Removing mangroves often doesn't end well, but Niwa has found out what does work
Removing mangroves rarely results in the desired return of a sandy beach, a Niwa study has found, but there are ways to improve the chances of success.
A guide based on the research was launched on Friday, and warns that mangrove removal often leaves areas looking unattractive and smelling bad.
NIwa researchers examined around 40 removal sites with varying characteristics - small to large, sheltered to exposed, muddy to sandy. They found removal of mangroves rarely resulted in a return of sand flats.
Many mangrove removal areas showed immediate and long-term adverse impacts, the report said. Those impacts included sediments that lacked oxygen, were often black in colour and smelled of sulphur.
There was only minimal dispersal or decomposition of plant remains, high levels of hydrogen sulfide associated with rotting plant material, bacterial mats, and large and prolonged algal blooms. Vehicle track marks could be seen for years after removal.
While sandier sediment could return, it was unlikely in areas with little tidal and wave action.
Plant material should always be taken right away from removal sites, the report said. "Post-removal monitoring and observations have revealed that the mulched material often does not get washed away and decomposes slowly, resulting in long-term adverse impacts, which have been observed to continue at least five years after removal."
At some places where mangroves were removed illegally, vegetation was left behind, and studies showed woody debris was likely to take decades to decompose.
The best chance for a return of sand was when a strip of plants was removed from the seaward side of a mangrove forest, the report said.
"Strip clearances on the shoreward side of a mangrove forest have limited flow and are slow to recover. Strip pathways through the middle of mangrove forests are rapidly recolonised by seeds from neighbouring trees. Large clearances are also slow to recover."
The part of mangroves below ground pose a big problem.
Up to 87 per cent of total biomass in New Zealand mangrove forests was below ground, and could take decades to decompose after the above-ground part of plants was removed, the report said. High densities of root material remained at many sites years later.
A trial was carried out using a digger to remove the main stump and surrounding dense root material of mangroves. But the root material was fine and soft, and the efforts to remove it were largely unsuccessful and caused considerable sediment disturbance.
Mangroves live in the northern half of the North Island and have expanded the extent of their coverage during the past half century, mainly seaward across tidal flats, the report said.
The expansion was thought to be a result of increased sediment runoff from surrounding land, and such things as the building of causeways.
"This has increased the suitability of many estuarine areas for mangroves, through increasing muddiness, reducing current flows and exposure, and increasing the height of tidal flats."
While applications for permission to remove mangroves had increased in recent decades, removal of the plants was only a "temporary fix" unless the amount of land-based sediments and nutrients entering waterways was minimised, the report said.
Mangroves were also an important bird habitat, used for foraging, roosting and nesting, but weren't so important for fish.
As a way to avoid the long-term problems caused by the use of heavy machinery and vehicles in the removal of mangroves, a hovercraft with a mower for controlling the spread of seedlings was being trialled on estuaries within Tauranga Harbour.
The cost of resource consents for mangrove removal ranged between $12,000 and $230,000 per operation, the report said. It put the cost of removal at between $1000 and $5000 per hectare, but also said removal costs were known to have exceeded $50,000 per hectare. Ongoing seedling removal cost about $1000 per hectare annually.