Saltmarsh-mangrove Ecotone Research Articles

Contrasting stocks and origins of particulate and mineral-associated soil organic carbon in a mangrove-salt marsh ecotone

The global warming-driven poleward expansion of mangrove habitats (e.g., Avicennia germinans and Rhizophora mangle) into temperate salt marshes (e.g., Spartina alterniflora and Juncus roemerianus) has been shown to alter coastal soil organic carbon (SOC) storage. However, the taxa-specific consequences of this vegetation shift on the origin and size of SOC sub-fractions (particulate OC (POC); mineral-associated OC (MAOC); and reactive iron-associated OC (FeR-MAOC)) remain largely unexplored. In this study, we used a particle size-based SOC fractionation method to compare quantity and δ13C composition of bulk and each SOC sub-fractions in soil cores collected from Apalachicola Bay barrier islands in Florida, USA, the highest latitude where monospecific communities of all four aforementioned plants co-occur. Depth-dependent variation of bulk soil δ13C clearly showed the global warming-driven replacement of S. alterniflora by mangroves, as well as reciprocal substitutions of S. alterniflora and J. roemerianus, probably driven by changes in wetland elevation. Higher OC burial rates in mangrove habitats suggested that mangrove soils were principally developed by particle deposition. In contrast, comparatively lower OC burial rates but higher OC stocks in salt marsh habitats illustrated subsurface OC input from salt marsh roots. POC was primarily derived from contemporary plant detritus; its concentration was higher in salt marsh habitats (58.8 ± 9.0 % of SOC) relative to mangroves (38.4 ± 6.0 % of SOC). In contrast, MAOC content did not vary across plant habitats (53.5 ± 10.9 % of SOC), and principally originated from microbially-transformed OC and pre-existing plants. FeR-MAOC was essentially absent in R. mangle soils (2.9 ± 3.6 % of SOC) while representing a minor fraction of MAOC in three other plant habitats (7.8 ± 7.0 % of SOC). The δ13C of FeR-MAOC was more like the present-day surface plants, highlighting the in situ FeR-MAOC formation in their active oxidizing rhizospheres.

Carbon sequestration potential of transplanted mangroves and exotic saltmarsh plants in the sediments of subtropical wetlands

Coastal blue carbon ecosystems offer promising benefits for both climate change mitigation and adaptation. While there have been widespread efforts to transplant mangroves from the tropics to the subtropics and to introduce exotic saltmarsh plants like Spartina alterniflora in China, few studies have thoroughly quantified the chronological records of carbon sequestration with different organic carbon (OC) sources. To understand how variations in OC sources can affect the carbon sequestration potential of coastal wetland environment over time, we conducted a study on typical islands with two scenarios: S. alterniflora invasion and mangrove transplantation. Our study determined chronological records of carbon sequestration and storage from five sediment profiles and traced changes in the OC sources using carbon stable isotope (δ13C) and C:N ratios in response to these scenarios. The S. alterniflora invasion resulted in an 84 ± 19 % increase in the OC burial rate compared to unvegetated mudflats, while mangrove transplantation resulted in a 167 ± 74 % increase in the OC burial rate compared to unvegetated mudflats. S. alterniflora and mangroves showed greater carbon sequestration potential in areas with high supplies of suspended particulate matter, while mangroves needed to grow to a certain scale to display obvious carbon sequestration benefits. In the mangrove saltmarsh ecotone, mature mangrove habitats exhibited resistance to the S. alterniflora invasion, while mangrove transplantation in the environment invaded by S. alterniflora had a significant effect on OC contribution. Besides, plant-derived OC can be exported to the surrounding environment due to the rapid turnover of sediments. The blue carbon chronosequence-based estimation of OC sources and burial rates provides a useful reference for establishing carbon accounting policies.

Tidal exchange of dissolved metal(loid)s and organic matters across the sediment–water interface in a salt marsh-mangrove ecotone

Coastal wetlands are considered as efficient barriers for pollutants such as dissolved metal(loid)s between land and sea. However, there remains a challenge in understanding how tidal hydrology regulates the source-sink patterns of metal(loid)s across the sediment–water interface, especially in the salt marsh-mangrove ecotone that commonly exists in subtropical coastlines. In this study, we conducted detailed time-series observations of dissolved metal(loid)s (Cr, Cu, Zn, Ba, Pb, Hg, and As) and organic matter (DOM) characteristics (bulk concentrations, aromaticity, and molecular size) in southeast China over a spring-neap tidal cycle. The results demonstrated that both DOM and metal(loid)s fluctuated with tide levels, and higher concentrations exited in groundwater than that in the surface water. The concentrations of bulk DOM and chromophoric DOM both in surface water varied oppositely with tide levels, suggesting the potential role of groundwater discharge at low tides. DOM aromaticity also had tidal time scale fluctuations, but the molecular weight of DOM was insensitive to tide changes. The dispersion fluxes were one to four orders of magnitude lower than advective fluxes with the ranges of 0.14–24.8 μg m−2 d-1 for Cr, 6.2–22.4 μg m−2 d-1 for Cu, 14.7–387.4 μg m−2 d-1 for Zn, 0.7–25.4 μg m−2 d-1 for As, 259.2–2308 μg m−2 d-1 for Ba, 0.1–6.0 μg m−2 d-1 for Pb, 0–1.7 μg m−2 d-1 for Hg and 3.1–116 mg m−2 d-1 for DOM, respectively. Mangrove sediments tended to be the sources of DOM and all dissolved metal(loid)s, but the salt marsh was only the source of Cu, As, and DOM. These results have implications on the geochemical cycling of DOM and dissolved metal(loid)s in the salt marsh-mangrove ecotone which has contrasting pattern of surface water-groundwater exchange.

Influence of climate-mediated shifts in wetland vegetation on prey fish habitat use and schooling dynamics

Intertidal wetlands provide vital habitat for numerous species by serving as a basal resource and refuge for shelter-seeking fauna. These environments are particularly important for transient nekton that trophically link marine and terrestrial food webs via migration into the intertidal during the flooding tide. Changes in coastal vegetation due to a warming climate, such as the replacement of temperate saltmarshes with tropical mangroves, may alter habitat use by transient species due to differences in food provisioning and the structural attributes of mangrove and saltmarsh vegetation. In northeast Florida (USA), Fundulus heteroclitus (mummichog) and F. majalis (striped killifish) are abundant, trophically important, transient fishes that may exhibit behavioral changes with coastal habitat shifts. We experimentally manipulated predator presence in a large-scale mesocosm containing an array of 4 habitats found at the mangrove-saltmarsh ecotone: marsh vegetation (smooth cordgrass Spartina alterniflora), 2 species of mangroves (red mangrove Rhizophora mangle and black mangrove Avicennia germinans), and sand, and measured prey fish habitat use and schooling dynamics. Mummichogs utilized black mangrove habitat to the same degree as cordgrass regardless of the presence of a predator, indicating that mummichogs perceive both habitats as providing equivalent protection. Killifish exhibited little affinity for any particular habitat and exhibited schooling behavior over seeking refugia for predator avoidance, suggesting that vegetation changes may have little effect on killifish behavior. Further research on how such patterns of habitat use combined with habitat-specific dietary constituents of fundulids will help to predict how saltmarsh loss at the ecotone will influence nutrient and energy flows in coastal food webs.

Porewater exchange drives nutrient cycling and export in a mangrove-salt marsh ecotone

Coastal wetlands regulate nutrient fluxes from the continents to the oceans. Salt marshes are rapidly encroaching into mudflat area in mangrove wetlands, shaping a mangrove-salt marsh ecotone, with unknown implications to coastal biogeochemical cycles. Here, we hypothesized that nitrogen and phosphorus cycling varied in mangrove and salt marsh, having significant implication on coastal waters. We investigated a tidal creek with a marked mangrove-salt marsh gradient in China using high-frequency time-series sampling of dissolved nutrients and observations of porewater exchange rate across the sediment–water interface over a spring-neap tidal cycle. The nitrogen transformation rates and microbiological activities were also investigated to explain the variability in nitrogen concentrations. The mangrove had net groundwater outflow rates of 3.6–4.3 mm d-1 while the salt marsh had net infiltration of surface water with rates of 0.5–2.9 mm d-1. Salt marsh had less capacity for ammonium (NH4-N) production (mineralization and dissimilatory nitrate reduction to ammonium DNRA) than mangrove. Denitrification dominated nitrogen removal reaching 97% and 83% in mangrove and salt marsh, respectively. Microbe distributions were consistent with nitrogen transformations with larger nirS and nrfA abundances for denitrification and DNRA in the mangrove than salt marsh. The mangrove had a net export of NH4-N but a net import of NOx-N (sum of nitrate and nitrite) and dissolved inorganic phosphorus (DIP) during the monitoring period. In contrast, the salt marsh had lower efflux of nutrient than influx leading to a net nutrient import during the monitoring period. Porewater released from the mangrove had a large DIN:DIP mole ratio (706 ± 236) due to high NH4-N concentrations, while NH4-N in the salt marsh were lower than in the mangrove. Overall, this study revealed that mangrove-salt marsh ecotone will push the native mangrove wetlands from being a source towards a sink of NH4-N to coastal waters by decreasing porewater exchange, modifying the nutrients stoichiometry, and ultimately alleviating the potential of N-associated eutrophication in nearby coastal waters.

Carbon and nitrogen pools and mobile fractions in surface soils across a mangrove saltmarsh ecotone

In the subtropics, climate change is pushing woody mangrove forests into herbaceous saltmarshes, altering soil carbon (C) and nitrogen (N) pools, with implications for coastal wetland productivity and C and N exports. We quantified total C and N pools, and mobile fractions including extractable mineral N, extractable organic C and N, and active (aerobically mineralizable) C and N, in surface soils (top 7.6 cm) of adjacent mangrove (primarily Avicennia germinans) and saltmarsh (Juncus roemerianus) vegetation zones in tidal wetlands of west-central Florida (USA). We tested whether surface-soil accumulations of C, N, and their potentially mobile fractions are greater in mangrove than in saltmarsh owing to greater accumulations in the mangrove zone of soil organic matter (SOM) and fine mineral particles (C- and N-retaining soil constituents). Extractable organic fractions were 39–45% more concentrated in mangrove than in saltmarsh surface soil, and they scaled steeply and positively with SOM and fine mineral particle (silt + clay) concentrations, which themselves were likewise greater in mangrove soil. Elevation may drive this linkage. Mangrove locations were generally at lower elevations, which tended to have greater fine particle content in the surface soil. Active C and extractable mineral N were marginally (p < 0.1) greater in mangrove soil, while active N, total N, and total C showed no statistical differences between zones. Extractable organic C and N fractions composed greater shares of total C and N pools in mangrove than in saltmarsh surface soils, which is meaningful for ecosystem function, as persistent leaching of this fraction can perpetuate nutrient limitation. The active (mineralizable) C and N fractions we observed constituted a relatively small component of total C and N pools, suggesting that mangrove surface soils may export less C and N than would be expected from their large total C and N pools.

Stable Isotopes Suggest Limited Role of Wetland Macrophyte Production Supporting Aquatic Food Webs Across a Mangrove-Salt Marsh Ecotone

Mangroves and salt marshes are considered important habitats for a diversity of fishery and other species. One of the key presumed values of these systems is their role in fuelling aquatic food webs that support many fisheries species. The mangrove-salt marsh ecotone along the NE Atlantic coast of Florida provides a natural experiment to examine the relative importance of mangrove and marsh production in supporting aquatic food webs. Representative food web components comprising key functional groups were sampled across the ecotone to test the prediction that species or functional groups that rely on the dominant wetland producers should show large differences in carbon stable isotope values across sites that span C4-dominated Spartina marshes in the north to C3-dominated mangrove wetlands in the south. Source contributions were estimated for fishery species (blue crabs, penaeid shrimp), wetland specialists (cyprinodontiformes fishes, grass shrimp, fiddler crabs), zoobenthivores (pinfish, spot), phytodetritivores (mullet) and planktivores (menhaden, anchovy, silversides). The range of wetland macrophyte source contributions had minima < 5% for most groups at most sites, despite the shift from C3- to C4-dominated wetlands, suggesting that Spartina and mangroves contributed minimally to the trophic support of these groups. Spatial correlations between sources and individual consumer species that spanned the ecotone showed the strongest correlations with micro-algal sources (microphytobenthos/phytoplankton), weak correlations with a ‘wetland’ source reflecting the transition from Spartina to mangroves across the ecotone, and no correlation with either mangrove or Spartina sources. These findings suggest a limited role of mangrove or Spartina production in supporting the aquatic consumers examined.

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

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

Limited Mangrove Propagule Retention at a Latitudinal Range Limit: Spatiotemporal Patterns at the Patch Scale

Dispersal and establishment dynamics are critical in understanding shifts in species’ ranges. We seek to illuminate patch-level dispersal dynamics by examining the shifting salt marsh-mangrove ecotone. Specifically, we ask the following: (1) How are mangrove propagules dispersed, retained, and exported within a discrete patch? (2) How do differences across a flooding gradient influence propagule dispersal dynamics? (3) How does the distribution of established seedlings compare to propagule movements? Avicennia germinans is the most temperate mangrove species in the northern Gulf of Mexico forming an ecotone with Spartina alterniflora marshes in coastal Louisiana. Sets of 500 distinctively marked mangrove propagules were placed at five different elevations. After their release, we observed dispersal dynamics for 1 month. Retention was limited in the study area (< 10%) with ~ 70–80% of propagules exporting out of the system and ~ 20% propagule predation. Retained propagules largely remained at their original elevations and were generally found at the highest elevation. Seedling establishment was also studied and unlike propagule dispersal distributions, peak seedling density occurred at elevations flooded 20–40% of the time. Our study highlights the mass export of mangrove propagules, the disparity between dispersal and establishment dynamics, and the need to explore dispersal at biologically relevant temporal and patch-level spatial scales. By understanding dispersal and establishment dynamics within the ecotonal boundary, we provide one of the first studies on dispersal at a temperature-controlled latitudinal limit for mangroves and highlight some of the drivers needed to better connect plot-, patch-, and landscape-level dynamics at this and other range limits.

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

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

Short-lived effects of nutrient enrichment on Avicennia germinans decomposition in a saltmarsh-mangrove ecotone

Rates of litter decomposition, an important component of the global carbon (C) cycle, often depend on litter quality and decomposer activity, which can then drive differences in C sequestration and storage in ecosystems. Nutrient enrichment may alter C storage in wetlands through its effects on litter quality. We investigated the effects of long-term nitrogen (N) and phosphorus (P) enrichment on Avicennia germinans litter quality and above- and belowground decomposition rates in a saltmarsh-mangrove ecotone in St. Augustine, Florida, U.S.A. Senescent leaf litter collected from Control, +N-, and +P-fertilized shrubs was placed in litterbags for use in a reciprocal transplant experiment, with each litter treatment placed on the soil adjacent to each fertilized shrub (Control, +N, and +P). Leaf litterbags were collected at 0, 14, 30, 60 and 180 days, while roots collected from a common, unfertilized location were incubated belowground at each shrub for 180 and 365 days to determine mass loss and decay rates. Nutrient enrichment did not alter %N, C:N, or %lignin in treatment leaves, and there were subsequently no significant differences in decay rates of treatment leaves, likely due to the homogeneity of litter quality. Initial decay rates (30 and 60 days) were significantly higher in +N and +P fertilized soils compared to controls, but by 180 days decay rates had stabilized to 0.009 ± 0.0003 k day−1. Labile fractions were rapidly decomposed and net immobilization of litter was favored after 60 days of incubation irrespective of nutrient treatments. Additionally, belowground root decomposition with roots from unfertilized plants did not vary across nutrient treatments. Thus, nutrient enrichment of A. germinans did not alter litter quality considerably or increase decomposition in the longer term, and therefore, we do not expect a substantial decline in sediment C sequestration as a result of eutrophication in this mangrove system.

A comparison of soil carbon pools across a mangrove-salt marsh ecotone at the southern African warm-temperate range limit

Mangroves and salt marshes are globally recognized for their ability to efficiently sequester and store carbon in biomass and soil carbon pools. An increase in soil carbon has been found at range limits where mangroves are expanding along ecotones into salt marsh, as mangroves store more carbon per unit area. However, the magnitude of this trend is variable at different mangrove range limits depending on the rates of expansion and the local climatic and geomorphological factors that influence current mangrove distribution. This study presents the first account of soil carbon storage across a mangrove-salt marsh ecotone at the Nahoon Estuary, a warm-temperate range limit for the east coast of the African continent. Soil carbon (to 50 cm depth) was compared between mangrove, salt marsh, and a transitional ecotone habitat. We found that soil carbon was highest at 30–50 cm below the surface across all habitats. The average soil carbon in the upper 50 cm was similar in mangrove (110.14 MgC.ha−1), ecotone (114.50 MgC.ha−1), and salt marsh habitats (109.62 MgC.ha−1). These results show that although mangroves have expanded at the Nahoon Estuary over the past 50 years, the soil carbon content is still similar across the mangrove-salt marsh ecotone. This is in contrast to other studies from southern hemisphere mangrove range limits in Australia. However, these trends have been reported at northern hemisphere range limits in the USA. Further studies in this region are needed to identify environmental factors influencing soil carbon variability.

Effects of Changing Vegetation Composition on Community Structure, Ecosystem Functioning, and Predator–Prey Interactions at the Saltmarsh-Mangrove Ecotone

Decreasing frequency of freeze events due to climate change is enabling the poleward range expansion of mangroves. As these tropical trees expand poleward, they are replacing herbaceous saltmarsh vegetation. Mangroves and saltmarsh vegetation are ecosystem engineers that are typically viewed as having similar ecosystem functions. However, few studies have investigated whether predation regimes, community structure, and ecosystem functions are shifting at the saltmarsh-mangrove ecotone. In this study, we manipulated predator access to marsh and mangrove creekside habitats to test their role in mediating vegetation and invertebrate structure and stability in a two-year experiment. We also conducted a survey to evaluate how shifting vegetation is modifying structural complexity, invertebrate communities, and ecosystem functioning at the ecotone. Excluding larger (&gt; 2 cm diameter) predators did not affect vegetation or invertebrate structure or stability in either saltmarsh or mangrove habitats. The survey revealed that the two habitat types consistently differ in structural metrics, including vegetation height, inter-stem distance, and density, yet they support similar invertebrate and algal communities, soil properties, and predation rates. We conclude that although mangrove range expansion immediately modifies habitat structural properties, it is not altering larger predator consumptive effects, community stability, community composition, or some other ecosystem functions and properties at the ecotone.

Climate-driven regime shifts in a mangrove–salt marsh ecotone over the past 250 years

Climate change is driving the tropicalization of temperate ecosystems by shifting the range edges of numerous species poleward. Over the past few decades, mangroves have rapidly displaced salt marshes near multiple poleward mangrove range limits, including in northeast Florida. It is uncertain whether such mangrove expansions are due to anthropogenic climate change or natural climate variability. We combined historical accounts from books, personal journals, scientific articles, logbooks, photographs, and maps with climate data to show that the current ecotone between mangroves and salt marshes in northeast Florida has shifted between mangrove and salt marsh dominance at least 6 times between the late 1700s and 2017 due to decadal-scale fluctuations in the frequency and intensity of extreme cold events. Model projections of daily minimum temperature from 2000 through 2100 indicate an increase in annual minimum temperature by 0.5 °C/decade. Thus, although recent mangrove range expansion should indeed be placed into a broader historical context of an oscillating system, climate projections suggest that the recent trend may represent a more permanent regime shift due to the effects of climate change.

Nitrogen Enrichment Accelerates Mangrove Range Expansion in the Temperate–Tropical Ecotone

Climate change and nutrient enrichment are two phenomena impacting coastal ecosystems. In coastal wetlands, mangroves in temperate–tropical ecotones are encroaching on adjacent saltmarshes, a pattern that is primarily attributed to warmer winter temperatures. Climate change is also expected to increase the vulnerability of coastal wetlands to eutrophication, and increases in nutrient availability may further mediate the rate of mangrove expansion. We investigated the consequences of nutrient enrichment on coastal wetlands in the mangrove–saltmarsh ecotone near the temperate edge of mangrove distribution along the northeast coast of Florida. We tested the hypothesis that nutrient enrichment enhances the ongoing, climate-driven expansion of mangroves into areas historically dominated by saltmarshes by increasing mangrove growth and cover, allowing them to outcompete and overgrow adjacent saltmarsh plants. We manipulated nitrogen (N) and phosphorus (P) availability and measured the effects on growth, cover, diversity, leaf traits, and nutrient dynamics of Avicennia germinans. We found that A. germinans shrubs growing in the saltmarsh–mangrove ecotone in northern Florida grew taller, increased their canopies, and had higher reproductive output when enriched with N compared to control plants and P-enriched plants. Nutrient enrichment did not alter Sarcocornia perennis growth and increased Batis maritima height but did not alter density or biomass. Nitrogen addition caused an increase in A. germinans cover and decrease in B. maritima cover and Simpson’s index of diversity, suggesting that N enrichment, an ongoing phenomenon, can hasten the invasion of mangroves into saltmarshes by favoring mangrove growth and reproduction without significantly enhancing saltmarsh plant growth.

Tipping Points in the Mangrove March: Characterization of Biogeochemical Cycling Along the Mangrove–Salt Marsh Ecotone

Coastal wetland vegetation communities can respond to sea level rise via the encroachment of more salt- and inundation-tolerant species into existing vegetation communities. Black mangroves (Avicennia germinans L.) are encroaching on saltgrass (Distichlis spicata L.) within the Merritt Island National Wildlife Refuge in east central Florida (USA). Nine soil cores collected along three transects captured the transitions of both perceived abiotic drivers (salinity and inundation) and vegetation communities during both high- and low-water seasons to investigate patterns in soil biogeochemical cycling of carbon (C), nitrogen (N), and phosphorus (P). Results showed no change in soil carbon dioxide production along the ecotone during either season, though changes in enzyme activity and mineralization rates of N and P could indicate changes in C quality and nutrient availability affecting C degradation along the ecotone. All parameters, excluding microbial biomass carbon, showed higher rates of activity or availability during the low-water season. Long-term soil nutrient stores (total C, N, P) were greatest in the saltgrass soils and similar between the mangrove and transition zone soils, indicating a ‘tipping point’ in biogeochemical function where the transition zone is functionally equivalent to the encroaching mangrove zone. Indicators of current biogeochemical cycling (that is, enzyme activity, potentially mineralizable N rates, and extractable ammonium concentrations) showed alterations in activity across the ecotone, with the transition zone often functioning with lower activity than within end members. These indicators of current biogeochemical cycling change in advance of full vegetation shifts. Increases in salinity and inundation were linked to mangrove encroachment.

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.

Microspatial ecotone dynamics at a shifting range limit: plant–soil variation across salt marsh–mangrove interfaces

Ecotone dynamics and shifting range limits can be used to advance our understanding of the ecological implications of future range expansions in response to climate change. In the northern Gulf of Mexico, the salt marsh-mangrove ecotone is an area where range limits and ecotone dynamics can be studied in tandem as recent decreases in winter temperature extremes have allowed for mangrove expansion at the expense of salt marsh. In this study, we assessed aboveground and belowground plant-soil dynamics across the salt marsh-mangrove ecotone quantifying micro-spatial patterns in horizontal extent. Specifically, we studied vegetation and rooting dynamics of large and small trees, the impact of salt marshes (e.g. species and structure) on mangroves, and the influence of vegetation on soil properties along transects from underneath the mangrove canopy into the surrounding salt marsh. Vegetation and rooting dynamics differed in horizontal reach, and there was a positive relationship between mangrove tree height and rooting extent. We found that the horizontal expansion of mangrove roots into salt marsh extended up to eight meters beyond the aboveground boundary. Variation in vegetation structure and local hydrology appear to control mangrove seedling dynamics. Finally, soil carbon density and organic matter did not differ within locations across the salt marsh-mangrove interface. By studying aboveground and belowground variation across the ecotone, we can better predict the ecological effects of continued range expansion in response to climate change.

Mangrove microclimates alter seedling dynamics at the range edge.

Recent climate warming has led to asynchronous species migrations, with major consequences for ecosystems worldwide. In woody communities, localized microclimates have the potential to create feedback mechanisms that can alter the rate of species range shifts attributed to macroclimate drivers alone. Mangrove encroachment into saltmarsh in many areas is driven by a reduction in freeze events, and this encroachment can further modify local climate, but the subsequent impacts on mangrove seedling dynamics are unknown. We monitored microclimate conditions beneath mangrove canopies and adjacent open saltmarsh at a freeze-sensitive mangrove-saltmarsh ecotone and assessed survival of experimentally transplanted mangrove seedlings. Mangrove canopies buffered night time cooling during the winter, leading to interspecific differences in freeze damage on mangrove seedlings. However, mangrove canopies also altered biotic interactions. Herbivore damage was higher under canopies, leading to greater mangrove seedling mortality beneath canopies relative to saltmarsh. While warming-induced expansion of mangroves can lead to positive microclimate feedbacks, simultaneous fluctuations in biotic drivers can also alter seedling dynamics. Thus, climate change can drive divergent feedback mechanisms through both abiotic and biotic channels, highlighting the importance of vegetation-microclimate interactions as important moderators of climate driven range shifts.

Mangrove Range Expansion Rapidly Increases Coastal Wetland Carbon Storage

The climate change-induced expansion of man- groves 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 occur- ring 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 gra- dients of marsh- to mangrove-dominance, and use unique chronological maps of vegetation cover to estimate C stock changesfrom2003to2010ina 567-km 2 wildlife refugeinthe mangrove-salt marsh ecotone. We report that over the 7-yr. period, total wetland C stocks increased 22 % due to man- grove 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 ,m ore than doubling the naturally high coastal wetland C sequestration rates. Mangrove expansion could account for a globally sig- nificant increase of terrestrial C storage, which may exert a considerable negative feedback on warming.