Go slow with the mangrove clearing
Mangroves get a bad rap and even Dr Erik Horstman admits he dislikes them sometimes. The researcher from the University of Waikato’s Coastal Marine Group says mangroves play important roles in North Island estuaries. New Zealand should be conservative about clearing mangroves because they will almost certainly help fight climate change, sea-level rise and coastal inundation.
Research shows clearing mangroves does not return the area to its former condition quickly. ‘‘In contrast to the global trend of mangrove decline, New Zealand mangroves are rapidly expanding,’’ Horstman and colleagues wrote in the New Zealand chapter of a new global book on the species called Threats to Mangrove Forests: Hazards, Vulnerability, and Management.
That expansion is down to widespread deforestation and the subsequent erosion has greatly increased the availability of the sediments that mangroves flourish in, Horstman wrote, relying on earlier research.
Rapid expansion is not caused by global warming, he said in an interview. New Zealand mangroves are confined to the northern half of the North Island. These places are generally free of frost, which is the critical limiter for the mostly tropical species.
Their range, covering about 26,050 hectares, ends at Ohiwa on the east coast and Kawhia on the west coast. There’s little habitat immediately south of those places that suit mangrove; there’s too much rock and not enough mudflat, he said.
Where they do thrive – in the estuaries, lagoons, tidal creeks and river mouths of Northland, Auckland, Coromandel and Tauranga – mangroves act as ‘‘ecosystem engineers’’. Their branches and roots create ‘‘drag’’
There is little scientific evidence to suggest that a return to a sandflat system will occur following mangrove clearance. Erik Horstman
that removes energy from the system and dissipates swell and wave energy.
Sea-level rise and increased coastal inundation are expected outcomes of climate change. Mangrove ‘‘can act as an ‘ecodefence’, providing a protective and resilient barrier against wave impact and mitigating against coastal erosion’’, Horstman and colleagues wrote.
His future research will in part look at whether mangroves living in front of a stopbank could protect the stopbank. If so, then smaller and cheaper stopbanks could be built. But it’s also important to know if the mangrove forests’ resilience will allow them to last and to keep up with sea-level rise, he said.
Mangrove ecosystems also capture carbon. It’s been estimated they account for as much as 14 per cent of carbon sequestration by the global ocean, the chapter reports.
Mangroves are often blamed for lowering the amenity value of coastal areas and the word ‘‘muddification’’ has been used to describe their effect.
Mangrove removal has been allowed in recent years. ‘‘However, there is little scientific evidence to suggest that a loss of mud and return to a sandflat system will occur following mangrove clearance,’’ Horstman wrote.
Five years after 110ha of mangroves were removed from three Tauranga Harbour sites, a study found degraded macrofaunal communities, anoxic sediments, low rates of sediment erosion, and extensive macroalgal blooms – none of which were positive.
A type of mangrove was present on Zealandia 19 million years ago. The single species here now, Avicennia marina, is a native that was present at least 14,000 years ago, pollen records show. It’s also present in southeast Australia and New Caledonia. Ma¯ ori call it ma¯ nawa and traditionally gathered fish and shellfish in mangrove forests.