mangrove tree on a dark green background
Many mangroves make a mighty forest (Image: Getty/Tina Tiller)

ScienceJanuary 27, 2023

The magic of mangroves

mangrove tree on a dark green background
Many mangroves make a mighty forest (Image: Getty/Tina Tiller)

In the northern part of Aotearoa, mangroves occupy mudflats and river mouths. They’re not always loved – but given our rising sea level, maybe they should be. 

It took a long time for Mere Kepa (Ngati Raka, Ngati Ira) to learn to love the manawa. She grew up around their forests, glossy green leaves lining the harbour around her father’s kainga. But when she was a child, these mangrove forests weren’t appreciated. “Previous generations treated the mudflats like a rubbish dump,” she says.

Kepa, a landowner in Takahiwai near Whangārei, only started paying attention to the mangroves when she was putting together a pest control strategy for her community to reduce the number of feral pigs on their land. That required thinking about the different habitats around her, including the mangrove forests bordering the estuaries. 

“I became friendlier with the manawa,” she says. “They take up a lot of the sunshine in Whangārei harbour – you can’t miss them.” 

Mangroves fill estuaries in the northern part of Aotearoa. They’re unique plants, says Julia Mullarney, an associate professor of marine science at the University of Waikato who studies how mangroves impact water flows and sediment mixing. Avicennia marina australasica, the species of mangrove found in New Zealand, has roots that spread like a web through the mud, with pneumataphores – breathing roots – popping up like pencils. Their shiny leaves protect them from seawater on one side, while their pores exude excess salt on the other, the fuzzy coating preventing the plant from losing their precious supply of water to the wind. Mangrove flowers are pollinated by land insects, including bees, then the plant grows propagules, germinated buoyant seeds which can float on the currents while finding a place to take root.

These trees have adapted to survive in an ever-changing intertidal zone, which can be bare earth in the morning and sloshing brackish water by afternoon – like scuba divers prepared for a regular cycle of immersion then exposure.

shiny reflective water and clear sky - early in the morning - a gravelly beach and water rimmed with short mangroves
Mangroves grow on the beach near Mere Kepa’s home (image: supplied)

Still, the mangrove is often underappreciated – perhaps because it blocks access to the coast, or because the muddy land where it grows only welcomes humans who really want to wade in (“I personally like walking around in mud,” Mullarney says). But just because humans can’t directly use the areas where mangroves grow doesn’t mean that they don’t enhance human life – or that their existence isn’t profoundly shaped by human activity. 

Manawa are native to Aotearoa. The grey mangrove species that grows here also lines southern Australian estuaries. In the tropics, there are dozens more species, with the trees growing up to eight or ten metres high in the warm water near the equator.

But even in Aotearoa, at the southernmost end of the mangrove’s ecological range, these trees form a forest. “I’ve walked through the manawa forest and thought ‘I’m going to get lost!’ because it’s an eerie space, thick with branches – it’s quite beautiful and scary,” Kepa says. She likes looking at the forest from above, on her land, too. “There are so many different greens, softly painted.” 

While mangrove coverage is decreasing in many parts of the world, these trees are expanding in Aotearoa. Kepa sends me a photo of a nearby beach, mirror-still water holding the sky while the manawa add a green texture wherever the water is shallow enough for them to grow. This land used to be a bare beach where people raced horses – or, more recently, quad bikes. Kepa sees the mangroves expansion as a way for this delicate ecosystem to protect itself.

Mangrove expansion is mostly due to land-use changes, says Mullarney. More intensive farming, or loss of soil cover, washes more sediment into the sea, especially the fine sediment that creates nutritious mud where the mangroves can grow. In the diaries from Captain Cook’s first visits to New Zealand, there is a description of some mangroves around the mouth of the Waihou River in the Firth of Thames, where Mullarney does field work. Today, they fill the entire river mouth. 

The impact of other factors, including warming temperatures and water pollution, on mangroves is less clear, Mullarney says. It’s possible that warmer temperatures in Aotearoa will see mangroves growing south of Kāwhia Harbour, their current southernmost point. But there are many other factors at play. 

“There’s a complex web of feedback through the system,” Mullarney says. Changing hydrodynamic conditions, meaning different patterns of water movement, could impact where mangroves are able to grow; the succulent wee propagules need settled conditions to take root. “But when mangroves establish, it changes the hydrodynamics again – it’s difficult to tease out these non-linear effects.”

Ultra-sensitive instruments help: to understand the way that water might eddy around a protruding mangrove root, Mullarney uses an instrument that spits out sounds, and measures how the flowing current changes the noise that reflects back; a pen sized instrument which sends light into the water determines the quantity of sediment based on what reflects back. She’s thinking about using drones next, but isn’t quite sure of the logistics. 

Measuring mangroves matters, because these ocean forests are a prime example of a “nature based solution” to some of the effects of climate change. Field work, lab experiments, numerical modelling and evidence from communities with and without mangroves all point to the same thing: mangroves are incredibly important for protecting coastlines. As the climate crisis causes sea level rise and more storm surges, their ability to retain sediment can limit erosion, while their trunks dissipate the power of storm surges. It’s the kind of thing that Mullarney gets excited about: her research focuses on how water movement can change in coastal settings. 

 “They offer some great ecosystem services,” Mullarney says. The settled waters around the forest are safe harbours for fish to breed in. The dense mud absorbs carbon, as do the trunks of the mangrove trees, although measuring the effect of these carbon sinks is difficult. Kaimoana, including crabs and snails, clamber along the surface, food for birds, fish, and people. The mangroves drop their leaves to help lose salt, adding lots of nutrients to the ecosystem for algae and bacteria and the rest of the food web.

Mere Kepa, who still lives beside the mangroves, hopes that other people can start to see the plants as more than tangled bushes in salty water. “I think of them as little gods,” she says. “This manawa forest is a habitat – it’s a home.” 

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