Mangrove Dieback Research Articles

Florida mangrove dieback on a decadal and centennial timescales

Hurricanes are considered some of the most devastating weather phenomena, causing deaths and destruction along the coast. Mangroves are perceived as natural defenses against coastal hazards. However, mangrove gaps, known as mangrove dieback, have occurred in southwestern Florida. This study implemented a spatial-temporal analysis based on satellite, lidar and unmanned aerial vehicle (UAV) images collected along the southwestern Florida coast between Tampa and Naples from 1994 to 2020 CE, and multi-proxy analyses on sediment cores sampled from two mangrove dieback sites to determine the cause(s) of mangrove death and the implications for the sustainability of these forests. The data indicated that out of the 86 identified dieback, 78 % arose between 2004 and 2017 CE, when one tropical storm and three hurricanes made landfall on the studied coast, suggesting hurricanes as the main driver for this mangrove degradation. Multi-proxy analysis indicated mangrove expansion since ∼600 cal yr BP due to relative sea-level (RSL) rise. However, hurricanes caused losses in the mangrove area between 1930 and 1950 CE, and during the last two decades in the studied sites. Ground topographic data and aerophotogrammetry revealed circular dieback with permanent ponds (∼30 cm depth) inhibiting mangrove recovery. The mangrove death occurred in <3 years, with a gradual dieback expansion due to positive feedback involving physicochemical processes in the sediments. In addition, recent hurricanes increased pre-existing dieback areas. Therefore, dieback sites are vulnerability hotspots for mangroves in the face of projected RSL rise and higher frequency of intense hurricane strikes. Mangrove dieback should be monitored on a temporal (seasonal - annual) and spatial (cm-m) resolution only possible through the combination of satellite and UAV data. Integrating remote sensing and stratigraphic analysis enabled us to identify mangrove diebacks on a decadal to centennial timescale, improving our comprehension of how catastrophic weather events affect these forests.

Tidal Impoundment and Mangrove Dieback at Cabbage Tree Basin, NSW: Drivers of Change and Tailored Management for the Future

Major storms can cause significant changes to coastal and wetland environments. A series of storm events in 2020 resulted in closure of the historically open estuary at Cabbage Tree Basin, Port Hacking, New South Wales (NSW), Australia. Prolonged tidal impoundment (3 months) led to substantial changes in hydrological and sedimentological processes, resulting in widespread mangrove dieback. This study aimed to quantify the degree of impact and recovery for mangroves, identify factors contributing to dieback, and consider the implications for carbon sequestration. This was achieved using remotely piloted aircraft structure-from-motion approaches, aerial photography, and field-based assessments of vegetation health and above-ground biomass (AGB). Mangroves were classified as ‘dead’, ‘partially dead’, and ‘live’. In October 2019, there was 10.8 ha of live mangroves, with this reduced to 6.6 ha by August 2022. Digital surface models (DSMs) were intersected with classified mangroves to assess the vertical distribution of each zone. All mangroves classified as ‘dead’ were distributed at elevations < 0.4 m with respect to the Australian Height Datum (AHD), suggesting these regions were persistently inundated, which was confirmed by water level loggers (inundated during logger deployment). Field data confirmed substrate elevation related to dieback with the proportion of ‘live’ mangroves greatest at elevations > 0.6 m AHD. Substrate elevation and distance to the estuary mouth were significantly correlated with species, with Avicennia marina located at lower tidal positions and closer to the entrance compared to Aegiceras corniculatum. The dieback event equated to a loss of 81.5 ± 48 Mg of above-ground biomass, 38.1 ± 22.5 Mg C, or 140 ± 82 Mg CO2 equivalence (CO2e). This study provides an important baseline for monitoring dieback events. Continued monitoring is crucial to assess recovery and to tailor management strategies.

Massive mangrove dieback due to extreme weather impact - case of Maputo River Estuary, Mozambique

This study documents one of the first massive mangrove dieback in Africa caused by hailstorm, occurring in Maputo Bay, Mozambique. Field observation and satellite imagery prompted this observation of extensive mangrove dieback out of Maputo River Estuary, a regionally significant large mangrove stand, the southernmost large mangrove area in the Western Indian Ocean.This study aimed to determine the causes and extent of mangrove dieback in Maputo River Estuary that occurred in September 2019, identify, and quantify the mangrove loss, describe the structure of the mangrove forest, describe the soil composition in healthy and impacted areas. To address this Sentinel 2 imagery were assessed and calculated the Normalized Difference Vegetation Index (NDVI) to estimate the mangrove dieback area and cover change. This was supported by an extensive field survey, impacted and pristine or natural areas were assessed by sampling 233 plots of 10×10 m, established along transects separated by 50 m and set perpendicularly to the coastal line. Structural parameters assessed included species composition, height, diameter at the breast height (DBH), number of live and dead trees, density of stumps and seedling per species.The mangrove cover reduced nearly half (48 %) of mangrove area in just a year, from 1377.4 Ha in 2019 to barely 716.2 Ha in 2020, representing a loss of 661.2 Ha, and 38.7 % of trees were completely dead. Five mangrove species were identified, Avicenna marina being largely dominant. The regeneration ratio was 142:10:1, way beyond the minimum ecological ratio of standard mangrove forest, 6:3:1. According to local communities and meteorological data, it seems plausible that this event was caused by a hailstorm.These results contribute to understanding climate-related impacts on mangrove forests and its response which is crucial for adopting adequate management measures, recovery and climate change adaptation actions and monitoring of mangrove forests in Mozambique.

Future climate change will increase risk to mangrove health in Northern Australia

Mangroves of the wet-dry tropical Gulf of Carpentaria, Australia, survive in a harsh environment. One of the worst recorded mangrove dieback events occurred during the El Niño of 2015 following an extreme, two-year sea level drop, illustrating that enhanced climate variability can exacerbate major stressors for these ecosystems. As well as sea level variability, maximum daily temperatures in the Gulf of Carpentaria are also linked to climate variability and change, and may play an important role in overall mangrove health. Here we address how these two factors: sea level variability and maximum daily temperatures, are projected to change under several future emissions scenarios. Climate projections from the sixth generation of Coupled Model Intercomparison Project indicate an increased occurrence of anomalously low and high sea level events in the coming century. This, alongside enhanced temperature stress, is likely to significantly increase risk to mangrove health in this region. The rate of increase of low and high sea level events, and high temperature events, is scenario-dependent, and is largest for a high-emissions scenario.

Land cover changes, biomass loss, and predictive causes of massive dieback of a mangrove plantation in Lampung, Sumatra

A new massive dieback case of Avicennia marina was observed in the area of the Sunda Strait, which started in 2020, and the cause is still unknown. This research was conducted to report the rate of mangrove degradation, measure biomass loss, and predict the causes of the dieback. A sequential imagery analysis (2019–2022) was conducted to interpret the area changes using a research area of 200 ha from the total 528.69-ha mangrove, using the plant senescence reflectance index (PSRI) and normalized difference vegetation index (NDVI). Field data were collected before and during the dieback event (2020 and 2021) at seven research stations including, live mangroves, partial dieback, and full dieback sites. Biomass loss was measured as standing stock volume (SSV) and total biomass carbon (TBC). A literature study was conducted to predict the cause of the dieback. Based on the results, the mangrove dieback caused a rapid and significant loss of healthy stands (results of the PSRI) and very-high-density stands (results of the NDVI). The rate of healthy stand loss was 13.43 ha month−1 (during July–October 2020) and that of very-high-density stand loss was 14.99 ha month−1 (October 2020–April 2021), which affected 126.62 ha or 24% of the total area (last measurement in January 2022). The SSV before dieback was 118.70 (±46.1) m3 ha−1, but within 14 months, it decreased to 20.8 (±6.0), 79.80 (±52.3), and only 1.0 (±1.0) m3 ha−1 for live, partial dieback, and full dieback stands, respectively. The TBC of live mangroves (before dieback) was 51.6 (±24.4) Mg C ha−1, but within the same period, it changed to 30.70 (±3.80), 69.50 (±14.6), and 51.60 (±9.4) Mg C ha−1 for live, partial dieback, and full dieback stands, respectively. Based on the literature study and field observation, the dieback was probably related to less flushing and long-term inundation by the decrease in rainfall intensity, in addition to nitrogen (NO3−N and NO2−N) enrichment, an over-supply of N from fishpond wastes. Sustainable silvofishery may be introduced to ameliorate water and soil quality.

Tidal control and mangrove dieback impact on methane emissions from a subtropical mangrove estuary

AbstractMangrove ecosystems with high sediment deposition and active carbon cycling are a source of methane (CH4) to the coastal atmosphere. We investigated diurnal and seasonal variations in CH4 emissions from a subtropical mangrove estuary in southern Texas, northwest Gulf of Mexico. Tidal processes, amplitude (spring vs. neap tides) and topographic characteristics are crucial factors controlling CH4 cycling in mangrove creeks. Higher CH4 concentrations were observed during the ebb in spring tides due to the combination of processive export of CH4 along the creeks during ebb tides and the addition of porewater CH4 in upper intertidal sediment under water inundation in spring tides. The annual CH4 emissions offset approximately 0.15% of the carbon stock in normal years, indicating that these mangrove creeks are a weak CH4 source. However, significantly elevated CH4 emissions were observed from mangrove dieback after the extreme cold‐freezing event in February 2021. The average CH4 flux from the mangrove creeks (126.1 ± 128.3 μmol [m2·d]−1) increased 45% in 3 months after mangrove die‐off in comparison with the overall average in normal years (87.0 ± 64.4 μmol [m2·d]−1). It is obvious that the previous small CH4 offset of the healthy mangrove forest was enlarged by the dieback event. Because the mangrove forests in this study live close to the limit of their survival range, our study highlights the important management considerations for blue carbon projects in vulnerable areas.

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.

Multiscale Diagnosis of Mangrove Status in Data-Poor Context Using Very High Spatial Resolution Satellite Images: A Case Study in Pichavaram Mangrove Forest, Tamil Nadu, India

Highlighting spatiotemporal changes occurring within mangrove habitats at the finest possible scale could contribute fundamental knowledge and data for local sustainable management. This study presents the current situation of the Pichavaram mangrove area, a coastal region of Southeast India prone to both cyclones and reduced freshwater inflow. Based on the supervised classification and visual inspection of very high spatial resolution (VHSR) satellite images provided with a pixel size of &lt;4 m, we generated time-series maps to analyze the changes that occurred in both the natural and planted mangroves between 2003 and 2019. We achieved a high mapping accuracy (&gt;85%), which confirmed the potential of classification techniques applied to VHSR images in capturing changes in mangroves on a very fine scale. Our diagnosis reveals variable expansion rates in plantations made by the local authorities. We also report an ongoing mangrove dieback and confirm progressive shoreline erosion along the coastline. Despite a lack of field data, VHSR images allowed for the multiscale diagnosis of the ecosystem situation, thus constituting the first fine-scale assessment of the fragile Pichavaram mangrove area upon which the coastal community is dependent.

Influence of the 2015–2016 El Niño on the record-breaking mangrove dieback along northern Australia coast

This study investigates the underlying climate processes behind the largest recorded mangrove dieback event along the Gulf of Carpentaria coast in northern Australia in late 2015. Using satellite-derived fractional canopy cover (FCC), variation of the mangrove canopies during recent decades are studied, including a severe dieback during 2015–2016. The relationship between mangrove FCC and climate conditions is examined with a focus on the possible role of the 2015–2016 El Niño in altering favorable conditions sustaining the mangroves. The mangrove FCC is shown to be coherent with the low-frequency component of sea level height (SLH) variation related to the El Niño Southern Oscillation (ENSO) cycle in the equatorial Pacific. The SLH drop associated with the 2015–2016 El Niño is identified to be the crucial factor leading to the dieback event. A stronger SLH drop occurred during austral autumn and winter, when the SLH anomalies were about 12% stronger than the previous very strong El Niño events. The persistent SLH drop occurred in the dry season of the year when SLH was seasonally at its lowest, so potentially exposed the mangroves to unprecedented hostile conditions. The influence of other key climate factors is also discussed, and a multiple linear regression model is developed to understand the combined role of the important climate variables on the mangrove FCC variation.

Mangrove Dieback and Leaf Disease in Sonneratia apetala and Sonneratia caseolaris in Vietnam

Even though survival rates for mangrove restoration in Vietnam have often been low, there is no information on fungal pathogens associated with mangrove decline in Vietnam. Therefore, this research was undertaken to assess the overall health of mangrove afforestation in Thanh Hoa Province and fungal pathogens associated with tree decline. From a survey of 4800 Sonneratia trees, the incidence of disorders was in the order of pink leaf spot &gt; shoot dieback &gt; black leaf spot for S. caseolaris and black leaf spot &gt; shoot dieback &gt; pink leaf spot for S. apetala. Approximately 12% of S. caseolaris trees had both pink leaf spot and shoot dieback, while only 2% of S. apetala trees had black leaf spot and shoot dieback. Stem and leaf samples were taken from symptomatic trees and fungi were cultured in vitro. From ITS4 and ITS5 analysis, four main fungal genera causing leaf spots and shoot dieback on the two Sonneratia species were identified. The most frequently isolated fungal taxa were Curvularia aff. tsudae (from black leaf spot),Neopestalotiopsis sp.1 (from stem dieback), Pestalotiopsis sp.1 (from pink leaf spot), and Pestalotiopsis sp.4a (from black leaf spot). The pathogenicity of the four isolates was assessed by under-bark inoculation of S. apetala and S. caseolaris seedlings in a nursery in Thai Binh Province. All isolates caused stem lesions, and Neopestalotiopsis sp.1 was the most pathogenic. Thus, investigation of fungal pathogens and their impact on mangrove health should be extended to other afforestation projects in the region, and options for disease management need to be developed for mangrove nurseries.

Compound climate extremes driving recent sub-continental tree mortality in northern Australia have no precedent in recent centuries

Compound climate extremes (CCEs) can have significant and persistent environmental impacts on ecosystems. However, knowledge of the occurrence of CCEs beyond the past ~ 50 years, and hence their ecological impacts, is limited. Here, we place the widespread 2015–16 mangrove dieback and the more recent 2020 inland native forest dieback events in northern Australia into a longer historical context using locally relevant palaeoclimate records. Over recent centuries, multiple occurrences of analogous antecedent and coincident climate conditions associated with the mangrove dieback event were identified in this compilation. However, rising sea level—a key antecedent condition—over the three decades prior to the mangrove dieback is unprecedented in the past 220 years. Similarly, dieback in inland forests and savannas was associated with a multi-decadal wetting trend followed by the longest and most intense drought conditions of the past 250 years, coupled with rising temperatures. While many ecological communities may have experienced CCEs in past centuries, the addition of new environmental stressors associated with varying aspects of global change may exceed their thresholds of resilience. Palaeoclimate compilations provide the much-needed longer term context to better assess frequency and changes in some types of CCEs and their environmental impacts.

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.

ENSO feedback drives variations in dieback at a marginal mangrove site

Ocean–atmosphere climatic interactions, such as those resulting from El Niño Southern Oscillation (ENSO) are known to influence sea level, sea surface temperature, air temperature, and rainfall in the western Pacific region, through to the north-west Australian Ningaloo coast. Mangroves are ecologically important refuges for biodiversity and a rich store of blue carbon. Locations such as the study site (Mangrove Bay, a World Heritage Site within Ningaloo Marine Park and Cape Range National Park) are at the aridity range-limit which means trees are small in stature, forests small in area, and are potentially susceptible to climate variability such as ENSO that brings lower sea level and higher temperature. Here we explore the relationship between mangrove dieback, and canopy condition with climatic variables and the Southern Oscillation Index (SOI)—a measure of ENSO intensity, through remote sensing classification of Landsat satellite missions across a 29 year period at a north-west Australian site. We find that the SOI, and seasonal mean minimum temperature are strongly correlated to mangrove green canopy (as indicator of live canopy) area. This understanding of climate variations and mangrove temporal heterogeneity (patterns of abundance and condition) highlights the sensitivity and dynamics of this mangrove forest and recommends further research in other arid and semi-arid tropical regions at mangrove range-limits to ascertain the extent of this relationship.

Alkalinity Production Coupled to Pyrite Formation Represents an Unaccounted Blue Carbon Sink

AbstractBlue carbon ecosystems, including mangroves, saltmarshes, and seagrasses, mitigate climate change by storing atmospheric carbon. Previous blue carbon research has focused on organic carbon stocks. However, recent studies suggest that lateral inorganic carbon export might be equally important. Lateral export is a long‐term carbon sink if carbon is exported as alkalinity (TAlk) produced via sulfate reduction coupled to pyrite formation. This study evaluates drivers of pyrite formation in blue carbon ecosystems, compares pyrite production to TAlk outwelling rates, and estimates global pyrite stocks in mangroves. We quantified pyrite stocks in mangroves, saltmarshes, and seagrasses along a latitudinal gradient on the Australian East Coast, including a mangrove dieback area, and in the Everglades (Florida, USA). Our results indicate that pyrite stocks were driven by a combination of biomass, tidal amplitude, sediment organic carbon, sediment accumulation rates, rainfall, latitude, temperature, and iron availability. Pyrite stocks were three‐times higher in mangroves (103 ± 61 Mg/ha) than in saltmarshes (30 ± 30 Mg/ha) and seagrasses (32 ± 1 Mg/ha). Mangrove pyrite stocks were linearly correlated to TAlk export at sites where sulfate reduction was the dominant TAlk producing process. However, pyrite generation could not explain all TAlk outwelling. We present the first global model estimating pyrite stocks in mangroves, giving a first‐order estimate of 197 Mg/ha (RMSE = 24 Mg/ha). In mangroves, estimated global TAlk production coupled to pyrite formation (∼3 mol/m2/y) is equal to ∼24% of their global carbon burial rate, highlighting the importance of including TAlk export in future blue carbon budgets.

Reconstructing extreme climatic and geochemical conditions during the largest natural mangrove dieback on record

Abstract. A massive mangrove dieback event occurred in 2015–2016 along ∼1000 km of pristine coastline in the Gulf of Carpentaria, Australia. Here, we use sediment and wood chronologies to gain insights into geochemical and climatic changes related to this dieback. The unique combination of low rainfall and low sea level observed during the dieback event had been unprecedented in the preceding 3 decades. A combination of iron (Fe) chronologies in wood and sediment, wood density and estimates of mangrove water use efficiency all imply lower water availability within the dead mangrove forest. Wood and sediment chronologies suggest a rapid, large mobilization of sedimentary Fe, which is consistent with redox transitions promoted by changes in soil moisture content. Elemental analysis of wood cross sections revealed a 30- to 90-fold increase in Fe concentrations in dead mangroves just prior to their mortality. Mangrove wood uptake of Fe during the dieback is consistent with large apparent losses of Fe from sediments, which potentially caused an outwelling of Fe to the ocean. Although Fe toxicity may also have played a role in the dieback, this possibility requires further study. We suggest that differences in wood and sedimentary Fe between living and dead forest areas reflect sediment redox transitions that are, in turn, associated with regional variability in groundwater flows. Overall, our observations provide multiple lines of evidence that the forest dieback was driven by low water availability coinciding with a strong El Niño–Southern Oscillation (ENSO) event and was associated with climate change.

Impacts of wetland dieback on carbon dynamics: A comparison between intact and degraded mangroves

Mangroves are effective blue carbon sinks and are the most carbon rich ecosystems on earth. However, their areal extent has declined by over one-third in recent decades. Degraded mangrove forests result in reduced carbon captured and lead to release of stored carbon into the atmosphere by CO2 emission. The aim of this study was to assess changes in carbon dynamics in a gradually degrading mangrove forest on Bonaire, Dutch Caribbean. Remote sensing techniques were applied to estimate the distribution of intact and degraded mangroves. Forest structure, sediment carbon storage, sediment CO2 effluxes and dissolved organic and inorganic carbon in pore and surface waters across intact and degraded parts were assessed. On average intact mangroves showed 31% sediment organic carbon in the upper 30 cm compared to 20% in degraded mangrove areas. A loss of 1.51 MgCO2 ha−1 yr−1 for degraded sites was calculated. Water samples showed a hypersaline environment in the degraded mangrove area averaging 93 which may have caused mangrove dieback. Sediment CO2 efflux within degraded sites was lower than values from other studies where degradation was caused by clearing or cutting, giving new insights into carbon dynamics in slowly degrading mangrove systems. Results of water samples agreed with previous studies where inorganic carbon outwelled from mangroves might enhance ecosystem connectivity by potentially buffering ocean acidification locally. Wetlands will be impacted by a variety of stressors resulting from a changing climate. Results from this study could inform scientists and stakeholders on how combined stresses, such as climate change with salinity intrusion may impact mangrove's blue carbon sink potential and highlight the need of future comparative studies of intact versus degraded mangrove stands.

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.

The role of multiple stressors in a dwarf red mangrove (Rhizophora mangle) dieback

Mangroves are habitat-forming foundation species that provide the framework for entire coastal communities. Unfortunately, mangrove forests are declining globally, often due to multiple stressors. We report a recent mangrove dieback on Abaco Island, The Bahamas, that appears to be the result of multiple stressors. First, we investigated the role of hurricane and drought events in relation to green vegetation in the dieback area, as measured using Normalized Difference Vegetation Index (NDVI). We used historical Landsat imagery of the dieback region from 1989 to 2013 to identify changes in green vegetation and to determine when the dieback likely began. We determined that the dieback began around 2008, but found little evidence of a relationship between NDVI and hurricane or drought events, particularly in relation to the estimated start of the dieback. Preliminary observations in the dieback area suggested grazing by leaf-chewing organisms and disease were both present at high levels. We surveyed individual mangrove trees to determine the intensity of leaf-chewing herbivory and foliar disease on mangroves in the dieback area. We found leaf-chewing herbivory ranged from 0 to 79%, with a mean of 29% of leaves chewed per tree and disease incidence ranged from 0% to 97% with a mean of 47%. Finally, we conducted a simulated grazing experiment to test if experimental opening mimicking grazing could facilitate disease infection in mangroves . Experimental opening of plant tissues showed that grazing can increase disease. Our results suggest that drought and hurricanes did not initiate this dieback, but that herbivory likely facilitated the spread of disease thereby contributing to the dieback.

Annual variations in regional mangrove cover in southern China and potential macro- climatic and hydrological indicators

Abstract Most previous studies on mangrove cover have focused on a single study site or environmental factor. There is a lack of regional studies that consider the integrated effects of multiple climatic and hydrological factors on mangrove cover. Multi-year-averaged data can reveal general relationships between mangrove cover and macro- climatic and hydrological conditions but often obscures the relationships at a finer temporal scale. Using data from 1990 to 2011, we extracted the annual mangrove area for one tropical and six subtropical important mangrove nature reserves in southern China. Annual and categorized macro- climatic and hydrological variables (temperature, precipitation, solar radiation and evapotranspiration) were also derived for each site. We then applied Kendall rank correlation and Bayesian regression to identify the relationships between mangrove variables and macro- climatic and hydrological variables. To investigate and validate the relevance of key factors that affect mangrove growth, we analyzed the changes in patterns of macro- climatic and hydrological conditions using the Pettitt test and a t-test. The tropical mangrove area exhibited only limited correlations with these factors, while the subtropical mangrove areas exhibited relatively strong correlations with precipitation and temperature. At the annual scale, the effects of precipitation were highly dependent on the local salinity background and the salt tolerance of dominant species and were stronger in drier regions and in drier periods. In subtropical regions, an increase in temperature was always positively correlated with mangrove expansion. The percentage of hot days (>30 °C) in a year, as opposed to cold days, can better explain mangrove area variations. Our results indicate that subtropical regions are more influenced than tropical regions by macro- climatic and hydrological factors. Precipitation and temperature are the governing macro- climatic and hydrological factors associated with mangrove area variations. Although previous studies have reported a positive relationship between precipitation and mangrove growth, the relationship is not necessarily positive at finer temporal resolutions. While freezes are recognized as one of the main causes of mangrove die-back in many other parts of the world, indices based on hot weather may be more suitable for explaining and predicting changes in mangrove areas in warmer regions such as southern China.

Land Cover Mapping using Digital Earth Australia

This study establishes the use of the Earth Observation Data for Ecosystem Monitoring (EODESM) to generate land cover and change classifications based on the United Nations Food and Agriculture Organisation (FAO) Land Cover Classification System (LCCS) and environmental variables (EVs) available within, or accessible from, Geoscience Australia’s (GA) Digital Earth Australia (DEA). Classifications representing the LCCS Level 3 taxonomy (8 categories representing semi-(natural) and/or cultivated/managed vegetation or natural or artificial bare or water bodies) were generated for two time periods and across four test sites located in the Australian states of Queensland and New South Wales. This was achieved by progressively and hierarchically combining existing time-static layers relating to (a) the extent of artificial surfaces (urban, water) and agriculture and (b) annual summaries of EVs relating to the extent of vegetation (fractional cover) and water (hydroperiod, intertidal area, mangroves) generated through DEA. More detailed classifications that integrated information on, for example, forest structure (based on vegetation cover (%) and height (m); time-static for 2009) and hydroperiod (months), were subsequently produced for each time-step. The overall accuracies of the land cover classifications were dependent upon those reported for the individual input layers, with these ranging from 80% (for cultivated, urban and artificial water) to over 95% (for hydroperiod and fractional cover). The changes identified include mangrove dieback in the southeastern Gulf of Carpentaria and reduced dam water levels and an associated expansion of vegetation in Lake Ross, Burdekin. The extent of detected changes corresponded with those observed using time-series of RapidEye data (2014 to 2016; for the Gulf of Carpentaria) and Google Earth imagery (2009–2016 for Lake Ross). This use case demonstrates the capacity and a conceptual framework to implement EODESM within DEA and provides countries using the Open Data Cube (ODC) environment with the opportunity to routinely generate land cover maps from Landsat or Sentinel-1/2 data, at least annually, using a consistent and internationally recognised taxonomy.