Mangrove Mortality Research Articles

Enhancing Mangrove Resilience: Assessing Rhizophora sp. Survival in Davao Occidental’s Conservation and Rehabilitation Zones, Philippines

Three distinct sampling locations within Davao Occidental, namely Brgy. Sto. Rosario in Sta. Maria municipality, Sitio Tawang in Brgy. Buhangin, and Sitio Agdao in Brgy. Tubalan in Malita municipality were investigated to evaluate the survival of planted mangroves (Rhizophora sp.). The study revealed a notable discrepancy in survival rates among the sites, with Sitio Agdao exhibiting the highest survival with 97.58%, followed by Sto. Rosario (91.84%), and Sitio Tawang registering 70.46% survival rate. No significant variance was observed between survival rates in Sta. Maria and Malita. Findings indicated diverse causes of mangrove mortality, including strong waves during northeasterly winds impacting Sto. Rosario, flashfloods affecting Sitio Tawang, and the presence of epiphytes (specifically barnacles) in Sitio Agdao. Among the sites, Sitio Tawang exhibited significantly lowest survival rates compared to Sto. Rosario and Sitio Agdao. Substrate analysis revealed clay loam in Sto. Rosario, sandy loam in Sitio Agdao, and sandy clay in Sitio Tawang. The study recommends extending mangrove survival rate monitoring to other conservation and rehabilitation areas, continuing mangrove rehabilitation efforts, and manually removing epiphytes to enhance survival rates.

The Culprit behind the Mass Death of Mangroves: Egrets or Rats (Rattus losea)?

Mangroves play a crucial role in maintaining coastal ecological balance. This study focused on the impact of branch-breaking behavior on the mortality of Rhizophora stylosa in the Guangxi Shankou Mangrove Reserve. However, we found mangrove mortality in areas devoid of egret habitation, prompting a reevaluation of our research hypothesis. Further investigation suggested that nesting behavior was the primary cause of mangrove mortality. A comparison of the data from areas with egrets (Egretta garzetta, Ardea intermedia) and lesser rice-field rats (Rattus losea) activity indicated significant mechanical damage caused by rats to mangroves as the main cause of mortality. Additionally, we found that the biological characteristics of R. stylosa, particularly its stunted growth and recovery abilities after branch breaking, were key factors affecting its survival. These findings imply that rat-induced mortality may not occur in other less susceptible mangrove species. The results contradict assumptions regarding the impact of egret behavior and highlight the importance of the biological characteristics of R. stylosa. This offers fresh insights into mangrove conservation and management, emphasizing the need for ongoing observation and hypotheses verification. Future studies should explore the influence of lesser rice-field rats’ activity and the intrinsic characteristics of R. stylosa on the ecosystem’s long-term stability.

Impacts and implications of anthropogenic activities on mangrove forests: A review

The purpose of this paper is to review and evaluate published literature on the causes, impacts, implications and mitigation strategies of anthropogenic activities on mangrove forests. A systematic method was utilized to access research works of literature on “Impacts and implications of anthropogenic activities on mangrove forests”. A total of fifty-three (53) research papers published between the years 1975 to 2024 was accumulated and used for this review. A subjective approach was used to select the topics: impact and implications of anthropogenic activities on mangrove forests. In this paper, five (5) roles of mangroves were assessed and presented. Anthropogenic activities that threaten mangrove forests such as land conversion, mining and logging activities and household and industrial activities were also evaluated. Further, four (4) detrimental anthropogenic activities and the potential impacts they have on natural mangrove forests and the ecosystems they thrive in was also reported on. In addition, this paper reviewed some mitigation strategies for sustainable management and conservation of our mangroves, their ecosystems and the coastlines. The published works of literature established that reduced mangrove forests, altered mangrove composition, unequal species abundance, mass mangrove mortality, pollution in mangrove areas, rising sea levels, global climate change and coastal erosion or abrasion are all effects of anthropogenic activity that have a negative impact on mangrove forests and the ecosystems. This review highlights the fact that more studies on the impact and implications of anthropogenic activities on mangroves should be done in neotropical countries since there are paucity of such information on research and published data in these biodiversity rich regions.

Mangrove vegetation response to alteration in coastal geomorphology after an earthquake in Andaman Islands, India

The 2004 Sumatra-Andaman earthquake caused a devastating tsunami and shifts in land elevations, which had an instantaneous as well as the long-term impact on the Andaman Islands. Especially, the coastal uplift and subsidence has altered the tidal regime, leading to mass mortality and degradation of mangroves (∼150 sq. km). The tectonic shift in the Andaman archipelago has created three broader geographical units: North Andaman with coastal uplift (∼1.3 m) – sea level drop (SLD) scenario; Middle Andaman with minor to no change in coastal geomorphology – no change scenario; and South Andaman with coastal subsidence (∼1.1 m) – sea level rise (SLR) scenario. How these changes in the tidal regime have influenced mangrove vegetation remains less understood. Therefore, we assessed mangrove vegetation characteristics across the Andaman Islands to examine mangrove response to coastal uplift and subsidence after 18 years since the disturbance. The uplifted sites mimicking the SLD scenario appeared more destructive to mangroves (tree density - 771/ha) in comparison to subsided sites mimicking the SLR scenario (tree density - 937/ha), while no change sites with the highest tree density (1288/ha) showed lowest impacts to mangrove. The species diversity (H’=Shannon-Wiener index) within recruitment cohorts (seedlings: <1 cm DBH and saplings: ≥1 cm and <5 cm DBH) sapling (H’=1.57), and seedling (H’=1.48) was significantly higher than the trees (H’=1.20). Rhizophora apiculata and Ceriops tagal were the dominant species (cumulative IVI 60%) found in survived mangrove forests demonstrating their high resilience for sudden changes in tidal regime. Our study provided a comprehensive account of mangrove vegetation response after a major disruptive event in 2004. Long-term monitoring of these novel sites would enhance our understanding of mangrove response to sea level change, which can also provide significant insights for the restoration and management of mangroves.

Addition of iron does not ameliorate sulfide toxicity by sargassum influx to mangroves but dampens methane and nitrous oxide emissions

Sargassum spp. strandings in the tropical Atlantic harm local ecosystems due to toxic sulfide levels. We conducted a mesocosm experiment to test the efficacy of iron(III) (hydr)oxides in (a) mitigating sulfide toxicity in mangroves resulting from Sargassum and (b) reducing potentially enhanced greenhouse gas emissions. Our results show that iron addition failed to prevent mangrove mortality caused by highly toxic sulfide concentrations, which reached up to 15,000 μmol l−1 in 14 days; timely removal may potentially prevent mangrove death. Sargassum-impacted mesocosms significantly increased methane, nitrous oxide, and carbon dioxide emissions, producing approximately 1 g CO2-equivalents m−2 h−1 during daylight hours, thereby shifting mangroves from sinks to sources of greenhouse gasses. However, iron addition decreased methane emissions by 62 % and nitrous oxide emissions by 57 %. This research reveals that Sargassum strandings have multiple adverse effects related to chemical and ecological dynamics in mangrove ecosystems, including greenhouse gas emissions.

Estimasi sebaran konsentrasi total suspended solid (TSS) di Tahura Ngurah Rai Denpasar tahun 2016 dan 2020

The large number of mangrove land conversions can cause various impacts, such as causing coastal abrasion to affect sedimentation rates. One of the variables used to determine the level of pollution and sedimentation that occurs in aquatic ecosystems is Total Suspended Solid (TSS). Based on research conducted by Sebastian (2020), mangrove land cover in Tahura Ngurah Rai decreased by 25.58 hectares in 2016 - 2020 due to the mass mortality of mangroves in 2019. The decline in mangrove land in this location allows an increase of TSS in the surrounding waters due to the lack of mangroves which function as a barrier and sediment trap. One of the satellite that have been widely used in observing the distribution of TSS concentrations is the Landsat-8 OLI. Therefore, researchers are interested in knowing the changes in TSS concentrations that occurred in Tahura Ngruah Rai in 2016 and 2020 using Landsat 8 imagery. This study used a gravimetric method to analyze TSS concentrations in situ at 45 sample points carried out by random sampling. To find out the spatial estimation of TSS in 2016 and 2020, an empirical algorithm is used. The best TSS estimation model in this study is the combination of the blue band/ SWIR-1 band using linear equations. The processing results show the concentration of TSS in 2016 has a range between 66 mg/L to 264 mg/L with an average of 127.74 mg/L, while . In 2020 TSS has a range between 72 mg/L to 209 mg/L with an average of 133.04 mg/L.

Ghosts of dry seasons past: Legacy of severe drought enhances mangrove salinity tolerance through coordinated cellular osmotic and elastic adjustments.

The incidence and severity of global mangrove mortality due to drought is increasing. Yet, little is understood of the capacity of mangroves to show long-term acclimation of leaf water relations to severe drought. We tested for differences between mid-dry season leaf water relations in two cooccurring mangroves, Aegiceras corniculatum and Rhizophora stylosa before a severe drought (a heatwave combined with low rainfall) and after its relief by the wet season. Consistent with ecological stress memory, the legacy of severe drought enhanced salinity tolerance in the subsequent dry season through coordinated adjustments that reduced the leaf water potential at the turgor loss point and increased cell wall rigidity. These adjustments enabled maintenance of turgor and relative water content with increasing salinity. As most canopy growth occurs during the wet season, acclimation to the 'memory'of higher salinity in the previous dry season enables greater leaf function with minimal adjustments, as long-lived leaves progress from wet through dry seasons. However, declining turgor safety margins - the difference between soil water potential and leaf water potential at turgor loss - implied increasing limitation to water use with increasing salinity. Thus, plasticity in leaf water relations contributes fundamentally to mangrove function under varying salinity regimes.

Coordination of hydraulic thresholds across roots, stems, and leaves of two co-occurring mangrove species.

Mangroves are frequently inundated with saline water and have evolved different anatomical and physiological mechanisms to filter and, in some species, excrete excess salt from the water they take up. Because salts impose osmotic stress, interspecific differences in salt tolerance and salt management strategy may influence physiological responses to drought throughout the entire plant hydraulic pathway, from roots to leaves. Here, we characterized embolism vulnerability simultaneously in leaves, stems, and roots of seedlings of two mangrove species (Avicennia marina and Bruguiera gymnorrhiza) along with turgor-loss points in roots and leaves and xylem anatomical traits. In both species, the water potentials causing 50% of total embolism were less negative in roots and leaves than they were in stems, but the water potentials causing incipient embolism (5%) were similar in roots, stems, and leaves. Stomatal closure in leaves and turgor loss in both leaves and roots occurred at water potentials only slightly less negative than the water potentials causing 5% of total embolism. Xylem anatomical traits were unrelated to vulnerability to embolism. Vulnerability segmentation may be important in limiting embolism spread into stems from more vulnerable roots and leaves. Interspecific differences in salt tolerance affected hydraulic traits from roots to leaves: the salt-secretor A. marina lost turgor at more negative water potentials and had more embolism-resistant xylem than the salt-excluder B. gymnorrhiza. Characterizing physiological thresholds of roots may help to explain recent mangrove mortality after drought and extended saltwater inundation.

Effects of Extreme Salinity Stress on a Temperate Mangrove Ecosystem

Mangrove forests provide essential ecosystem services, but are threatened by habitat loss, effects of climatic change and chemical pollutants. Hypersalinity can also lead to mangrove mortality, although mangroves are adapted to saline habitats. A recent dieback event of &amp;gt;9 ha of temperate mangrove (Avicennia marina) in South Australia allowed to evaluate the generality of anthropogenic impacts on mangrove ecosystems. We carried out multidisciplinary investigations, combining airborne remote sensing with on-ground measurements to detect the extent of the impact. The mangrove forest was differentiated into “healthy,” “stressed,” and “dead” zones using airborne LIDAR, RGB and hyperspectral imagery. Differences in characteristics of trees and soils were tested between these zones. Porewater salinities of &amp;gt;100 were measured in areas where mangrove dieback occurred, and hypersalinity persisted in soils a year after the event, making it one of the most extreme hypersalinity cases known in mangrove. Sediments in the dieback zone were anaerobic and contained higher concentrations of sulfate and chloride. CO2 efflux from sediment as well as carbon stocks in mangrove biomass and soil did not differ between the zones a year after the event. Mangrove photosynthetic traits and physiological characteristics indicated that mangrove health was impacted beyond the immediate dieback zone. Normalized Difference Vegetation Index (NDVI), photosynthetic rate, stomatal conductance and transpiration rate as well as chlorophyll fluorescence were lower in the “stressed” than “healthy” mangrove zone. Leaves from mangrove in the “stressed” zone contained less nitrogen and phosphorous than leaves from the “healthy” zone, but had higher arsenic, sulfur and zinc concentrations. The response to extreme hypersalinity in the temperate semi-arid mangrove was similar to response from the sub-/tropical semi-arid mangrove. Mangrove in semi-arid climates are already at their physiological tolerance limit, which places them more at risk from extreme hypersalinity regardless of latitude. The findings have relevance for understanding the generality of disturbance effects on mangrove, with added significance as semi-arid climate regions could expand with global warming.

Annual characteristics of gross primary productivity (GPP) in mangrove forest during 2016-2020 as revealed by Sentinel-2 remote sensing imagery

Carbon dioxide (CO2) is the most responsible gas in the phenomenon of global warming on earth because of its greatest concentration and longevity in the long atmosphere. Meanwhile mangrove is one of the blue carbon parameters that can take CO2 for photosynthesis and store it into biomass and sediment, so the existence of Mangrove plays a key role in the balance of the global carboncycle. Gross Primary Productivity (GPP) is one of the key variables in conducting a carboncycle study because the GPP values constitute the total value of carbon fixation by terrestrial ecosystem through vegetation photosynthesis. The aim of this study is to recognize the annual characteristics of GPP values in the mangrove ecosystem using remote sensing satellite Sentinel-2 during the period of 2016 to 2020 in TAHURA Ngurah Rai, Bali, Indonesia. The vegetation photosynthesis model (VPM) model is used to calculate annual GPP by using remote sensing indices of Enhanced Vegetation Index (EVI) and Land Surface Water Index (LSWI). Both remote sensing indices are supported by temperature and solar radiation data to determine photosynthesis fraction. The results of the current study indicated that the annual GPP values in the mangrove forest of TAHURA Ngurah Rai have decreased during the observation year. The total value of GPP in 2016 is reached to 28790 tC m−2 year−1, while the total amount of GPP value in 2020 decrease to 26223 tC m−2 year−1. The decline in GPP values may be due to changes of land cover and mangrove mortality that occurred around Benoa port in 2018.

Ecosystem carbon losses following a climate-induced mangrove mortality in Brazil

Drought events may induce mangrove mortality and dieback events worldwide as a result of climate extremes. As mangroves sequester large quantities of carbon, quantifying the losses of these stocks following climate disturbances may guide wetland governance strategies globally. In Southeast Brazil, we determined the total ecosystem carbon stocks (TECS) of pristine mangroves that were up to 1851 Mg of carbon per hectare (Mg C ha−1), which are the highest stocks measured from South American and raising estimates of Brazil's mangrove TECS to 0.52 Pg C. A mangrove mortality event in the same estuary resulted in a 14.6 % decrease in TECS (270.5 Mg C ha−1) and loss of 20 % of mangrove soil carbon within less than 2-years. Carbon dioxide emissions from this impact were 992.8 Mg CO2e ha−1, which are slightly lower than emissions from land use disturbances on mangroves worldwide. Our results suggest that climate effects on mangroves can become significant sources of greenhouse gases globally.

Storm surge and ponding explain mangrove dieback in southwest Florida following Hurricane Irma

Mangroves buffer inland ecosystems from hurricane winds and storm surge. However, their ability to withstand harsh cyclone conditions depends on plant resilience traits and geomorphology. Using airborne lidar and satellite imagery collected before and after Hurricane Irma, we estimated that 62% of mangroves in southwest Florida suffered canopy damage, with largest impacts in tall forests (>10 m). Mangroves on well-drained sites (83%) resprouted new leaves within one year after the storm. By contrast, in poorly-drained inland sites, we detected one of the largest mangrove diebacks on record (10,760 ha), triggered by Irma. We found evidence that the combination of low elevation (median = 9.4 cm asl), storm surge water levels (>1.4 m above the ground surface), and hydrologic isolation drove coastal forest vulnerability and were independent of tree height or wind exposure. Our results indicated that storm surge and ponding caused dieback, not wind. Tidal restoration and hydrologic management in these vulnerable, low-lying coastal areas can reduce mangrove mortality and improve resilience to future cyclones.

Effect of Restoration Actions on Organic Carbon Pools in the Lagoon—Delta Ciénaga Grande de Santa Marta, Colombian Caribbean

Mangroves provide multiple ecosystem services and are essential for mitigating global warming owing to their capacity to store large carbon (C) stocks. Due to widespread mangrove degradation, actions have been implemented to restore them worldwide. An important representative case in Colombia is the Ciénaga Grande de Santa Marta’s restoration plan. This management intervention focused on restoring the natural hydrological functioning after massive mangrove mortality (~25,000 ha) due to soil hyper-salinization after river water input from the Magdalena River was eliminated. A partial recovery occurred during subsequent years, and hydrological management is still being implemented today. To understand how the degradation and subsequent management have affected mangrove C stocks, we compared C stocks in stands with different intervention levels reflected in their current forest structure. We found that the total C stock (398–1160 Mg C ha−1) was within the range measured in other neotropical mangroves without vegetation deterioration. The aboveground C was significantly higher in the stands where hydraulic connectivity was restored. By contrast, the belowground C was higher in the stands with low hydraulic connectivity due to channel clogging and a lack of sufficient maintenance. Our results show that hydrological management measures influenced above- and belowground C stocks, even at a 2 m depth. In addition, a strong indirect relationship useful for estimating carbon content from organic matter content was found.

Coastal carbon cycle changes following mangrove loss

AbstractGlobal mangrove loss is occurring from deforestation and extreme climatic events, but changes to the coastal carbon cycle following mangrove mortality and/or loss are not well understood. In 2015/2016, a massive climate‐driven mangrove dieback event occurred over ~ 1000 km of Australian coastline. To assess carbon loss following mortality, carbon fluxes in adjacent living and dead forest areas were compared 8 and 20 months postforest dieback. Dead areas experienced an increase in soil CO2 efflux by ~ 189%, and a decrease in oceanic dissolved inorganic carbon (DIC) outwelling of ~ 50% relative to living areas. DIC outwelling (predominantly carbonate alkalinity) and soil CO2 efflux accounted for 81% and 16% of losses from the living forest, in comparison to 51% and 47%, respectively, from the dead forest. The dieback drove a shift from a dominance of oceanic carbon outwelling to increased atmospheric CO2 emissions and decreased alkalinity exports. This shift was likely driven by increased oxygen sediment permeation and the loss of mangrove net primary productivity. Combining our new observations with literature data, we found a logarithmic relationship between soil carbon loss and time since mangrove loss. Using this relationship, we estimate ongoing global carbon losses from historical mangrove deforestation and dieback could be 13.7 ± 9.4 Tg C yr−1, which is eightfold higher than previous estimates and offsets global mangrove carbon burial by ~ 60%. Even if no future deforestation occurred, we estimate ongoing carbon losses to the atmosphere and ocean from current global mangrove losses of 27 Tg C over the next 30 yr.

Frequency of extreme freeze events controls the distribution and structure of black mangroves (Avicennia germinans) near their northern range limit in coastal Louisiana

AbstractAimClimate 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.LocationCoastal Louisiana, USA.MethodsWe 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.ResultsWe 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 conclusionsOur 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.

Massive loss of aboveground biomass and its effect on sediment organic carbon concentration: Less mangrove, more carbon?

Mangroves play an important role in the nutrient cycle of coastal areas, contributing to the health of adjacent ecosystems by retaining carbon, nitrogen and phosphorous. Different studies have shown that sediment organic carbon (SOC) stock decrease with the loss of above-ground forest biomass. Our objective was to assess the effect of deforestation on the ability of mangroves to store carbon and retain nutrients in the sediment. We hypothesized that conserved forests contain the highest values of SOC compared to sites where the vegetal cover was completely lost. The sampling was carried out in a highly altered ecosystem in the South Caribbean, where a freshwater diversion project had been implemented between 1993 and 1998 to rehabilitate 290 km2 of a deforested mangrove area after hypersalination of the sediment. Based on our results, we reject our initial hypothesis; SOC stores in areas where massive mangrove mortality occurred were between 258 ± 2 and 339 ± 19 Mg C ha−1, i.e., two times higher than those in conserved mangrove forest areas (156 ± 16 Mg C ha−1). Nitrogen (N) followed a trend similar to that of SOC, in which sites with degraded mangroves had greater N stores (between 15.5 ± 0.4 and 19.1 ± 1.0 Mg N ha−1) than conserved site (11.7 ± 0.8 Mg N ha−1). In contrast, phosphorus (P) presented higher values at the conserved site (3.2 ± 0.3 Mg P ha−1) than at degraded sites (between 0.8 ± 0.2 and 2.0 ± 0.2 Mg P ha−1). The higher concentration of SOC in highly degraded sites may be driven by the balance between the large accumulation of dead plant material due to hypersalination and the low decay rates due to anoxia sediment conditions.

Mangrove Mortality: Analyses of Natural and Anthropic Causes and their Effects on Forest Dynamics

Lopes, D.M.S.; Chaves, F.O., and Tognella, M.M.P., 2020. Mangrove mortality: Analysis of natural and anthropic causes and their effects on forest dynamics. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 102-107. Coconut Creek (Florida), ISSN 0749-0208.Located in the north coast of Espirito Santo State, Brazil, mangrove forests are associated with estuarine systems of variable flow, changing according to the climate regime of their origin. The complexity of this ecosystem is also a function of different factors, such as the physiography, the geology of the coastal plain and pollution. Natural and anthropogenic stressors can drain energy by removing structures and restricting the development, which causes variation in the number of dead individuals. In this study is reported the dynamics of the Barra Nova mangroves (in of Espirito Santo) through forest mortality investigations, either for natural or anthropic reasons. At total, eleven phytosociological data collection stations were performed along the estuary. Structural development showed an average height ranging from 3.41 to 16.69 m, average DBH of 2.69 to 21.68 cm, and trunk density of 853 to 6,360 ha-1. Measurements of the salinity of water in the estuary ranged from 25 to 68. A large number of dead and cut trunks occurred in most stations, with the highest percentage of dead basal area (84.18 %) occurring in the station 1. Four stations harbored more than 50 % of dead basal area. Overall, the structural values were extremely variable. Although sufficiently anthropized and submitted to natural environmental changes, the results obtained here show normal and no-massive mangrove mortality, which indicate a good structural development in the analyzed areas when compared to other mangrove forests. Finally, the information about the mortality rates and processes presented in this study are helpful tools to detect pressures on ecosystems, contributing to the knowledge and protection of these natural systems.

Untargeted metabolomics in halophytes: The role of different metabolites in New Zealand mangroves under multi-factorial abiotic stress conditions

Mangroves are halophytes which live in harsh coastal environments, and can serve as excellent model organisms to understand mechanisms of stress tolerance. Metabolomics is a useful approach to investigate the role different metabolites play during physiological responses of plants to abiotic stress factors. Previous studies indicate that levels of non-structural carbohydrates in plants may be associated with stress tolerance. Here, we manipulated levels of non-structural carbohydrates (NSCs) in the New Zealand mangrove, Avicennia marina subsp. australasica, through a light swapping regime. In a subsequent drought × salinity experiment, we then monitored leaf metabolite profiles of two NSC groups (high-NSC and low-NSC). Our results show that fourteen metabolites, belonging to multiple biochemical pathways, were significantly affected by one or more of these factors. The manipulation of non-structural carbohydrates led to increased abundance of amino acids in the low-NSC (L-NSC) plants compared to the high-NSC (H-NSC) plants. Under high drought conditions, the L-NSC plants had higher abundances of leucine and valine in comparison to those with the H-NSC phenotype. The L-NSC plants also had higher abundances of putrescine and aminoadipic acid when exposed to high salinity. Under the combination of drought and salinity, α-ketoglutarate was reduced in plants with the H-NSC phenotype, and soluble sugars accumulated compared to those with the L-NSC phenotype. The increased soluble sugar content in the H-NSC plants can facilitate osmotic adjustment, thereby aiding their survival during low soil water potential conditions. We also detected the presence of stress-protective phenolic compounds (syringic and sinapic acids) not previously reported in mangroves. Relationships between plant growth and the mangrove metabolome were also established, with levels of glucose and phosphoenol pyruvate being primarily responsible for phenotypic associations. Our findings show that the metabolites detected in this study, and their respective metabolic pathways, play a significant role in salinity and drought stress tolerance of A. marina subsp. australasica, providing new information to better understand mangrove mortality.

Mangrove Damage, Delayed Mortality, and Early Recovery Following Hurricane Irma at Two Landfall Sites in Southwest Florida, USA

Mangrove forests along the coastlines of the tropical and sub-tropical western Atlantic are intermittently impacted by hurricanes and can be damaged by high-speed winds, high-energy storm surges, and storm surge sediment deposits that suffocate tree roots. This study quantified trends in damage, delayed mortality, and early signs of below- and aboveground recovery in mangrove forests in the Lower Florida Keys and Ten Thousand Islands following direct hits by Hurricane Irma in September 2017. Mangrove trees suffered 19% mortality at sites in the Lower Florida Keys and 11% in the Ten Thousand Islands 2–3 months post-storm; 9 months post-storm, mortality in these locations increased to 36% and 20%, respectively. Delayed mortality of mangrove trees was associated with the presence of a carbonate mud storm surge deposit on the forest floor. Mortality and severe branch damage were more common for mangrove trees than for mangrove saplings. Canopy coverage increased from 40% cover 1–2 months post-storm to 60% cover 3–6 months post-storm. Canopy coverage remained the same 9 months post-storm, providing light to an understory of predominantly Rhizophora mangle (red mangrove) seedlings. Soil shear strength was higher in the Lower Florida Keys and varied with depth; no significant trends were found in shear strength between fringe or basin plots. Rates of root growth, as assessed using root in-growth bags, were relatively low at 0.01–11.0 g m−2 month−1 and were higher in the Ten Thousand Islands. This study demonstrated that significant delayed mangrove mortality can occur 3–9 months after a hurricane has passed, with some mortality attributable to smothering by storm surge deposits.

Ghost forests of Marco Island: Mangrove mortality driven by belowground soil structural shifts during tidal hydrologic alteration

Land use changes often create in situ stress and eventual mortality in mangroves as unsuitable hydroperiods are created through tidal flow alterations. Here, we document mangrove forest and soil structural changes within transects established in tidally restricted areas on Marco Island (Collier County, Florida, USA), which has broad swaths of dead-standing or unhealthy mangroves (“ghost forests”). Transects (N = 4) were arranged to include full canopy, transitional, and open canopy (dead) forests, and compared to nearby reference forests. Aboveground and belowground carbon (C) stocks (Total C) ranged from 288 to 304 Mg C ha−1 on full canopy, transitional, and reference sites, which did not differ from each other. However, Total C was lower for dead sites (233 Mg C ha−1) dictated entirely by differences in aboveground C (live and dead trees, downed wood); no differences were found among forest condition in belowground C stocks. This belowground C has been persistent in the soil for 85 years since initial tidal restriction. Nevertheless, hydrologic rehabilitation has the potential to increase total C stocks on dead sites by 70–110 Mg C ha−1. Collapse of the soil surface by 6–8 cm just under the active root zone in chronically stressed mangroves was evident within the bulk density profiles from transitional versus dead sites, suggesting that surface elevation loss as root turnover ceases may work correlatively with chronic stressors (anoxia, P limitation) to affect rapid mortality of forests over short periods of time many years after stress initiation. Hydrologic rehabilitation of stressed or denuded mangroves must also include an understanding of how these soil processes might be re-established.