Staying Afloat  

April 1, 2024

By Sandy Ong

The Hawaiʻi of today is very different from what Seana Walsh remembers growing up.  

“The highest tides now are much higher than they used to be in the past, washing onto roads and beaches disappearing,” says Walsh, a conservation scientist and scientific curator of living collections at the National Tropical Botanical Garden (NTBG). “There’s lots of areas experiencing coastal erosion too.”

Walsh isn’t alone in her observations. Similar scenes are playing out across the world, from Bangladesh to Tuvalu, as coastal communities grapple with rising sea levels. 

On average, ocean levels across the globe have swelled more than eight inches since the 1880s. Along Hawaiʻi’s coasts, the sea is now ten inches higher than it was 75 years ago. And because of unabated global warming, it continues to rise at a rate unprecedented in human history — 0.18 inches a year, nearly double that of two decades ago and triple the average throughout the twentieth century.

To Walsh, these changes mark a worrying development for the plants she cares so deeply about. As sea levels rise, coastal plant communities are increasingly submerged in salty water. “Even though something occurs along the coast, they might not, up until now, be inundated with water,” she says. 

The ramifications on plant health are something that scientists, such as Walsh and NTBG’s scientific curator of seed conservation Dustin Wolkis, are working hard to understand in a bid to protect plant species from climate change.

Breaching Thresholds

Global warming is driving sea level rise in two main ways: thermal expansion (ocean water heating and expanding) and the melting of ice sheets and mountain glaciers. The latter adds an estimated 750 billion tons of water to the world’s oceans every year.

By 2050, we can expect sea levels to rise a foot, regardless of how much carbon emissions we manage to slash today. In the worst-case scenario — one where we keep burning fossil fuels indiscriminately and all glaciers and ice-sheets end up melting — ocean levels could rise 216 feet in a couple of centuries.

Even absent such a doomsday scenario, sea level rise will have far-reaching consequences, with the danger especially acute for the approximately 900 million people, or nearly 10 percent of the world’s population, living in low-lying coastal areas. “We would witness a mass exodus of entire populations on a biblical scale, and we would see ever-fiercer competition for fresh water, land, and other resources,” warned United Nations Secretary-General António Guterres last year. 

Not only will higher sea levels threaten critical infrastructure such as roads, water supplies, and Internet cables, they will also result in more frequent high-tide flooding, more powerful storm surges, and more dangerous cyclones.  

Animals, too, will be affected as their habitats are encroached upon — with rising seas estimated to threaten the survival of one in six federally protected species in the U.S., including the Hawaiian monk seal and loggerhead sea turtle.

Coastal plant communities won’t be spared either. Protecting plants from this influx is critical for both guarding against climate change, as well as adapting to it. Plants growing along the coast provide important ecosystem services that humans rely on: they sequester massive amounts of carbon from the atmosphere, help stabilize the shoreline, and protect people against cyclones and storms. 

Plus, many such plants tend to be native species that occur in narrow coastal habitats, making their protection even more paramount. “They have ecological, economic, and cultural value,” says Walsh.

Mangroves and tidal marshes, as well as coral reefs and other coastal communities, “act as a buffer between the ocean and land,” explains Neil Saintilan, an environmental scientist at Macquarie University in Sydney, Australia. “They absorb the impact of wave action, prevent erosion, and are crucial for biodiversity of fisheries.”

Saintilan offers up the example of the tea tree (Melaleuca alternifolia), which grows near streams and in swamplands of eastern Australia. The Indigenous Bundjalung people use leaves and oil from the tree to treat infections, coughs and colds, as well as to repel mosquitoes. Its antibacterial and medicinal properties are so culturally important that tea tree lakes, where the leaves fall into and slowly ‘stew’, are used for child birthing ceremonies.

“Historically, they’ve been very important to Indigenous communities and there’s a lot of uses for these trees,” says Saintilan. “But of all our vegetation types, they’re the ones that have so far been most heavily impacted by sea level rise.”

Swampy forests aside, the phenomenon is also “one of the biggest threats” to mangroves, says Daniel Friess, a coastal scientist at Tulane University in New Orleans.

In 2022, Friess co-authored a paper that predicted how various climate change stressors — including atmospheric carbon dioxide concentrations, temperature, precipitation change, and increased cyclone activity — would impact mangrove ecosystems. “Certainly, globally, we think mangroves are going to be more affected by sea level rise than by anything else,” he says.

Tolerance, not Love

A common misconception is that coastal plants, by virtue of their habitat, love seawater. “The thing is, they hate it. The salinity, the flooding, are all really stressful on them,” says Friess. “They’ve just managed to adapt to it in ways that other plants haven’t been, so they tolerate it.”

But with rising sea levels, “we’re pushing them really close to or beyond their tolerance threshold,” he cautions. 

Saintilan estimates that the upper limit for most mangroves is roughly 0.28 millimeters of sea level rise annually, “beyond which the ecosystem fails to keep up with the change.” We’re currently just over half that rate, he says.

But some places seem to have already reached a tipping point. Take the Myall River north of Sydney, for instance. “The mangroves look completely healthy except for maybe the lower 20 meters where they’re all dead. You’ve just got these ghost trees without any leaves, looking quite white and bleached,” Saintilan says. “They’re just too low now, given the water levels there remain sustained.”

Judith Weis, a marine biologist at Rutgers University in New Jersey, has witnessed something similar happen with salt marshes halfway across the world. “In the high marsh, we see some very large expanses of water that remain at low tide, which is not supposed to be the case,” she says. When they’re waterlogged, mangroves and salt marshes can’t get the oxygen they need from the air. “Those plants are not adapted and will not survive if they’re underwater for hours and hours at a time.”

For seagrasses, it’s “not so much a submergence issue,” says Friess, but rather the increased amounts of water above them. “Seagrass is a plant so they still need to photosynthesize. The more water you have above your plant, the less sunlight that’s able to penetrate.”

Coping with Change

Higher sea levels can also affect plant growth in other ways, as NTBG’s Walsh, Wolkis, and other researchers from the University of Hawaiʻi reported in 2023. In a series of experiments, the team exposed the seeds of 21 Hawaiian coastal plant species (10 indigenous, 8 endemic, 3 non-native), found across a variety of coastal habitat types, to varying concentrations of seawater and studied how germination was affected. 

“It sounds a little bit like a high school science fair project, where we put seeds in seawater and see what happens,” says Wolkis with a laugh. But essentially, the team sought to answer two key questions: Can seeds germinate in salinity? And if not, can they recover when rinsed with fresh water?

Their findings were illuminating. “Although different species behaved differently, salinity exposure reduced and slowed down germination overall,” explains Wolkis. The higher the salinity, the greater the effects. For two-thirds of Hawaiian species tested, germination was inhibited by more than 90%, suggesting that coastal systems in Hawaiʻi are highly vulnerable to increases in salinity exposure.

With regards to mangroves, Friess says, “some species are more tolerant than others, and even by the end of the century, will be completely fine.” These include mangroves found in India, Bangladesh, and northern Australia, according to an analysis he co-authored in 2015. “They’re able to respond to sea level rise and keep pace because they have sediment coming down rivers that helps them build up their surface by a couple of millimeters a year. Or they’re able to track mud and produce roots to increase their elevation.”

In particular, supratidal forests, a type of coastal wetland unique to Australia, excel in this coping mechanism of “trying to keep their feet dry by building up all their mass,” says Saintilan. Tidal marshes have this ability too, although they’re less effective at doing so compared with their coastal cousins.

Apart from increasing their elevation, the other way coastal plants cope with rising sea levels is to move to higher ground. “I don’t mean a mangrove tree getting up and walking,” says Friess. 

Rather, plants at the front slowly die out and new mangroves begin to grow further inland — a process called lateral colonization that can happen within a few short years. “Over time, you’ll see that system as a whole migrate up the slope to try and keep up with sea level rise.”

Other coastal plants are capable of such landward migration too. But there’s a key limiting factor: “In so many places, there’s nowhere to go up because the land behind is developed in some way,” says Weis. There might be roads and houses; or in Southeast Asia, an aquaculture pond or palm oil plantation. “So plants get subjected to what we call ‘coastal squeeze.’”

NTBG’s Walsh adds: “In Hawaiʻi’, a lot of the lowlands are developed and even beach access for humans is an issue.”

Some cities have already begun to address this issue. In Queensland, New Jersey, and New York, for instance, there are programs that buy out land adjacent to existing wetlands. This proactive approach to creating plant ‘migration pathways’ in anticipation of future sea level rise is an important step of coastal planning, says Friess.

Another critical aspect of good coastal management, he adds, is to ensure that ecosystems like salt marshes and mangroves get sufficient new sediment so that they can build up elevation or lay down new roots inland in response to rising water levels. “Sediment supply in many rivers is declining because we’re damming rivers for other uses, which limits the amount of sediment that can be transferred from the uplands to the coastal zones,” Friess says. When scientists analyzed nearly 5,000 rivers and estuaries across the U.S. in 2023, they found that 72 percent had watersheds that were too small to generate sufficient sediment to match sea level rise.

“So what people are doing is to get elements from elsewhere and spray layers of them on top of the marsh as a way to keep them up,” says Weis about the salt marshes along the eastern U.S. coast. “This has been going on experimentally in a number of places and it’s looking fairly successful.”

But the practice is pricey and must be repeated every couple of years. Instead, she advocates alternative approaches, such as reducing erosion along the marsh edge by building ‘living shorelines’ comprising rocks, oyster reefs, or other hard materials. She also supports growing the reed grass phragmites — a controversial stance given that the plant is an invasive species in the U.S. But Weis says it “enables a marsh to elevate faster as it collects more sediments.” Phragmites also offer the added benefits of increased carbon and nitrogen sequestration, and are able to deal with metal pollutants better. “It does a lot of good things.”

One of Many

Meanwhile, conservation actions such as seed banking can help conserve threatened coastal plants. “It’s the most cost-effective and efficient means of ex situ plant conservation,” says Wolkis. “It provides a genetic safety net for safeguarding species against local extinction, and buys us some time while we figure out best management practices.”

But when it comes to protecting coastal plants, he cautions we shouldn’t just be looking at rising sea levels in isolation, but rather consider other threats induced by a warming planet. “With climate change, you’ll also get an increase in the frequency and intensity of storms, both of which will increase storm surges,” says Wolkis.

“And then in some areas, we’re experiencing drier conditions,” adds Walsh. “Which means less rainfall to rinse away salt spray so you can have more salt accumulating on plants and in the soil.”

To complicate things, there are other non-climate related factors to consider too when it comes to whether coastal plants can thrive in this era of rapid change. “In Hawaiʻi, we have problems with native pollinator loss,” says Wolkis. Throw in the persistent, ongoing threat from non-native animals and plants. “Sea level rise is another problem that’s just compounding on top of other problems that we already have.”

Friess agrees: “It’s the synergies between the different stressors that might push plants over the edge.”

To give coastal plants the best fighting chance, we need to alleviate as many of these other pressures as possible, says Friess. “So that when they have to adapt, they can focus all their energy and resources on sea level rise.”

Sandy Ong is an independent science journalist based in Singapore. She participated in NTBG’s Environmental Journalism Program in 2023. Read more of her work at www.sandyong.com

Seana and Dustin’s article was published in Annals of Botany https://doi.org/10.1093/aob/mcad129