Ken Rayward from the Mangawhai Harbour Restoration Society is working with his community to free their estuary from mangroves. Photos / Michael Cunningham
New Zealand is home to 602 different species of native trees and shrubs today.
For the past century, Kiwis have fought hard to protect their fauna after the arrival of Europeans and later, the spread of devastating plant diseases cost us large amounts of biomass.
Yet there is one tree that stands out in the endeavor to preserve our natural environment – the mangrove.
Mangroves have noticeably spread along New Zealand's coast and have long sparked heated discussions amongst different lobby groups questioning whether or not they should be allowed to grow.
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While generations of Kiwis learned that mangroves are a vital part of our ecosystem and provide a habitat for wildlife, it has been a thorn in the side of coastal communities who see their harbours being taken over by the saltwater tree.
Increasingly in the past two decades, local authorities and researchers have questioned the value of mangroves and it has come to light that we don't know all that much about the manawa, as called in te reo.
Most of our assumptions derive from the knowledge we have about foreign mangroves species creating a conservative stance on the mangrove population.
With newly emerging data and public debates, the notion is on the verge of change. But what do we actually know of the manawa in New Zealand?
Seventy per cent of all mangroves species grow in tropical climates, between latitudes 25° N and 25° S, and gather in thick saline woodlands.
Walking through a tropical mangrove forest is an ear-shattering experience.
The abundance of insects, birds and other animals create a harmonised orchestra that reaches incredible volumes. It is undeniably a biotope that is home to countless organisms.
Walking through mangroves in our local realms is quite the contrast. It's a silent, almost eerie experience.
Kiwi mangroves are clearly lacking the avian and marine orchestra, indicating life isn't as abundant here.
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Subspecies Avicennia marina, our indigenous mangrove that expands in coastal areas between Northland and the Waikato, grows at as far as 38° S – some 1500 km away from their tropical cousin.
Here, the trees reach a maximum height of 6m whereas black mangroves in the Americas and Africa, for example, can grow to 25m and higher.
From a visual and acoustic point of view, there are apparent differences between both.
Despite their geographical distance and variation in appearance, New Zealanders have been attributing the same biological properties to our local mangroves as known from their distant counterparts.
Presumably, this is because there was no empirical data about the Avicennia marina until recently.
Andre LaBonte is an ocean and coastal engineer from Waipu who has been advocating a shift in perception towards mangroves for the past two decades.
He and his wife Robin, who is also a coastal engineer, had moved to Northland in 1984.
Knowing mangroves from home in Florida, their interest was sparked when they noticed the large expansion of manawa here.
"Since there was not a lot of data available, we studied historical aerial photos and talked to people who had been living here all their lives," LaBonte says.
"There was this profound moment when we realised that back in the 40s, there were no mangroves around the Mangawhai harbour area.
"People said during their childhood, they would go swimming along sandy beaches. Then the mangroves spread explosively."
Research about the spread of mangroves pre-European settlement is inconclusive. It is believed by some that farming, deforestation and land development initially caused the mangrove population to retreat.
However, studies of pollen trapped in sediment cores dating back 2500 years showed that the New Zealand temperate mangrove was only present in small numbers in the upper reaches of estuaries.
Northlanders are witnessing a massive boost of growths since the 70s. The mangrove expansion is favoured by road constructions, housing developments, and growing livestock numbers, as they increase sediments in our waterways.
Mangroves roots spread wide rather than deep into the ground to anchor the plant in the muddy, intertidal ground.
The extensive root system gradually stabilises the surrounding soil creating more land for mangroves to grow. They eventually build up a buffer zone that stops erosion along the shore.
While conservation groups and councils regard this as an asset to the coastal environment, LaBonte says the mangrove expansion caused ecological issues.
"It's a misconception that mangroves stop erosion. They only establish where there is no erosion in the first place as they need calm conditions," LaBonte says.
"Instead, they are displacing the habitat of other organisms. Shellfish, fish and birds don't live in the mangroves but in or on the sand flats that are being taken over by mangroves."
LaBonte is working with Mangawhai Harbour Restoration Society to manage the mangrove expansion in their estuary.
In 2013, they received resource consent from the Northland Regional Council to remove 20ha of mangroves.
Ken Rayward is part of the executive committee and spokesman of the Mangawhai Harbour Restoration Society.
He says 25 years ago, the historic inlet was closed and the lower harbour was stagnant.
The infilling of the upper harbour with mangroves above the causeways likely contributed to the closure of the inlet.
"The mangroves are a tragedy in the making," Rayward says.
"There are mangroves that are useful to the environment. But not in Mangawhai; they have no advantages here. They have been clogging up our harbour and waterways."
He says as guardians of the harbour, the society protected their local birdlife by moving harbour sediment out to the Wildlife Refuge, a sand spit on the coastal side of the inlet.
The sand spit is a popular bird nesting site, however, it's prone to erosion and "every grain of sand" was required to keep that habitat alive, Rayward explains.
Not only the geological benefits and deficits are a controversial topic between different agencies.
Wildlife protection is another wildly discordant debate which is complicated by the lack of fauna related research.
A report by Niwa for the Auckland Regional Council records two invertebrate species – the mangrove leafroller and an eriophyid mite – that fully depend on the mangroves.
Further, they mention yellow-eyed and grey mullets, short-finned eels, as well as sand and yellow-belly flounders which have been sighted inside the mangroves.
Yet, the fish have to migrate daily with the tidal flow and Niwa concludes that it was unlikely the mangroves are used as a spawning ground for fish.
According to the report, birds will use the margins of mangrove forests for foraging. While some bird species made "extensive use for roosting, feeding and breeding", it says none were "totally dependent" on them.
On the other hand, LaBonte says they had not come across a single bird's nest while removing trees in Mangawhai.
Recent surveys conducted for the Mangawhai Harbour Restoration Society show, instead, that mangrove removal sites are extensively adopted by birds.
Four bird counts – one pre- and three post-removal – were conducted as part of the Mangawhai mangrove management.
Results show that the population of the endangered eastern bar-tailed godwit has increased in the area since the mangroves had been removed.
The godwit migrates from Alaska to New Zealand and is at risk in New Zealand, and threatened overseas.
The summer population suffered an 18 per cent decrease between 1993 and 2003, presumably because their foraging grounds had been taken over by mangroves.
Since mangroves have been kept at bay in the estuary, the number of godwits landing in Mangawhai has grown again.
According to the survey, critical numbers of New Zealand dotterels are also utilising the restored estuary, which is considered significant by ornithologists.
However, conservation groups fear that the mangrove removal causes loss of habitat for other birds.
Forest and Bird have been appealing the mangrove removal since its proposal in 2011 to protect the critically endangered fairy tern.
"Mangroves are an integral part of fairy tern (tara-iti) habitat," Nick Beveridge, regional manager at Forest and Bird, says.
"One of their important functions is to trap the sediment resulting from upstream land disturbance, enabling tara-iti to forage for fish in the clear water of the main channels."
When the mangroves are removed, the trapped sediment is released. Because of the increased turbidity, the birds cannot see the fish and are not able to get enough food, according to Beveridge.
The Wildlife Refuge – the sand spit at the coastal site of the harbour, opposite of where the mangroves have been removed – is one of the breeding areas for fairy tern.
Their total population has fallen to just 35 birds; they are at risk of becoming extinct.
In 2012 the Environment Court ruled that the mangrove removal in Mangawhai had no impact on the fairy terns' food resources.
The harbour society points towards extensive commercial sand mining close to the Wildlife Refuge as a possible disturbance of the fairy tern habitat.
The Mangawhai Harbour Restoration Society and Forest and Bird are currently appealing proposed amendments to the NRC Regional Plan which would see changes in the mangrove management.
Currently, no mangroves can be removed, pruned or otherwise interfered without the council's consent.
Apart from Mangawhai, several other communities in the region, including Ruakaka, Totara North in Whangaroa Harbour, Chuck's Cove in Doubtless Bay, Pataua, Kaimaumau in Rangaunu Habour, Tauranga Bay and Whananaki, have been removing mangroves.
The proposed changes are aimed to make small-scale mangrove removal more accessible for coastal communities.
"[The] Proposed Regional Plan includes a rule allowing the removal of seedlings," Ben Lee, NRC policy and planning manager, says.
However, there was less relaxation for larger-scale removal, which is in the aspirational focus of the Mangawhai Harbour Society.
Lee says the changes were taking a more strategic approach towards the mangrove removal and aimed at more clearly identifying areas where mangroves are vital for the environment.
It will remain a case-by-case decision whether council deems mangrove removal appropriate or not.
While Forest and Bird are lobbying to conserve the fairy tern habitat, the harbour society says the proposed changes were in no way more relaxed and attaining resource consent was still an unreasonably large and cost-intensive bureaucratic obstacle.
A final, more subjective, point of discussion is the aesthetic nature of mangroves.
Boardwalks through the manawa offer an appealing outing for some, yet others complain that the mangroves hinder recreational use of the shoreline.
While that is a matter of personal preference, the issue remains that mangroves have to be managed in one way or the other.
Since the trees cunningly create their own ground to grow on, we can expect to see more mangroves along the Northland coast if they're not kept at bay.
Decision-makers will have to abandon historic myths and study the data at hand to make an informed choice that'll ultimately benefit our natural environment.
Until then, the silent mangroves will spark more fire and fury.
Statistics
6300 hectares of mangroves spread through Northland
80 per cent of Northland's mangroves are found in Rangaunu, Bay of Islands, Hokianga, Kaipara and Whangārei
There are 70 different species of mangroves with 30 occurring in Australia and one in New Zealand
The root system of a mangrove tree can spread five times the diameter of its canopy
The latest news bites from around the region.