Mangroves

Forests of the harbour

Northland is home to 6300 hectares of mangroves (manawa) which are spread throughout the region.

Eighty per cent of Northland’s mangroves are found in the following five harbours - Rangaunu, Bay of Islands, Hokianga, Kaipara and Whangārei. Each contains about 1000 hectares of mangroves – meaning a total area of about 5000 hectares.

Northland is one of only two regions in New Zealand which has extensive forests of large mangrove trees (the other is Auckland).   

There are also another 1300 hectares of mangroves spread through another 13 Northland harbours and major estuaries which each contain over 100 hectares of the plant.

Mangroves have been in New Zealand for about 19 million years.

Hokianga mangrove forest.Hokianga mangrove forest.

Researchers believe mangroves may have come here with sporadic arrivals of mangrove propagules (baby mangrove plantlets).

The kiwi mangrove is an indigenous New Zealand plant. Its extensive forests are a major Northland landscape feature.

Mangrove forests are made up of a number of different types of habitats - mangrove stands, pneumatophore zones, seagrass beds, low-tide channels, channel banks, and sandflats.

The mangrove has adapted to living in the harshest of conditions - a dunking in salt water twice a day when the tide comes in and heavy, stinky mud with no oxygen for its roots.

There are not many other flowering trees that could survive in these conditions.

Council's policy on mangroves

Mangroves are protected in Northland.

This means they cannot be removed, pruned or otherwise interfered with without Northland Regional Council consent.

Communities and organisations may however apply to Northland Regional Council to manage local areas of mangroves.

Northland’s mangroves are now managed under a 2010 change to the rules governing how the region’s 3200 kilometre coastline is used. This is Plan Change 3 (Mangroves) to the Regional Coastal Plan for Northland. It allows for mangrove management resource consents to be granted in a wider range of situations than previously.

Northland mangroves.Several Northland communities have been granted resource consent to manage their mangroves since this change.

Management options include mangrove removal and pruning along with plucking seedlings out of the ground.

Owners of an historic Totara North foreshore gum store on Whangaroa Harbour were the first in Northland to gain mangrove management resource consent under this 2010 change. This was in September 2011. The Historic Places listed building was built in 1890 and serviced gum diggers in the local area with mangrove management aimed at protecting the building.

Other consents have been granted for mangrove management in areas including Chuck’s Cove (Doubtless Bay), Mangawhai, Pataua, Kaimaumau in Rangaunu Harbour, Tauranga Bay and Whananaki. 

Where they grow

New Zealand is the most southern mangrove growing country in the world.

Its mangroves are found only in the top half of the North Island.

Hokianga mangroves.New Zealand mangroves grow in an area from near Cape Reinga in the north southwards to Bay of Plenty’s Ohiwa Harbour (near Opotiki) on the east coast and in the Kawhia Harbour on the west coast.

This is north of about latitude 38 degrees south.

Northland’s mangroves grow throughout the region’s coastal shores on the west and east coast.

There are about 70 species of mangroves world-wide.

Some countries are home to many mangrove species. New Zealand however has only one species - Avicennia marina.

Australia is home to more than 30 different mangrove species. There are 50 species in South-East Asia. The word mangrove refers to all types of trees that have adapted to living in the sea. Worldwide there are 23 genera from eight different families that have species that are described as mangroves.

Mangroves mostly grow in warmer climates, reaching their greatest size and diversity in the tropics.

Mangroves viewed from a kayak.Mangroves enjoy life without frosts.

The world’s mangroves are usually restricted to tropical climates where the coldest air
temperatures are warmer than 20°C and where the seasonal temperature range is less than 10 degrees celcius. Their geographic limits are closely linked to ground frost. Their global distribution appears closely linked with the 20 degrees celcius winter isotherm for seawater.

However, the occurrence of mangroves in New Zealand is a notable exception to this.

Mangrove map

This Northland map shows the region’s main mangrove growing areas.

A map of Northland showing where mangroves grow.

Every Northland harbour and estuary has its own area of land feeding it fresh water from rivers and streams. These areas are called catchments. 

Northland Regional Council is working with a number of communities to improve land management within these catchments.

This work is aimed at reducing the sediment and nutrients getting into harbours and estuaries. Sustainable land use practices are continually encouraged.

Increased sedimentation and nutrient quantities flowing into Northland harbours and estuaries is thought to be a cause of mangrove spread. Northland Regional Council's integrated catchment management work is being done to help manage this.

The quantity of sediment flowing into in estuaries and harbours has increased following human settlement through native vegetation clearance linked to logging, mining, farming and urban development.

Needs

Mangroves need to grow in quiet waters on gently shelving shorelines such as those inside harbours and estuaries. This is because rough, exposed waters would either uproot them or carry away the silt in which they would take root.

They can tolerate being completely submerged in seawater, but need to be uncovered for at least half of each tide so they can absorb oxygen.

Optimum growth occurs within a 60 centimetre vertical rise/fall of the tide.

Mangroves hate frost. This is why they like Northland’s warm Northland climate. The warmer the climate, the bigger they grow. 

What mangroves need.

Mangrove adaptations

Mangroves have many adaptations to live in their extremely salty environment.

These allow them to survive even when they’re standing in sea water.

The main purpose of the mangrove’s adaptations is to help the tree handle its salty inter-tidal environment. 

Too much salt
The mangrove has to regulate the amount of salt in its sap to be able to survive in seawater.

One way of doing this is to develop adaptations that manage the flow of salt and fresh water in and out of its plant cells.

This means reducing the ability of salt water to flow into its plant cells through the process known as osmosis.

Osmosis means that when two solutions of different concentrations are separated by a membrane like the type which surrounds plants cells, the more diluted liquid will flow from the less concentrated solution to the more concentrated solution until the two solutions have equal osmotic pressure. Water therefore tends to flow out of the mangrove into the sea washing around it, placing the tree in danger of dehydration.  Ironically, this danger is greatest when the tide is in.

The concentration of salt in New Zealand mangrove’s cell sap is higher than land-only plants. It has about two grams per litre in comparison with 0.2 grams per litre for land plants. The mangrove’s higher salt sap concentration helps it stop osmotic water loss from its plant tissues into the sea around it, helping it to survive in such a salty environment.

Mangroves can live in a wide range of salinity from brackish water – where the salinity is low because sea water is mixed with fresh water – through to very salty water which has been concentrated through evaporation to more than twice the salinity of sea water.

Full strength seawater has a salt concentration of about 34 grams per litre or 34 parts per thousand.

The leaves of the mangrove also secrete salt. This is another way the plant uses to regulate its salt content. This is done through hundreds of tiny salt-secreting glands on the mangrove leaf’s upper surface. These show up under a microscope as tiny pores in the leaf.

They get rid of extra salt by exuding a salty brine that is more concentrated than full-strength seawater.

The brine is easily washed away by rain due to another mangrove adaptation – tough leathery upper leaf surfaces which allow water to easily slip across their surface.

This adaptation also makes it easier to wash off salty sea splash and spray residues. 

Another mangrove adaptation for getting rid of salt is by shedding leaves.

Mangrove leaves.Mangrove leaves have several adaptations for salty living.

Mangroves lose about 60 percent of their leaves in a year. The drop is mainly of older leaves which contain the greatest salt concentrations. 

This annual litter drop can be as high as about five to six tonnes per hectare.

Leaf drop increases during summer which also helps the mangrove reduce the impact of losing water through evaporation. This increase can be 10 times the usual winter leaf drop.

Many of the leaves are trapped among the mangrove’s pneumatophores (vertical corky breathing roots that stick up like fingers out of the mud). The leaves are quickly broken down by soil fungi and bacteria to make their nutrients available for reabsorption by the mangrove. By this stage their salt content has leached out into the seawater.

These leaves and other falling plant matter play a key role in food webs of the mangrove forest and surrounding areas. The plant matter includes leaves, twigs and other woody material, flowers and fruit.

Photosynthesis in adult mangrove plants is affected by surrounding salinity. Extremes of low and high salinity both slow photosynthesis but in spite of this New Zealand’s mangrove can handle a wide range of salt-tolerance, growing in waterlogged soils across the spectrum.

Waxy leaves

Waxy leaves.A waxy coating helps mangroves survive.

Mangroves have developed a wetsuit to help them survive living in seawater. The upper surface of mangrove leaves has a thick, waxy cuticle that makes the leaves waterproof. As the tide falls away, the salty water drains away quickly from the pointed tips of the leaves.

However, the waterproof covering cannot encase the leaves entirely because the leaves still need to breathe to allow photosynthesis to take place.   (Photosynthesis is a process all plants go through to transfer sunlight and gases so they can stay healthy and grow).

Mangrove breathing pores (stomata), are tucked in sunken pits on the underside of the leaves. This protects the plant from air currents that would carry away water vapour.

These stomata allow carbon dioxide from the air to diffuse into the photosynthetic leaf tissue inside. The leaves’ underside is also coated with fine hairs that form an insulating blanket to reduce air flow.

 

Root system
The mangrove’s shallow root system spreads over a large area, extending much further than land-based trees.

Mangrove root system.Mangroves produce extensive root systems.

These extensive root rafts radiate out from the trunk, often up to five times the diameter of the canopy. Trees on the land usually have root systems that match the diameter of the canopy.

Most flowering trees only have to cope with wind. Mangroves also have to contend with water forces as tides rise and fall or when rivers are in flood after heavy rain.

The root system anchors the tree firmly in the ground and absorbs water and dissolved soil nutrients for the plant to use.

The mangrove cannot send its roots deep into the ground. This is because fine estuary silt in which it grows is usually packed so tightly it contains virtually none of the air which the tree needs to survive.

The mangrove roots intertwine forming a mesh of support.

Snorkel city
Snorkel city.A key feature of the mangrove is its many snorkel-like breathing roots that stick up above the mud. These grow along the mangrove’s shallow radiating roots. One tree may have thousands of snorkels sticking up above the mud. Pneumatophores – the name for these snorkels - means breath carriers. They serve as snorkels by carrying air down to the roots. Each pneumatophore has small port holes or lenticles in its outer surface which let air into the root when the tide is out.

Pneumatophores vary in thickness (from six mm to 12mm in diameter) and in length depending on the conditions in which the tree is growing. The more aerated the ground, the shorter the pneumatophore. A pneumatophore might grow only five centimetres tall in an open sandy site whilst it might grow up to half a metre tall in deep, sloppy dark mud. 

Children laying on the boardwalk checking out the mangroves.Checking out the mangroves' world.

Ground stabilisers

The specialised root system acts as an obstacle, slowing the flow of the water and catching the silt particles floating in the water. Over time the root systems generally stabilise the surrounding soil so that other plants can take root.

Salt marsh plants such as rushes and sedges will settle first. These are followed by bushes and hardy, salt-tolerant grasses. Eventually, this process may lead to natural land reclamation.

Flowers

Mangrove trees rely on insects to fertilise their flowers. The New Zealand mangrove's small flowers are about six mm in diameter, growing in clusters of four to a dozen blooms at the tips of branches. The four-pointed petals have a tough brown, hairy outer surface and are golden and shiny inside.

The flowers’ sweet scent entices insects. The scent smells like a blend of tropical fruit and sweet sherry. Insects attracted include the introduced European honey bee, the small native common blue butterfly and several species of native bees and small flies.

Beekeepers often move their hives close to mangrove forests during the flowering season so the bees gather mangrove flower nectar which makes a distinctively fruity flavoured honey.

Bee on mangrove flower.Bees are attracted to the mangrove flower's sweet scent. A close-up of the golden flower is inset.

Each mangrove flower is only fertile for three or four days.

The main mangrove flowering period extends over several months in later summer and autumn - between February and April.

Baby mangroves

Mangrove propagule grow on their parent tree.Mangrove propagules grow on their parent tree for up to five months.

The mangrove tree gives its young mangroves or propagules the best chance of survival by letting them develop in protective casings on the tree.

The propagules develop for up to five months while still attached to their parent plant.

These propagules or plantlets begin life as a fertilised mangrove flower. They grow into a furry, golden-covered container of future life on the tree then drop into the surrounding water to spread by floating.

At this early stage of development a propagule is effectively a ‘packaged seedling’ consisting of a developing embryo, rudimentary root and leaf structures, and two fleshy cotyledons, all enclosed by a protective cover (the pericarp).

The mangrove uses its surrounding watery environment to spread its propagules which may float off many kilometres away from the parent tree.  

This adaptation of growing young plantlets or packaged seedlings on the parent plant is another key mangrove survival trick.

Growing on the parent plant means fallen propagules are quickly ready to take root - almost as soon as they fall.

This helps make sure they can quickly get anchored to the ground between tides.

A propagule may take root quite close to its parent tree or it might float off many kilometres away. It contains its own life jacket to help it float well and a portable food supply to keep it alive on its journey.

The pericarp helps the propagule to float when it first drops off its parent plant.

Air inside the pericarp allows the propagule to float for up to three days. The pericarp splits several days after dropping to reveal the bright green smooth, shiny contents inside.

Northland propagules start falling from mangrove trees as early as late October. Peak fall comes about Christmas when hundreds can be seen along quiet Northland beach tide lines.

Fallen propagule on the beaches at christmas.Fallen propagules, 'mangroves to go' on the beach at Christmas.

These propagules are not seeds. They are ‘mangroves to go’, ready-made future plants that have already done almost half a year’s growing on their parent tree.

Conventional seeds would have a very low success rate in the salty wet mud, and the constant movement of the tide would not give the young mangroves enough time to take root.

The bright green propagule may float for some time looking for a great place to start its new life.

It might find a new place quite quickly and take root straight away. 

It might however take longer to find just the right spot. Propagules can arrive at a potential new spot and survive there for a few days only to become buoyant again after a few days and float off again on the next high tide if that first location’s not suitable for growing in. Tests indicate that propagules remain viable for at least four months.

Mangrove seedling growth rates are greatest where the salinity around them is at between 10% and 50% seawater.

Tiny mangroves.Tiny new mangroves taking shape.

Growth rates

Mangrove growth rates, size and shape differ widely. These features depend on many factors including:

  • Climate
  • Temperature 
  • Soil type
  • Water salinity
  • Shoreline position
  • Length of time standing in water 
  • Density of surrounding mangrove forest.

New Zealand’s tallest mangroves are found in the Far North, commonly growing up to nine metres tall in harbours such as Whangaroa and Hokianga.

Meanwhile, they reach about five metres in height around Whangarei, four metres around Auckland and about half a metre tall at the southern limit of mangrove habitat.

Mangrove forests act as a breakwater to reduce coastal wave erosion.

Home sweet home

Algal scum floats amongts the mangroves. Algal growth is abundant in the warm waters of the mangrove estuary, and sometimes there is such an abundance of phytoplankton that the water has a greenish tinge.

The richness of the mangrove habitat makes it an ideal spot for fish, crabs, snails and crustaceans to set up home. These attract an abundance of sea birds eager for an easy meal.

As many as 30 different species of fish can be found in the mangrove forest when the tide is high.  These include yellow-eyed mullet, grey mullet, parore, pilchards and anchovies.

Birds found amongst mangroves include white-faced heron, pukeko, banded rail, harriers, kingfishers, grey warblers, fantails, shining cuckoos, bitterns, royal spoonbills pied and little black shags.

Shorebirds such as wrybills, golden plovers, red knots and whimbrels avoid feeding amongst or near mangrove habitat.

Mangroves provide bird feeding opportunities.Mangroves provide bird feeding opportunities.

Plants found in mangrove habitats include seaweeds such as Neptune’s necklace. Nearby there are many saltmarsh plants too.

Children learn about the creatures in the ecosystem.Children learn about the creatures in the ecosystem.

Other mangrove ecosystem creatures include worms, mud snails, whelks, cockles, barnacles, mussels and oysters.

Browsing farmed cattle have destroyed large areas of mangrove forest. Cattle eat the foliage, smash branches, damage root systems and crush pneumatophores underfoot.


 

Importance to Māori

The Māori name for mangroves is manawa. Māori traditionally gathered food from among the mangrove forests. This included mullet (kanae), oyster (parore tio), sea snail (karahu) and eel (tuna).

  • Māori manawa use also included using the black earth formed by rotted mangrove leaves to dye flaxes which were used for making kits and piupiu skirts
  • A green dye was made from the lichen on mangrove trees 
  • Branches were sometimes made into fern root pounders 
  • Mangrove leaves were sometimes used to keep fish cool on fishing trips 
  • Dry mangrove wood was not used for heating hangi stones because it gave out too much heat and burned with a pungent smell.

Under the boardwalk

A pleasant way to see mangroves is in a dinghy or kayak, where the surrounding tranquility can make the rest of the world seem miles away.

Boardwalks have been constructed in several places in Northland so mangroves can be studied at close quarters without getting your feet wet. Boardwalks protect the pneumatophores - the breathing roots - and radial root systems from being crushed.

 

Whangārei
Two boardwalks meander through mature mangrove trees in the city. One is in the Town Basin area, just up the Hatea River from the Whāngārei city swimming pool complex. Access is from the swimming pool carpark in Ewing Road.

The other is on the banks of Limeburners Creek and can be accessed from Kioreroa Road.

Bay of Islands
There are two boardwalks in the Bay of Islands. The Waitangi Mangrove Walk can be found between Haruru Falls and Waitangi. It is accessed across the road from the Waitangi Treaty Grounds.

Another is between Opua and Paihia as a part of the coastal walkway. The boardwalk is accessed near Smith's Holiday Camp, Opua.

Hokianga Harbour
There is a boardwalk at Rawene, south Hokianga which is accessed from Clendon Esplanade, Rawene.

Mangroves boardwalk.Boardwalks are a great way to see mangroves.

History

Mangroves in New Zealand caused confusion for the botanists on Captain Cook's first voyage. They would have seen mangroves before, because they grow in many other parts of the Pacific.

However, the botanists found large lumps of gum on the soft ground among the trees, and so named the trees Avicennia resinifera - which means "the resin-bearing Avicennia". They did not realise the gum had actually come from kauri trees and had been washed down in flood waters.

Mangroves faced many challenges over the years. Roads were formed around coastal shores, with causeways allowing too few channels to flush upper tidal areas properly. Silt built up quickly, smothering pneumatophores and killing trees.

Rubbish was often dumped legally and illegally around mangroves. Tip sites were still being permitted in mangrove forests in the late 1970s. Land reclamation took place. Large flat areas were created as cheap industrial land in many areas. However, this sometimes led to flooding problems, because mangroves and associated wetlands can act as a buffer zone to absorb stormwater in times of heavy rainfall.

Changes in mangrove distribution
Expansion of mangroves has been occurring at least since aerial photographic records began in the late 1930s but researchers believe it to have been most active since the late 1970s.

Sequences of historical aerial photographs have been used to document mangrove distribution changes.

In some places there has been little change in distribution, often because the mangroves occupy a small estuary where their spread is restricted by the presence of a low-tide channel along their seaward edge. This means propagules cannot grow there and mangrove growth along their habitat’s landward side is limited by elevation. In these cases the canopy may become more dense over time as more trees take root and existing trees mature, but the area occupied stays roughly the same.

There are also cases where little change has occurred even when apparently suitable habitat is present.

The area occupied by mangroves increases over time in many cases. This may involve the colonisation of small harbours such as Whangape Harbour and parts of the Kaipara Harbour.

Researchers believe that in the first part of the twentieth century there was probably a significant net reduction in the area of mangroves in New Zealand. This was because coastal areas were filled in to create farmland.

Change also took place due to urban and industrial development. Grazing of livestock in mangroves was common.

Infilling to create farmland was prevented by legislation in 1977, and destruction of mangroves, where permitted, became a much more controlled activity.

Mangroves - issues, effects and perceptions

There is a range of opinions about mangroves.

Mangrove ecosystem benefits include:

  • Homes for many marine and land animals
  • Nursery habitat for short-finned eels, grey mullet and parore
  • Erosion control and shoreline protection
  • Organic material for nearby habitats
  • Reducing urban stormwater pollution
  • Bird roosts
  • Amenity value (how a place looks)
  • Tourism opportunities
  • Storm surge protection
  • Major ecosystem food web component

Mangrove ecosystem adverse effects include:

  • Colonising the habitat of other indigenous coastal flora and fauna e.g. bird roosting sites
  • Accelerating estuary infilling
  • Restricting usability of coastal structures e.g. wharves
  • Limiting road visibility
  • Loss of beach areas and open harbour spaces
  • Restriction of public access to the harbour
  • Loss of amenity values for swimmers and small boats
  • Loss of feeding areas for wading and shorebirds
  • Restriction of tidal flushing
  • Obstruction of natural and manmade land drainage channels

 

Stilts in a Northland estuary.Stilts stalk the receeding tide line in a quiet Northland estuary.

Acknowledgements: NZ Geographic magazine, Strategies for the Management of Mangrove Forests in New Zealand and the Estuaries Kit, Plan Change 3 (Mangroves) to the Regional Coastal Plan for Northland 2010, The New Zealand Mangrove:review of the current state of knowledge. May 2007 ARCTP325