Nicolás Younes Cárdenas, James Cook University; Karen Joyce, James Cook University, and Stefan W Maier, James Cook UniversityAustralia is home to around 2% of the world’s mangrove forests and is the fifth most mangrove-forested country on Earth. Mangroves play a crucial role in the ecosystem thanks to the dizzying array of plants, animals and birds they feed, house and protect.
Mangrove forests help protect coastal communities from cyclones and storms by absorbing the brunt of a storm’s energy. They help our fight against climate change by storing vast amounts of carbon that would otherwise be released as greenhouse gases.
In other words, mangroves are some of our most precious ecosystems. Despite their importance, there is much we don’t know about these complex wetland forests. For example, when does their growing season start? And, how long does it last?
Usually, answering these types of questions requires frequent data collection in the field, but that can be costly and time-consuming. An alternative is to use satellite images. In the future, this will allow us to track the impacts of climate change on mangroves and other forests.
What is phenology?
Our research used satellite images to study the life cycles of mangrove forests in the Northern Territory, Queensland, and New South Wales. We compared the satellite images with field data collected in the 1980s, 1990s and 2000s, and found a surprising degree of variation in mangrove life cycles.
We’re using the phrase life cycle, but the scientific term is “phenology”. Phenology is the study of periodic events in the life cycles of plants and animals. For example, some plants flower and fruit during the spring and summer, and some lose their leaves in autumn and winter.
Phenology is important because when plants are growing, they absorb carbon from the atmosphere and store it in their leaves, trunks, roots, and in the soil. As phenology is often affected by environmental conditions, studying phenology helps us understand how climate change is affecting Australian ecosystems such as mangrove forests.
So how can we learn a lot in a short amount of time about mangrove phenology? That’s where satellite imagery comes in.
How we use satellites to study mangrove phenology
Satellites are an excellent tool to study changes in forest health, area, and phenology. Some satellites have been taking images of Earth for decades, giving us the chance to look back at the state of mangrove forests from 30 years ago or more.
You can think of satellite images much like the photo gallery in your smartphone: you can see many of your family members in a single image, and you can see how everyone grows and “blooms” over time. In the case of mangroves, we can see different regions and species in a single satellite image, and we can use past images to study the life cycles of mangrove forests.
For example, satellite images depicted below, which use data from the Australian government’s National Maps website, show how mangroves forests have changed in the Kimberley region of Western Australia between 1990 and 2019. You can see how the mangrove forest has reduced in some areas, but expanded in others. Overall, this mangrove forest seems to be doing pretty well thanks in large part to the fact this area has a reasonably small human population.
Our study of satellite images of mangrove forests in the Northern Territory, Queensland, and New South Wales – and how they compared with data collected on the ground – found not all mangroves have the same life cycles.
For instance, many mangrove species grow new leaves only once per year, while other species grow new leaves twice a year. These subtle, but important differences will allow us to track the impacts of climate change on mangroves and other forests.
How climate change affects mangrove phenology
Climate change is changing the phenology of many forests, causing them to flower and fruit earlier than expected.
Science cannot yet tell us exactly how mangrove phenology will be affected by climate change but the results could be catastrophic. If mangroves flower or fruit earlier than expected, pollinators such as bats, bees and birds may starve or move to a different forests. Without pollinators, mangroves may not reproduce and can die.
The next step in our research is to figure out how climate change is affecting the life cycles of mangroves. To do this, we will use satellite images of mangroves across Australia and factor in data on temperature and rainfall.
We think rising temperatures are causing longer periods of leaf growth, a theory we plan to test by studying data from now with satellite images from the 80s and 90s.
Satellite monitoring can’t do it all
Satellites can tell us a lot about how a mangrove forest is faring. For example, satellite images captured a dieback event (depicted below, using data from the Australian government’s National Maps website) that happened between 2015 and 2016, when around 7,400 hectares of mangroves died in the Gulf of Carpentaria due to drought and unusually high air and sea temperatures.
But satellite monitoring is not enough on its own and cannot capture the detail you can get on the ground. For example, satellites cannot capture the flowering or fruiting of mangroves because flowers are often too small and fruits are often camouflaged. Also, satellites cannot capture what happens under the canopy.
It is also important to recognise the work of researchers on the ground. Ground data allows us to validate or confirm the information we see in satellite images. When we noted some mangrove forests were growing leaves twice per year, we validated this observation with field data, and confirmed with experts in mangrove ecosystems. Field data is crucial to understand the life cycles of ecosystems worldwide and how forests are responding to changes in the climate.
Nicolás Younes Cárdenas, Postdoctoral research fellow, James Cook University; Karen Joyce, Senior Lecturer – Remote sensing and spatial information, James Cook University, and Stefan W Maier, Adjunct Research Fellow, James Cook University
When we think of mangrove forests, seagrass meadows and saltmarshes, we don’t immediately think of shark habitats. But the first global review of links between large marine animals (megafauna) and coastal wetlands is challenging this view – and how we might respond to the biodiversity crisis.
Mangrove forests, seagrass meadows and saltmarshes support rich biodiversity, underpin the livelihoods of more than a billion people worldwide, store carbon, and protect us from extreme weather events.
We know marine megafauna also use these habitats to live, feed and breed. Green turtles and manatees, for instance, are known to eat seagrass, and dolphins hunt in mangroves.
But new associations are also being discovered. The bonnethead shark – a close relative of hammerheads – was recently found to eat and digest seagrass.
The problem is that we’re losing these important places. And until now, we’ve underestimated how important they are for large, charismatic and ecologically important marine animals.
Counting wetland megafauna
Today our review of the connections between marine megafauna and vegetated coastal wetlands was published in the journal Trends in Ecology and Evolution. As it turns out, far more megafauna species use coastal wetlands than we thought.
Before our review, the number of marine megafauna species known to use these habitats was 110, according to the International Union for Conservation of Nature (IUCN) Red List, which assesses species’ conservation status.
We identified another 64 species from 340 published studies, bringing the total number to 174 species. This means 13% of all marine megafauna use vegetated coastal wetlands.
We predominantly documented these habitat associations by electronic tracking, direct observation or from analysing stomach contents or chemical tracers in animal tissues.
Less commonly, acoustic recordings and animal-borne video studies – strapping a camera on the back of turtle, for instance – were used.
Deepening our understanding of how species use their habitats
In recent weeks, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) released a damming assessment of humanity’s stewardship of the natural world. Up to 1 million species were reported to be facing extinction within decades.
We need to dramatically change how we relate to and engage with species and their habitats, if we are to fix this problem.
But the question is, how can we make global change real, relevant and feasible at local and regional scales? And, as the international community rises to this challenge, what information is needed to support such efforts?
Our study suggests a critical first step to addressing the global biodiversity crisis is to deepen our understanding of links between species and their habitats. We also need to elevate how the evidence is used to both assess extinction risk and prioritise, plan and deliver conservation actions.
More than half of all coastal wetlands have been lost globally and the rest are at risk from a range of serious threats, including deforestation. There is an urgent need to limit and reverse the loss of coastal wetlands to stop biodiversity loss, protect communities and tackle climate change.
Targeting places where high rates of mangrove loss intersect with threatened megafauna could lead to more efficient and effective conservation outcomes. Southeast Asia, Mexico and northern Brazil are such places.
In Southeast Asia, for example, the world’s largest mangrove forest is losing trees at a rate far exceeding global averages, largely due to aquaculture and agriculture. This is threatening the critically endangered green sawfish, which relies on these mangrove habitats.
Habitats should always be considered in assessments
The IUCN Red List assesses the extinction risk for almost 100,000 species. It provides comprehensive information on global conservation statuses, combining information on population sizes, trends and threats.
The wealth of data collected during species’ assessments, including habitat associations of threatened species, is one of the Red List’s most valuable features.
But our study shows many known associations are yet to be included. And for more than half of the assessments for marine megafauna, habitat change is yet to be listed as a threat.
This is concerning because assessments that overlook habitat associations or lack sufficient detail, may not allow conservation resources be directed at the most effective recovery measures.
But it’s also important to note habitat associations have varying strengths and degrees of supporting evidence. For example, a population of animals shown to consume substantial amounts of seagrass is clearly a stronger ecological link than an individual simply being observed above seagrass.
In our paper, we propose a simple framework to address these issues, by clarifying habitat associations in conservation assessments. Ideally, these assessments would include the following:
- list all habitat types the species is known to associate with
- indicate the type of association (occurrence, grazing, foraging or breeding)
- cite the source of supporting evidence
- provide an estimate of the level of habitat dependence.
Data for decision making
Habitat loss is accelerating a global extinction crisis, but the importance of coastal habitats to marine megafauna has been significantly undervalued in assessments of extinction risk.
We need to strive to protect remaining coastal wetland habitats, not only for their ecological role, but also for their economic, social and cultural values to humans. We can do this by strengthening how we use existing scientific data on habitat associations in species assessments and conservation planning.
Michael Sievers, Research Fellow, Global Wetlands Project, Australia Rivers Institute, Griffith University; Rod Connolly, Professor in Marine Science, Griffith University, and Tom Rayner, Science Communicator, Griffith University
Humans have become adept at destroying natural habitats. Indeed, we’re so good at it we’ve changed the very makeup and climate of our planet. But there may be signs the natural world is fighting back by protecting itself against rising temperatures and changing weather patterns, and we face the tantalising prospect of helping this process.
A recent study found that mangrove forests could be adapting to climate change by growing beyond their usual range. The risk of several days of continuous frost, which previously kept these trees in tropical and subtropical areas near the equator, is continuously shifting towards the poles. As average global temperatures rise, mangroves are able to increase their growth and expand their range beyond the equator.
Mangrove forests are coastal wetlands made up of a dense jumble of trees and shrubs capable of living in salt or brackish water. Famous for their tangle of roots sticking up from the ground and dropping down from branches, mangrove forests can grow out into the sea and create almost impenetrable mazes of narrow channels along shorelines.
Mangroves protect coastlines, treat polluted waters, provide livelihoods and resources for some of the world’s poorest people and are home to an impressive number of species – many of which are commercially important. It’s been suggested that the majority of the global fish catch relies, either directly or indirectly, on mangroves.
Despite their value, humans have also done an impressive job over the last century of destroying them to make way for coastal developments, aquaculture and by logging them for timber and fuel production. Not to mention destroying their natural water courses and polluting the ground they grow in.
So the possibility that climate change could be benefiting these habitats is promising indeed. In the long run, this could help society adapt to climate change and even reduce the concentration of greenhouse gases in the atmosphere.
Adapting to climate change
One feature of mangroves that we’ve long benefited from is the protection they offer to our coastlines. Waves lose their power passing through dense mangrove forests, and they can offer protection from storms, typhoons, hurricanes and tsunamis.
Their mass of roots –- both above and below ground – help to bind and build sediments, meaning mangrove areas can grow vertically, which is a clear asset in the face of rising sea levels. Expanding mangrove forests could therefore help protect us from the devastating effects of extreme weather that become more likely with climate change.
Mangrove forests are also incredibly productive ecosystems, which means that lots of carbon dioxide is taken in and used by the trees and shrubs as they grow. When this organic matter dies, a proportion of it forms the sediment underneath the mangrove forest. As a result, carbon remains trapped as semi-decomposed plant matter, and is unable to re-enter the atmosphere as a greenhouse gas. This ensures mangroves can actually act as giant stores – or sinks – of carbon.
Research suggests that mangroves could be better carbon stores than the coastal habitats they are encroaching on – opening the possibility for mangroves to combat the very causes of global warming. In this way, mangroves act as Earth’s natural defences to climate change –- protecting the planet by striking at the very cause of the problem.
Around the world, some mangrove forests are being given legal protection and large-scale restoration works are taking place with varying degrees of success, as one study in Sri Lanka found.
In America and Australia work is being undertaken to restore areas of mangrove dieback following ill-considered developments and the use of herbicides. Conservationists and academics are researching where mangrove restoration would be most beneficial, and developing the best methods for these projects around the world.
The knowledge that mangroves could both benefit from a changing climate and protect us from some of its worst effects demands a renewed vigour in promoting these wetlands. It also raises a question. Should resources be ploughed into maintaining ecosystems where regional changes in the climate are unlikely to help them prosper? Or should we concentrate our efforts on helping expand habitats that are not only resilient to climate change but can help mitigate climate change itself?
Perhaps it is time to move towards the latter and act as ecosystem physicians, giving healing and healable habitats like mangroves every opportunity to do what they do best.
One of the worst instances of mangrove forest dieback ever recorded globally struck Australia’s Gulf of Carpentaria in the summer of 2015-16. A combination of extreme temperatures, drought and lowered sea levels likely caused this dieback, according to our investigation published in the journal Marine and Freshwater Research.
The dieback, which coincided with the Great Barrier Reef’s worst ever bleaching event, affected 1,000km of coastline between the Roper River in the Northern Territory and Karumba in Queensland.
About 7,400 hectares, or 6%, of the gulf’s mangrove forest had died. Losses were most severe in the NT, where around 5,500ha of mangroves suffered dieback. Some of the gulf’s many catchments, such as the Robinson and McArthur rivers, lost up to 26% of their mangroves.
The gulf, a remote but valuable place
The Gulf of Carpentaria is a continuous sweep of wide tidal wetlands fringed by mangroves, meandering estuaries, creeks and beaches. Its size and naturalness makes it globally exceptional.
An apron of broad mudflats and seagrass meadows supports thousands of marine turtles and dugongs. A thriving fishing industry worth at least A$30 million ultimately depends on mangroves.
Mangroves and saltmarsh plants are uniquely adapted to extreme and fickle coastal shoreline ecosystems. They normally cope with salt and daily inundation, having evolved specialised physiological and morphological traits, such as salt excretion and unique breathing roots.
But in early 2016, local tour operators and consultants doing bird surveys alerted authorities to mangroves dying en masse along entire shorelines. They reported skeletonised mangroves over several hundred kilometres, with the trees appearing to have died simultaneously. They sent photos and even tracked down satellite images to confirm their concerns. The NT government supported the first investigative surveys in June 2016.
In the end, the emails from citizen scientists nailed the timing: “looks like it started maybe December 2015”; the severity: “I’ve seen dieback before, but not like this”; and the cause: “guessing it may be the consequence of the four-year drought”.
Our investigation used satellite imagery dating back to 1972 to confirm that the dieback was an unparalleled event. Further aerial helicopter surveys and mapping during 2016, after the dieback, validated the severity of the event extending across the entire gulf. Mangrove dieback has been recorded in Australia in the past but over decades, not months.
Mysterious patterns in the dieback
We still don’t fully understand what caused the dieback. But we can rule out the usual suspects of chemical or oil spills, or severe storm events. It was also significant that losses occurred simultaneously across a 1,000km front.
There were also a number of tell-tale patterns in the dieback. The worst-impacted locations had more or less complete loss of shoreline-fringing mangroves. This mirrored a general loss of mangroves fringing tidal saltpans and saltmarshes along this semi-arid coast.
Mangroves were unaffected where they kept their feet wet along estuaries and rivers. This, as well as the timing and severity of the event, points to a connection with extreme weather and climate patterns, and particularly the month-long drop of 20cm in local sea levels.
Extreme weather the likely culprit
We believe the dieback is best explained by drought, hot water, hot air and the temporary drop in sea level. Each of these was correlated with the strong 2015-16 El Niño. Let’s take a look at each in turn.
First, the dieback happened at the end of an unusually long period of severe drought conditions, which prevailed for much of 2015 following four years of below-average rainfall. This caused severe moisture stress in mangroves growing alongside saltmarsh and saltpans.
Second, the dieback coincided with hot sea temperatures that also caused coral bleaching along the Great Barrier Reef. While mangroves are known to be relatively heat-tolerant, they have their limits.
The air temperatures recorded at the time of the mangrove dieback, particularly from February to September 2015, were also exceptionally high.
Third, the sea level dropped by up to 20cm at the time of the dieback when the mangroves were both heat- and moisture-stressed. Sea levels commonly drop in the western Pacific (and rise in the eastern Pacific) during strong El Niño years: and the 2015-2016 El Niño was the third-strongest recorded.
The mangroves appear to have died of thirst. Mangroves may be hardy plants, but when sea levels drop, reducing inundation, coupled with already heat-and-drought-stressed weather conditions, then the plants will die – much like your neglected pot plants.
We don’t yet know what role human-caused climate change played in these particular weather events or El Niño. But the unprecedented extent of the dieback, the confluence of extreme climate events and the coincidence with the bleaching of the Great Barrier Reef mean the role of climate change will be of critical interest in the global response to mangrove decline.
What future for mangroves?
The future for mangroves around the world is mixed. Thanks to climate change, droughts are expected to become hotter and more frequent. If the gulf’s mangroves experience further dieback in the future, this will have serious implications for Australia’s northern fisheries including the iconic prawn fishery, mudcrab and fin fish fisheries. All species are closely associated with healthy mangroves.
We don’t know whether the mangroves will recover or not. But there is now a further risk of shoreline erosion and retreat, particularly if the region is struck by a cyclone – and this may have already begun with recent cyclonic weather and flooding in the gulf. The movement of mangrove sediments will lead to massive releases of carbon uniquely buried among their roots.
Mangroves are among the most carbon-rich forests in the tropics and semi-tropics and much of this carbon could enter the atmosphere.
Now we urgently need to understand how mangroves died at large and smaller scales (such as river catchments), so we can develop strategies to help them adapt to future change.
Australia’s top specialists and managers will be reviewing the current situation at a dedicated workshop during next week’s Australian Mangrove and Saltmarsh Network annual conference in Hobart.
Penny van Oosterzee, Principal Research Adjunct James Cook University and University Fellow Charles Darwin University, James Cook University and Norman Duke, Professor of Mangrove Ecology, James Cook University
Mangroves are some of the world’s most important trees. They provide food and resources for people and animals, protect coasts, and store huge amounts of carbon. The world’s largest mangrove forest – the Sundarbans in the Bay of Bengal – supports millions of livelihoods. In terms of the services they provide, they are worth nearly US$200,000 per hectare per year.
But these coastal forests are threatened by rising seas and human development. In a study published today in Nature, we show that some of these forests will drown unless we help them.
Getting to the root of it all
Mangroves grow along tropical coasts. Unique amongst the world’s plants, they can survive in salt water and can filter seawater. The rain of leaf-fall from tropical mangrove forests provides food for crabs and other herbivores, the foundation of a food web that extends to fish (and therefore people) right across the tropics.
One of the distinguishing characteristics of mangroves are their roots, used to anchor the plant on unstable ground and buttress against wind, waves and currents. The form of root architecture varies greatly between families of mangrove, including the dense prop-roots (Rhizophora), cathedral-like buttresses (Bruguiera), and numerous pneumatophores – literally narrow breathing–tubes – of the common grey mangrove of southeast Australia (Avicennia).
A high proportion of the living mass of mangroves exists below-ground. This means mangroves are the most efficient ecosystem globally in the capture and sequestration of atmospheric carbon dioxide. The uniquely oxygen-poor, salty characteristics of mangrove soil provides the perfect setting for long-term preservation of carbon below ground. The typical mangrove forest sequesters several times more carbon dioxide than a tropical rainforest of comparable size.
Mangrove roots trap sediment as currents carrying suspended particles are intercepted and slowed. Between the carbon sequestered below-ground, and the sediment trapped within the tangle of roots, mangroves are effectively able to raise the height of the land over time.
Keeping up with rising seas
Analysis of these sediments shows mangroves can deal with low to moderate sea-level rise by building up land. But how will mangroves respond to future rising seas when people are in the way?
We and other colleagues measured how fast mangrove forests in the Indo-Pacific region increase the height of the land. We used a tool called Surface Elevation Table-Marker Horizon, as you see in the video below.
Mangroves also build up land height by accumulating roots below ground. Previous studies have focused on this. Our study, using up to 16 years of data across a range of coastal settings, shows that sediment build up is also important.
We also compared the rate of land height increase in mangroves to local tidal gauges, to assess whether mangroves were keeping pace with the local rate of sea-level rise.
In most cases (90 out of 153 monitoring stations) mangroves were lagging behind. This is not an immediate problem if mangroves are already high enough to delay the effect of expected sea-level rise. However, mangroves at the low end of their elevation are highly vulnerable.
We used this insight to model how long mangroves might survive rising seas across the Indo-Pacific. We used a range of sea-level rise projections from the Intergovernmental Panel on Climate Change, including a low-range scenario (48 cm by 2010), high-range (63 cm by 2100) and extreme (1.4 m by 2100).
Mangrove forests with a high tidal range and/or high sediment supply such as Northern Australia, eastern Borneo, east Africa and the Bay of Bengal proved to be relatively resilient. Most of these forests will likely survive well into the second half of the century under low and moderate rates of sea-level rise.
The prospect of mangrove survival to 2070 under the 63 cm and 1.4 m scenarios was poor for the Gulf of Thailand, the southeast coast of Sumatra, the north coasts of Java and Papua New Guinea and the Solomon Islands.
Dams holding mangroves back
Our results imply that factors that prevent sediment building up may prevent mangroves responding to sea-level rise. This might include dams holding sediment within water catchments.
This impact is already being felt. An 80% reduction in sediment delivery to the Chao Phraya River delta has, for example, contributed to kilometres of mangrove shoreline retreat.
Similar developments are planned for the Mekong River. These threats compound those already being felt, including the widespread conversion of mangrove to aquaculture.
Appreciation of the financial contribution of mangroves has been slowing the trend of decline. However, long-term survival will require planning that includes both the continued provision of sediment supply, and in many cases the provision of retreat pathways, to allow mangroves to respond to sea level in ways they always have.