 180 experimental runs were conducted under 18 wave conditions with different wave steepness. The results showed that the fence with a lower infill porosity appears to increase wave transmission coefficient that comes at a cost of a higher reflection coefficient and less wave-energy dissipation inside the fence, and the fence with the highest porosity infills (90%) is nearly equivalent to the fence without any infills but a dense frame in terms of wave damping performance. Moreover, the wave transmission through both fences with and without infill can be remarkably affected by incoming wave steepness. The outcome of the research is not only indicating the importance of the appropriate infill porosity in attenuating incoming waves but also guiding the design of mangrove restoration project in offshore wetlands.

Eco-morphological evolution of the Bolivar Peninsula (Texas, U.S.A.) during the last 2,000 years: A multi-proxy record of coastal environmental changes

In the face of rapidly changing climate and increasing natural and anthropogenic disturbances, the long-term dynamics of coastal ecosystems in relation to hurricanes and human activities has become a focus of research for ecologists and geomorphologists across the Gulf of Mexico. This study integrates diatom, remote sensing, geochemical, and hydrodynamic analyses to document the ecological and morphological development of the barrier-island and estuary-wetland complex bordering Galveston Bay, Texas, from the late Holocene to the Anthropocene. The multi-proxy dataset revealed two major ecological shifts between 1320 and 840 cal yr BP and between 230 cal yr BP and 2008 AD in Galveston Bay. The first shift was driven by the landfall of a backdoor hurricane at ∼1000 cal yr BP in combination with the aperiodic and intense long-shore current that caused the barrier complex to retreat over fluvial deposits while the Bolivar Peninsula was growing toward the west. These morphological changes resulted in a narrower inlet, hence, restricting the marine influence from entering the bay and converting Galveston Bay to a wave-dominated estuary that was shallower and fresher. The second shift was caused by increasing modification to the waterway and infrastructures since 1800 AD. As a result, rapid sediment accretion was induced near the North Jetty, forming the current Bolivar Flats and setting the stage for mangrove colonization. The warming climate and hurricanes in recent decades acted as catalysts that accelerated mangrove expansion on the Bolivar Peninsula. Our results imply that the proposed coastal barrier system will likely increase the sediment deficit, alter the biogeochemical environment, and impact the primary productivity in the bay. However, on a regional scale, more and more mangrove stands are expected to appear around estuaries and lagoons, and along man-made jetties and barriers along the Gulf.

Mangrove colonization on tidal flats causes straightened tidal channels and consequent changes in the hydrodynamic gradient and siltation potential

A healthy mangrove ecosystem includes diverse landscape structures, such as tidal flats, tidal channels, and areas with circulating waters, in addition to mangrove stands. The complex structure of mangrove forests affects the hydrodynamics and sediment transport behaviour of tidal channels. Understanding the influence of the mangrove invasion of tidal flats on the pattern and stability of tidal channels is essential. In this study, two types of remote sensing images, Google Earth images and aerial photographs, were collected to analyze the relationship between mangrove colonization and changes in tidal channel patterns. After applying binary image processing, these two kinds of images show similar abilities to discriminate the locations, extents, and boundaries of mangroves and tidal channels. We found that the mangrove area was inversely proportional to the tidal channel sinuosity and width. The tidal channels exhibited a meandering pattern with a wider width before the mangroves invaded the tidal flats. After the expansion of the mangroves, the tidal channels gradually transformed into a straight shape with a narrower width. After the mangroves developed into forests, the tidal channels maintained a straight and stable pattern. Since mangroves promote siltation and increase the elevation of the surrounding mudflats, the habitat suitability for mangroves in the neighbouring tidal flat areas may vary. These processes may help expand mangrove habitats, thereby compressing the area of flats and changing the shape of tidal channels. Due to tidal current effects, the unit stream power of a straight tidal channel is approximately twice that of a meandering channel, indicating that straight tidal channels have a stronger anti-siltation capability. Our research also found that the tidal channels may return to a meandering pattern when mangroves are degraded or die and their area decreases. This study provides key evidence that mangroves affect tidal channel types and hydrodynamic characteristics, thus providing a useful reference for restoring and managing estuarine mangrove ecosystems.

First report on the occurrence of Avicennia marina (Forssk.) Vierh. (Acanthaceae) in the Nicobar archipelago

The 2004 Sumatra-Andaman earthquake (9.2 Mw) and tsunami that followed it resulted in uplift and subsidence across the Andaman and Nicobar archipelago. This unusual natural event severely affected the mangrove and coastal ecosystems across the Andaman and Nicobar Islands. The tsunami and land drowning of 1.1-2.85 m resulted in considerable loss of mangrove habitats in the Nicobar archipelago. Meanwhile, the land drowning also created new intertidal habitats in the earlier terrestrial zones that are now providing suitable conditions for the colonization of mangroves. During the long-term monitoring of mangrove colonization in these new inter-tidal zones, we identified the first occurrence of the Avicennia marina (Forssk.) Vierh. in the Nicobar archipelago. The distribution of A. marina and the characteristics of its colonizing sites are discussed herein.

Seasonal atmospheric and oceanographic factors influencing poleward mangrove expansion in the southeastern American coast

Mangrove ecosystems are distributed worldwide, along tropical and subtropical coastlines. For a long time, mangrove biogeographers have been challenged by the question: why is mangrove distribution restricted to its current latitudinal limits? The Araranguá estuary in Brazil is located ∼75 km beyond the eastern South America mangrove limit. Despite its geomorphology apparently being suitable for mangrove colonization, mangroves have been reported absent from this estuary. In this work, we analyze key environmental variables (such as the longest available observational in-situ records of air temperature) and provide an assessment of other environmental players (such as the adjacent ocean circulation and upwelling system) to better understand which factors could be determinant in the species range limits in eastern South America. Our results and assessment suggest that, depending on the season, multiple factors could combine to prevent a poleward dispersion of mangrove species. These are mainly the northward-directed longshore drift which dominates throughout the year and the high occurrence of chilling events during winter, although seasonal upwelling of cold waters in spring and summer could also influence the propagules’ viability.

Wave transmission and drag coefficients through dense cylinder arrays: Implications for designing structures for mangrove restoration

Mangrove vegetation constitutes a natural coastal defence against waves and erosion. Despite their protective role, mangrove ecosystems have experienced continuous degradation over the last decades due to human causes. At retreating mangrove coastlines, bamboo structures are built to create new habitat for mangrove colonization. Existing structures have experienced mixed rates of success due to the lack of a scientific basis in their design. Optimizing future structure designs requires investigating the effect of the bamboo poles on waves. We consequently conducted laboratory experiments to measure wave transformation, hydrodynamic forces, and flow velocities inside cylinder arrays, mimicking bamboo poles, with varying cylinder configurations and orientations. The experiments provided relationships for wave transmission, wave reflection, and the drag coefficients for configurations with volumetric porosities between n = 0.64 − 0.9. Configurations with a small lateral spacing (causing higher blockage) and a relatively longer streamwise spacing (causing less sheltering) exhibit larger forces and dissipation per element. Such arrangements enable optimizing wave dissipation at locations where the wave direction has low variability over the year. Placing the poles horizontally instead of vertically increases the forces and wave dissipation per element in relatively deeper water. Based on the experiments, we developed a conceptual analytical model that predicts wave reflection and dissipation through cylinder arrays, including blockage and sheltering. The model can reproduce the influence of cylinder arrangement on wave transformation, and it suggests that accurate predictions of sheltering and wave reflection are important to find optimal designs. Overall, these results provide useful insights on how to model and optimize the design of structures for mangrove restoration.

Mapping mangrove dynamics and colonization patterns at the Suriname coast using historic satellite data and the LandTrendr algorithm

Mangroves play an important role in protecting coasts against wave energy and storms. Mangrove ecosystems provide important habitats for fauna and flora and are an important carbon sink. Loss of mangroves forest may lead to enhanced coastal erosion. Mangroves are complex ecosystems and processes of settling and development are not fully understood. Characterizing the rates and patterns of mangrove gains and losses is needed to better understand the functioning of mangrove ecosystems, how mangrove dynamics are linked to coastal morphological behaviour and how human interference with the coastal system impacts mangroves. Here we present a study of the mangrove ecosystems at the Suriname coast, which are relatively pristine and characterized by strong dynamics due to migrating mudbanks along the coast. Satellite images between 2000 and 2018, available in the historic satellite image archives, were analysed using the LandTrendr (Landsat-based detection of trends in disturbance and recovery) algorithm to identify locations of mangrove erosion, mangrove colonization, surface areas of change and patterns of settlement, as indicated by (sudden) changes in NDVI. The algorithm requires careful setting of various parameters for successful detection of (abrupt) temporal changes in mangrove coverage. The algorithm was evaluated on its robustness using various parameter settings. Results show the value of the timeseries of Landsat imagery to detect locations of coastal erosion of up to 50 m/yr and accretion where loss or settlement of mangroves is prevailing between 2000 and 2018. Locally differences are very large. An overall westward mangrove progression along the coast is apparent from the images and probably linked to mud bank migration. Various patterns of mangrove colonization and development such as arc-, zonal- and patch- arrangements were identified, although at some locations the Landsat resolution of 30 m is somewhat coarse to allow detailed analysis. The success and robustness of the LandTrendr algorithm are controlled by NDVI threshold values, number of allowed breakpoints in the timeseries and fitting parameters. The presented method requires further testing and evaluation but is a promising tool for semi-automatic detection of coastal mangrove erosion and colonization that can be applied to other mangrove ecosystems in the world. The satellite timeseries analyses generate valuable information on coastal dynamics, which is helpful to identify coastal areas prone to erosion and mangrove retreat and provide as such a valuable tool for coastal management and protection.

Mangrove and sand cay dynamics on Australian and Indonesian low wooded islands: A 45 year comparison of changes from remote sensing

Changes to coral reef landscapes are driven by regional processes that are unique to particular localities, yet much of our global knowledge about how landscape changes manifest in coral reef environments is generalised from work undertaken on the Great Barrier Reef. We compare observations of 45 years of change on sand cays and mangroves associated with low wooded islands in Australia and Indonesia. We draw on field observations from ground referencing campaigns, alongside remote sensing technology, including satellite images and unmanned aerial vehicle campaigns. Four low wooded island sites are compared: two in the Great Barrier Reef (GBR), Australia (Nymph Island and Two Isles) and two in the Spermonde Archipelago, Indonesia (Sabangko and Tanakeke Island). The Spermonde and GBR sites can be distinguished in relation to the process regimes that entrain, distribute and deposit sediments on the reef surface thereby providing a substrate for further mangrove colonisation, particularly the presence or absence of cyclones as a key determinant of sediment transport. The influence of human populations inhabiting these sites is also an important control on their geomorphology. In the Spermonde Archipelago, local communities have altered sand cays through the development of infrastructure and converted mangroves to shrimp farms, while sand cays and mangroves have remained largely unaltered by humans on the GBR. This comparative evaluation of changes to sand cays and mangrove forest across low wooded islands emphasises the importance of considering changes within the context of their local geographic setting, inclusive of natural environmental and anthropogenic drivers of change.

Mangrove colonisation of the Mlalazi Estuary, South Africa: a response to artificial breaching

The Mlalazi Estuary has 40 ha of mangroves, but before the 1930s there were none. The purpose of this study is to understand why this change occurred, and how the understanding thus gained can provide ecological information that will help understand the dynamics of similar estuaries in the region. The approach was to accumulate evidence gathered from historical records, published literature and scientific investigations. In the past 6 000 years, the Mlalazi Estuary evolved from an open bay to a permanently open estuary and then to its current state of a temporarily closed estuary. There is evidence that the mangroves that occurred in the system were lost when the estuary mouth started closing for extended periods. Since 1896, Mlalazi has been artificially breached whenever closure resulted in the backing up of water. This mouth-breaching regime resulted in physical conditions suitable for mangrove growth. Mangrove propagules would have dispersed from adjacent estuaries; taking more than 30 years for Avicennia marina and Bruguiera gymnorrhiza to colonise, and approximately 100 years for Rhizophora mucronata. Once established the mangrove population expanded and there is evidence that available habitat is becoming limiting. Any alteration to the current mouth management strategy is likely to affect the mangroves and must not be implemented without a full understanding of its consequences. The Mlalazi Estuary is a unique estuary in South Africa in that mouth manipulation has led to the colonization of mangroves not previously current in the system.

Multi-decadal Changes on the Mud-dominated Coast of Western French Guiana: Implications for Mesoscale Shoreline Mobility, River-mouth Deflection, and Sediment Sorting

Jolivet, M.; Gardel, A., and Anthony, E.J., 2019. Multi-decadal changes on the mud-dominated coast of western French Guiana: Implications for mesoscale shoreline mobility, river-mouth deflection, and sediment sorting. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 185–194. Coconut Creek (Florida), ISSN 0749-0208.Mud supplied by the Amazon River forms large banks that migrate along the Amazon-Orinoco Guianas coast under the influence of waves and currents, separated by ‘inter-bank’ zones (phases). Bank-welding onto the coast creates new land, followed by rapid mangrove colonization, and westward deflection of the mouths of the smaller rivers. A fine example is Pointe Isère, a mud cape that deflected the mouth of the Mana River in French Guiana. During inter-bank phases, higher wave energy can result in rapid and massive shoreline erosion, except where sandy-shelly cheniers develop from winnowing of coarse-grained bedload from the surrounding muddy matrix. In order to contribute to a better understanding of the rates and scales of shoreline change on the French Guiana coast, we conducted a GIS analysis on Pointe Isère using a >60-year record of aerial photographs and satellite images. The results show significant and quasi-continuous erosion of this mud cape at the multi-decadal timescale. A phase of massive retreat of the shoreline (>1 km between 1955 and 1972), was followed by much slower erosion up to 2015, probably due to: (1) the alongshore passage of several mud banks, and (2) the formation of a sandy chenier through concentration, by waves, of contemporary sand of fluvial origin stored in the inner mud-dominated shoreface, and of sand derived from erosion of an old inland chenier exposed as a result of shoreline retreat. The new chenier has also been characterized by a westward-elongating spit. The progressive demise of Pointe Isère finally resulted, in 2001, in a breach that created a new direct outlet for the Mana River, and sealing of the former outlet through downdrift distal spit welding onshore. Over the study period (1955–2017), the demise of Pointe Isère has involved a loss of 41.8 km2, much of it mangrove wetlands. This corresponds to about 8% of the total area of mangroves in French Guiana in 2015. Since 2015, a new phase of large-scale muddy accretion has been associated with the on-going isolation of the active sandy chenier. This probably heralds a new cycle of mud-cape formation in the vicinity of the mouth of the Mana that will lead to a new westward diversion of this river. During the continuous erosion of Pointe Isère, the longshore passage of several mud banks did not mitigate or counter erosion. This has important implications regarding the unpredictability of shoreline change on the Amazon-Orinoco coast, and the need to go beyond the common vision of such change in terms of mudbank (shoreline accretion) and inter-bank (shoreline erosion) phases that cover timescales of the order of years to a decade.

2004 Post Tsunami Resilience and Recolonization of Mangroves in South Andaman, India

Mangrove resilience and the scope for new venues of mangrove colonization in the Southern part of south Andaman were studied using satellite interpretation and ground truthing due to after the devastating 26th December 2004 tsunami. 1069.82 ha of Tsunami created wetlands were found in thirty five different villages. Fimbrisstylis littoralis, Acanthus ilicifolius, Acrostichum aureum, Rhizophora spp. and Avicennia spp. were observed in abundance in all the villages. However, Nypa fruticans and Pandanus tectorius were found in four villages alone. Eight venues were found as newly colonized mangrove. Rhizophora mucronata was existing in all the eight new venues whereas Rhizophora apiculata and Avicennia spp. was absent in 3 venues. The present study will act as reference point information for future investigation. The results of the present study provides a huge insight and hope in restorative models for degraded mangrove systems.

Land‐based and climatic stressors of mangrove cover change in the Auckland Region, New Zealand

Abstract Changes in mangrove forest cover (increases and decreases) have been observed globally as a result of the influence of environmental changes and human impacts. Although mangrove forest increases are globally uncommon, in New Zealand mangroves are increasing in distribution at the expense of other intertidal estuarine habitats. This study quantified the change in mangrove area over a period of 74 years (1940–2014), and investigated the influence of land‐based (e.g. land use conversion in the catchments), climatic, and oceanic factors on the change in temperate mangrove forest cover in the Auckland Region of New Zealand over that period. An improved mapping approach using aerial photographs and Landsat images was applied to quantify changes in tall and dwarf mangrove cover, and to identify the mechanism of change (expansion, gap filling, and loss), across 38 estuaries in the Auckland Region, New Zealand. Mangrove area increased from 2313 ha in 1940 to 10 483 ha in 2014 (on average 3.2% yr−1), with higher rates in small estuaries (<1000 ha) than in large estuaries (>1000 ha). The area of the intertidal mudflat habitat, i.e. potential area for mangrove colonization, has doubled from 14 193 ha in 1974 to 28 764 ha in 2014. The increase in total mangrove area was explained primarily by gap filling followed by expansion. The proportion of expansion‐related increase was predominantly seaward (82%) but mangroves also expanded upstream (16%). Boosted regression tree analysis revealed that catchment forest cover, sea level, and mean annual air temperature explained around 85% of the variation in mangrove area. Periods of catchment forest clearing coincided with gains in mangrove forest area. Dwarf mangroves were more sensitive to changes in sea level, in particular over the past 25 years, when the annual sea‐level rise exceeded the sediment accumulation rates. Catchment land use, especially the clearing of forests, associated with increased sediment supply into estuaries is the strongest driver of mangrove forest gain. Thus, land‐use management is a necessary aspect of the management of New Zealand estuaries to conserve the diversity of estuarine habitats (including mangroves) and the ecosystem services that each habitat provides.

Mangrove dynamics and blue carbon sequestration.

We monitored coastal wetland vertical accretion, elevation gain and surface carbon (C) at Homebush Bay, Australia over 18 years (2000-2017) in three settings initially characterized by saltmarsh, mixed saltmarsh-mangrove ecotone and mangrove-dominated zones. During this time, the saltmarsh transitioned to mixed saltmarsh-mangrove ecotone, and the mixed saltmarsh-mangrove ecotone transitioned to mangrove, consistent with vegetation transitions observed across the east Australian continent in recent decades. In spite of mangrove recruitment and thickening in the former saltmarsh zone, and the dominance of mangrove root material as a contributing C source, the rate of C accumulation in the former saltmarsh zone did not change over the study period, and there was no significant increase in surface elevation. This contrasted with the response of sites with a longer history of mangrove colonization, which showed strong accretion and C accumulation over the period. The result suggests that the C accumulation and surface elevation gains made as a result of mangrove colonization may not be observable over initial decades, but will be significant in the longer term as forests reach maturity.

Interactive Effects of Intertidal Elevation and Light Level on Early Growth of Five Mangrove Species under Sonneratia apetala Buch. Hamplantation Canopy: Turning Monocultures to Mixed Forests

The introduced Sonneratia apetala Buch. Hamplantation plantations have occupied more than 3800 ha in China. The prevalence, fast growth rate, and high seed production of S. apetala have raised concerns about the risks to native mangrove habitats. Efforts are required to convert these introduced monocultures to mixed or native forests. In this study, we examined native mangrove colonization in the introduced S. apetala plantations at the Qi’ao Island, Zhuhai, China. A 12-month field study was conducted to evaluate the effects of intertidal elevation and light level on the survival and early growth of five native mangrove species, viz., Bruguiera gymnorrhiza (L.) Savigny, Kandelia obovata Sheue, Liu & Yong, Aegiceras corniculatum (L.) Blanco, Avicennia marina (Forssk.) Vierh., and Rhizophora stylosa Griff. Across intertidal elevations and light levels, the survival of B. gymnorrhiza was the highest. All the species had relatively higher survival rates under 30% canopy closure. Although the seedlings survived best at high intertidal elevation, the relative growth rate (RGR) was the highest at low intertidal elevation, and it was promoted by high light level. The stem height at low intertidal elevation was higher than that at high intertidal elevation, and it was the highest under 30% canopy closure. B. gymnorrhiza and R. stylosa at high intertidal elevation had relatively high leaf numbers, whereas K. obovata and A. marina showed a reverse tendency. The growth of stem diameter showed a decreasing trend initially and then increased with better performance at low intertidal elevations, and B. gymnorrhiza presented the best value under 30% canopy closure. Bruguiera gymnorrhiza showed the highest growth rate under similar conditions. Overall, intensive canopy thinning is an effective measure to promote native mangrove growth in S. apetala plantations. Additionally, increasing planting density especially at low intertidal elevations may improve native mangrove establishment and growth. Furthermore, Bruguiera gymnorrhiza is the best choice in the effort to plant native species in S. apetala plantations in the study area.

Tropicalization of the barrier islands of the northern Gulf of Mexico: A comparison of herbivory and decomposition rates between smooth cordgrass (Spartina alterniflora) and black mangrove (Avicennia germinans).

The expansion of black mangrove Avicennia germinans into historically smooth cordgrass Spartina alterniflora-dominated marshes with warming temperatures heralds the migration of the marsh-mangrove ecotone northward in the northern Gulf of Mexico. With this shift, A. germinans is expected to outcompete S. alterniflora where it is able to establish, offering another prevalent food source to first order consumers. In this study, we find A. germinans leaves to be preferable to chewing herbivores, but simultaneously, chewing herbivores cause more damage to S. alterniflora leaves. Despite higher nitrogen content, A. germinans leaves decomposed slower than S. alterniflora leaves, perhaps due to other leaf constituents or a different microbial community. Other studies have found the opposite in decomposition rates of the two species’ leaf tissue. This study provides insights into basic trophic process, herbivory and decomposition, at the initial stages of black mangrove colonization into S. alterniflora salt marsh.