Forests in the sea
Amphibians of the plants world, mangroves have adapted to life in one nature’s most demanding environments.
They stand alone where no other trees can survive; not outcasts, but pioneers breaking new ground at the margins of our land. Bathed twice daily by the sea, and rooted in waterlogged soils that are virtually devoid of oxygen, they are marvels of nature, flourishing in conditions that are intolerable to all other flowering trees, and forming dense, luxuriant forests. Yet, for the most part, these unusual trees of northern New Zealand have been disparaged and misunderstood.
There was confusion from the moment that botanists on Captain Cook’s initial voyage of discovery first found them. They would have seen such trees before, wherever they stopped in the islands of the Pacific, but never had they seen mangroves that had secreted such copious quantities of resin. The golden brown nuggets of gum lay in profusion on the soft ground amongst the trees, and, in respect of this observation, the tree was named Avicennia resinifera, the “resin-bearing” Avicennia.
In reality, the hard lumps of resin, which can still be found in mangrove forests of the far north today, came from another notable Northland tree, the kauri. The geographic distribution of the two species coincides, with both being more or less confined to parts of the country north of the 38th parallel. On the east coast they extend naturally as far south as the Ohiwa Harbour in the Bay of Plenty, and on the west coast Raglan Harbour is generally regarded as their southern limit, although from time to time occasional plants are recorded in the Aotea and Kawhia Harbours.
Some biologists believe that the range of mangroves might potentially extend a little further south were it not for the fact that their distribution is also limited to very sheltered shores: the quiet backwaters of harbours and the upstream banks of estuaries. As suitable inlets are not to be found for considerable distances further south along both east and west coasts, the actual range in New Zealand is possibly truncated.
Support for this theory comes from the survival of a few trees that were planted in Tolaga Bay near Gisborne on the east coast, and others planted in northern Taranaki on the west coast. A further anomaly in what has long been a simple distribution map is the recent discovery of two plants in Parapara Inlet, Golden Bay, at the northern tip of the South Island—some 200km south of Ka, hia Harbour.
Until recently, mangroves have been regarded as a poor relation in the plant world. No one waxes lyrical about the mysterious mangrove as they do about hosts of golden daffodils or groves of mighty oaks. In literature, mangroves have been almost completely ignored—unless one is liberal with the interpretation of Rudyard Kipling’s writing. It is tempting to think that the “bottlytree with the twisty roots” growing on the banks of the great grey-green, greasy Limpopo River in the story of the Elephant’s Child might well have been contorted mangroves near the lower reaches of that river in Mozambique, for it is a part of the world where they flourish.
Perhaps other cultures have a higher regard for them, for they are predominantly trees of warmer climes, and reach their greatest diversity and luxuriance in the tropics. In South-east Asia 50 species are known, and there are more than 30 in northern Australia, but towards the southern limits of their range the diversity declines. Around Australia’s southern shores and around our own there is just a single species. Some botanists consider it to be the same species in both countries, while others claim that there are sufficient subtle differences to merit calling them sub-species. Accordingly, the mangrove in southern Australia is named Avicennia marina var. marina, and the New Zealand mangrove, known to Maori as manawa, is A. marina var. resinifera.
The word “mangrove”, like the term “succulent”, refers to a lifestyle rather than to a particular plant classification. Mangroves are trees from any plant family that have adapted to living in the sea. Worldwide, 23 genera from eight different families have species that are described as mangroves. As with succulents, while the problems are standard in different places, the methods of coping with them are quite varied.
Mangroves are restricted to quiet waters such as are found inside harbours and estuaries, because on more exposed shores the stronger wave action would either uproot them or carry away all fine particles that might otherwise make up a soil for them to root in. As amphibious trees, they can tolerate being completely submerged in seawater, but need to be uncovered for at least half of each tidal period. Even when the tide is out, though, their roots remain soaked with seawater in the waterlogged mud.
These are difficult conditions for flowering plants to cope with, and almost every part of the mangrove tree shows some modification to enable it to survive.
The four main problems a mangrove must accommodate are establishing a root system in waterlogged ground that is largely without oxygen, standing upright when wind, tidal currents and, infrequently, moderate waves strain at shallow root anchorages, distributing seeds and establishing young plants in environments swept twice daily by the rising and falling tide, stopping water loss from the plant tissues.
That mangroves should have problems maintaining sufficient water in their tissues may seem surprising for a plant which spends its life standing in water. The problem has to do with the concentration of salts in cell sap compared with that of seawater. Full-strength seawater has a salt concentration of about 34 parts per thousand, or 34 grams per litre. The cell sap concentration in the New Zealand mangrove is about 2.0g/1—higher than in land plants (which have sap concentrations of up to 0.2 g/l) but still much lower than seawater.The natural laws of osmosis dictate that when two solutions of different concentrations are separated by a semipermeable membrane of the type which surrounds plant and animal cells there will be a net flow of water from the dilute to the concentrated medium until the concentrations on the two sides of the membrane are equal. Water will therefore tend to be drawn out of the mangrove tissues into the seawater that washes around the plants, placing them in danger of dehydration. Ironically, this danger is greatest when the tide is in.Mangroves must also fight against the opposite osmotic tendency: the influx of salt into their tissues from the surrounding sea.These battles with salt and water are fought in several ways in the roots and leaves of mangroves. Some species are salt excluders, expending energy at the roots to reduce salt intake, while others, including the New Zealand mangrove, are salt secreters, and use a number of ingenious mechanisms to regulate salt in the tissues.The upper surface of mangrove leaves bears a thick waxy cuticle that makes the leaves waterproof. As the tide falls the salty water drains away quickly from the pointed tips of the leaves, and any remaining droplets on the surface are well isolated from sensitive internal tissues.However, this waterproof skin cannot encase the leaves completely because, like the leaves of other trees, they must have breathing pores (stomata) to allow carbon dioxide in the air to diffuse into the photosynthetic tissues inside. To minimise water loss, the stomata of mangroves are confined to the lower leaf surfaces, and are located in sunken pits. This placement allows a moist micro-atmosphere to develop around each opening, and reduces the potential for water vapour to be carried away by air currents moving across the leaf surface.Water loss from the stomata is further reduced by the under surfaces of the leaves being clothed in a fawn to whitish felt of flat-topped hairs. These form an insulating blanket which reduces desiccation by restricting air flow.
Under a microscope, the upper leaf surface is seen to be perforated by many tiny pores: the openings of simple salt-secreting glands that may number as many as two thousand on a single leaf. These glands get rid of extra salt in the plant sap by secreting a brine that is more concentrated than full-strength seawater. Along with the salty residues of splash and spray, this is easily rinsed away by rain or the rising tide.
Isolation and removal of unwanted chemicals by storing them in older leaves is a common strategy in the plant kingdom. Mangroves, too, appear to use leaf drop as a mechanism for removing excess salt.
Monthly leaf litter collections from New Zealand mangroves have shown that leaf fall occurs throughout the year, but is ten times greater in summer than it is in mid-winter. Maximum leaf fall therefore coincides with the period when water loss from the leaves through evaporation would be greatest.
Mangroves lose about 60 per cent of their leaves in a year. In the far north, annual litter drop is in the range of five to six tonnes per hectare. Such a large turnover could be construed as a wasteful mechanism for disposing of salt. However, research on the fate of fallen leaves indicates that most are trapped amongst the pneumatophores around the trees, where they are quickly broken down by soil fungi and bacteria, and their nutrients made available for reabsorption by the mangroves. Excess salt stored in the leaves leaches out into the bathing seawater.
The root system of a mangrove, like that of other trees, has two functions: anchoring the tree firmly in the ground so that it can stand upright, and absorbing water and dissolved nutrients from the soil for conduction to other parts of the plant. As well as coping with wind stress, trees standing in tidal waterways must also resist the strains of strong water currents whenever the tide rises and falls or when rivers are in flood after heavy rain.
All plant roots, including those of mangroves, need oxygen to survive,but the fine silt of a swamp compacts so tightly that it effectively seals out air. As a result, the mud a few centimetres below the surface is a clogging, stinking anaerobic morass, stained black by the sulphide waste products released from dead soil bacteria. Mangrove roots cannot grow down into this hostile airless soil, and therefore cannot form the deep root anchor that is characteristic of land trees. Instead, they form extensive “root rafts” that radiate out from the trunk just below the surface, where the soil is partially oxygenated. This broad, interlaced mesh-work of roots anchors the tree and stabilises the soil.
Whereas the deep root systems of terrestrial trees are often not much broader in diameter than the diameter of the canopy, mangrove root systems are very much larger—often up to five times the diameter of the canopy (and therefore up to 25 times the area). When the trees grow close together, as is normal in dense mangrove forest, the roots of adjacent trees intertwine, forming a matrix that performs a similar function to the reinforcing mesh that binds and strengthens concrete. In areas of high wind stress accessory prop roots reminiscent of flying buttresses may arch down from the trunk to the substrate, providing extra support.
Although the mangrove’s roots are buried below the surface, it is easy to follow the layout and measure the extent of the root system because at regular intervals along each radiating root erect accessory roots project up through the soil. These are called pneumatophores—literally “breath carriers”.
True to their name, these pencil-like projections serve as snorkels, carrying atmospheric air down to the roots. Each has a number of small “portholes” (lenticels) in the outer surface which admit air into the root when the tide is out.
Pneumatophores vary in thickness (from 6 to 12mm diameter) and in length, depending on the substrate and the amount of exposure. In open, sandy sites, where the soil is better aerated, they are often only 5cm tall, but in extremely sheltered sites, where fine, sloppy mud clogs the vital lenticels, they may grow to half a metre tall.
These specialised roots have a second function besides gas transport. They act as obstacles to the streaming tide, slowing the flow, facilitating the settlement of the suspended particles of silt that cloud estuarine waters, and generally stabilising the surrounding soil.
Because the root systems of mangroves extend well beyond their canopies, the arrays of pneumatophores outside the canopy rim, like the front line of an advancing army, prepare the soil in that region, thus aiding the outward expansion of the forest.
In the long term, sediment accumulation around mangrove roots may raise the ground level to the point where the forest can be invaded first by salt marsh plants such as rushes and sedges, and eventually by fringing bush or hardy, salt-tolerant grasses. The process of sediment retention started by the pneumatophores can ultimately lead to natural land reclamation.
Mangroves have so many modifications for living in a marine habitat that it is easy to overlook the fact that they are flowering trees which, like many other higher plants, rely on insects to fertilise their flowers. The New Zealand mangrove’s flowers are about 6mm in diameter and are borne erect in clusters of four to a dozen blooms at the tips of branches. The four pointed petals have a tough brown, hairy outer surface, but inside they are golden and shiny, with small glossy nectaries at the bases. Four short stamens with plump cream anthers are positioned between the petals.
Although the main flowering period extends over several months of late summer and autumn, each flower is fertile for only three or four days. Flowers may remain open for much longer, but they will be spent, their anthers reduced to black withered stumps.
Mangrove flowers have the mouth-watering scent of tropical fruit blended with sweet sherry, so it is not surprising that they are attractive to insect pollinators. Most obvious are the introduced European honey bee and the small native common blue butterfly, but the blooms are also visited by several species of native bees and small flies.
Such is the lure of mangrove scent to these pollinators that at the peak of flowering in late summer and autumn, trees hundreds of metres from the shore in large harbours can be a-buzz with hundreds of bees collecting nectar.
A few apiarists appreciative of the distinctive fruity flavour of manawa honey, and aware that their bees will feed almost exclusively from mangrove trees at the peak of flowering, move hives to shoreline positions adjacent to large stands of mangrove forest.
Although it is not very showy, the mangrove flower is in all respects a normal bloom. What is exceptional is what follows fertilisation, for in response to its special habitat needs it does not produce seeds as other trees do. Instead, each embryo develops directly into what looks like a young seedling inside a protective case on the tree. When these well-developed embryos fall, around Christmas and New Year, they can take root very rapidly if they find themselves on a suitable substrate.
This method of propagation (the plant equivalent of “live birth”) is essential for a tree that lives along the margins of the sea, where the release of conventional seeds would have a very low success rate. Permanently wet mud is a very poor medium for seed germination, and the motion of waves and tidal currents would make it extremely difficult for small, freshly emerged seedlings to establish a root system.
Since the young mangrove plant does not develop from a conventional seed it cannot correctly be called a “seedling”. Nor is the pod within which it develops a true fruit. Instead, the voyaging embryo and its portable food supply is termed a propagule.
At maturity, a mangrove propagule looks like a young peach or almond, and has an olive-coloured coat with the velvety texture of suede leather. Inside are two large, bright green cotyledons, folded in such a way as to form a small leaf “book”. In the centre lies the young shoot, and protruding from the base is a stump covered with many fine hairs from which four lateral roots will sprout when the propagule is released from its case.
Mature propagules vary in weight and size from about ig up to 11g. The average size seems to be related to the local climate. In the far north, the average weight is 5g, while at Ohiwa, near the southern limit of mangroves in New Zealand, the average propagule weight is a little over 2g.
The propagule is beautifully adapted for both dispersal and for planting itself when it comes to rest on a suitable substrate. When it falls, a propagule is still enclosed in its leathery case. Air inside the case gives it buoyancy and allows it to float away from the parent trees at high tide.
After a time in the water, each case splits open along its longitudinal seam, and buoyancy is lost. The two halves of the case turn inside out, fully releasing the bright green propagule, which starts to unfold its cotyledons. Most propagules are released from their cases within three days.
If the naked propagule settles in a calm site with a soft substrate, it may unfold and take root straight away. However, the presence of large numbers of unfolded propagules in high tidal debris along northern shorelines shows that many do not root successfully where they first settle, either because the substrate is too hard or because wave and current disturbance is too great. These propagules get a second chance by becoming buoyant again after a few days of lying along the strand line. It is not certain how they regain their buoyancy, but it is possible that minute bubbles of oxygen produced during photosynthesis are somehow retained in the tissues.
Since mangrove forests are restricted to the calm waters of harbours and estuaries, areas suitable for mangrove colonisation are physically isolated from each other. Intervening promontories of open coast prevent any continuity from one mangrove area to the next. Propagules must therefore be capable of surviving sea transport of considerable distances and long duration. Tests indicate that propagules remain viable for at least four months, and their presence along the drift lines of beaches throughout the year suggests that they may be able to survive even longer.
However, once a suitable landfall has been made, it is important that the young plant rapidly establishes a secure root anchorage. Propagules settling between mid and high tide (the natural tidal range of mangroves) are rocked back and forth by movements at the water’s edge caused by wave ripples and small ebbs and flows of the changing tide. As the broad, stiff cotyledon leaves separate and unfold, their tips settle into the mud, reducing the rolling movement. Meanwhile, irrespective of which side the propagule is lying on when it comes to rest, one pair of the quadrant of new roots will stick into the substrate—giving additional stability—and start growing. Eventually, the other pair of roots also make contact with the substrate, penetrate the soil and pull the plant upright from its prostrate position.
Juveniles that root in the midst of existing mangrove stands may become successfully established, but many, shaded by the canopy of older trees, will remain stunted and small. However, in full daylight, along the edges of a mangrove stand where the pneumatophore fringe extends well beyond the outermost branches, newly settled plants are in a good position to survive and flourish.
Dense nursery stands of stunted one- and two-year-old juveniles within the forest may represent a form of “living dormancy”—the mangrove equivalent of the seed dormancy which occurs in terrestrial plants, where some seeds do not germinate at the first opportunity, and are “held in reserve” should the first crop of seedlings fail. In a mature mangrove forest the death of a large tree can open up a wind hole in the canopy, leading to damage to the adjacent trees. It is advantageous that such holes are filled quickly, and the stands of dormant juveniles are poised to flourish as soon as the extra light is let in. As they compete for space and nutrients, eventually one or two plants will dominate, and the hole in the canopy will close over again.
The growth rates of mangrove trees and the sizes and shapes they attain are affected by a number of factors: the type of substrate, salinity of the water, vertical position on the shore and the length of time spent standing in water. They are also affected by the density and height of the surrounding mangrove forest which provides wind shelter and shade. Pests and diseases and grazing cattle can also limit growth, but the greatest regulator is local climate, and this is generally related to latitude.
In the far north, in harbours such as Whangaroa and Hokianga, large trees up to nine metres tall are commonly found, but further south the average height of trees gradually decreases. At Whangarei, the maximum size is about five metres and around Auckland the largest trees are generally only four metres tall. At Raglan and Ohiwa, most trees are stunted bushes little taller than one metre.
Although there is a trend from tall trees with stout trunks in the far north to slower-growing, slendertrunked bushes in the south, there are some exceptions. In several inlets along the eastern side of the Coromandel Peninsula, there are some very fine mangrove trees. In the Purangi Estuary behind Cooks Beach, at the same latitude as Auckland, one stand has a number of trees that are seven metres tall, with trunks that, for mangroves, are straight and clean.
One specimen in this estuary is of remarkable stature. Around eight metres in height, it has a stout, clean trunk like a puriri, and a girth of 1.5m. Its root raft, as indicated by its radiating arrays of pneumatophores, is 45 metres in diameter. To manawaphiles, this tree would have a status among its peers equivalent to that of kauri like Tane Mahuta and its kin, but, unlike those trees, mangrove giants go largely unrecorded and unnoticed.
Access to stands of fine mangrove trees in the harbours of the north is easy enough if you have a small dinghy or canoe, or even a surf board to float on. Drifting or paddling amongst the trees at high tide is a most satisfying way to enjoy the tranquillity of these strange forests where dappled sunlight plays on water that is frequently almost mill-pond calm.
In summer the rhythmic plop of falling propagules punctuates the silence, as does the occasional splash of fish fins breaking the surface. Peering through the shallow water, you can often see schools of yellow-eyed mullet gliding effortlessly amongst the trunks and pneumatophores, searching for morsels to eat. Sometimes they will dash away, flicking up a cloud of sediment from the bottom as a smoke screen to confuse a larger fish pursuing them.
On the trunks and pneumatophores, modest barnacles, some of them ten times the size they grow on open rocky shores, rake their feathery legs through the water to strain out plankton. Around them grazing snails rasp away to remove algal films from the bark. On the bottom small semi-transparent shrimps skip about on tiptoe looking for even smaller crustaceans amongst the decaying leaf litter, and tubeworms display their delicate conical filter-feeding crowns.
When the tide is out a completely different world awaits discovery.Mud crabs emerge from their burrows to graze the algal film left on the mud by the receding tide. Wading birds and kingfishers move in to stalk or dive on prey.
Like mature terrestrial trees—tall totara or spreading puriri—mangroves are “islands” which provide homes for a large range of plants and animals.
On very sheltered shores there are seldom any intertidal rocks, so the pneumatophores, trunks, branches and even regularly immersed leaves of mangroves are favoured settling sites for encrusting plants and animals. In the shade of leafy canopies, and with either water or damp ground below, they also provide surfaces that remain moist—ideal for the lush growth of algal films. These surfaces are grazed by rocky shore snails, in particular the cat’s eye, but also by periwinkles, topshells, black nerita and the smaller of the two horn shells found on sheltered shores, Zeacumantus subcarinatus.
The vertical zonation of encrusting plants and animals that is familiar on rocky shores is often neatly and clearly compressed on the trunks of mature mangrove trees. Tubeworms and rock oysters are found around the trunk bases of the outermost trees—those rooted towards mid tide level. A little higher up, sheets of modest barnacles and clumps of black flea mussels often abound, sometimes weighing down slender hanging branches, or forcing long pneumatophores to bend over. At the same level the trunks occasionally have collars of small intertidal seaweeds such as Catanella and Gelidium.
Above the upper barnacle line are bands of lichen growth. Lowest, at about high tide mark, are black velvety tufts of Lichina confinis, while a little higher the bark is smudged with sooty patches of Verrucaria maura. Higher still, black grades into grey-green where several different flaky foliose lichens, often in rosettes, clothe trunks and branches in the maritime zone. Further up in large, old trees are the bearded lichen growths of Usnea, sometimes harvested for dyeing homespun wool or flax, hanging long and wispy from the branches.
There are other epiphytic plants at this level, too. Ferns creep along the trunks of open-branched trees and, occasionally, the pale yellow-flowered orchid Earina mucronata can be seen.
Mangals (mangrove forest ecosystems) are usually tranquil retreats at high tide, but when the water ebbs away the silence is often interrupted by the sound of small pistol shots. Each sharp crack, a sound characteristic of New Zealand mangrove forests, is made by a snapping shrimp, Alpheus sp., a bizarre crustacean with one of its two pincers so large that it is equal in size to its combined head and thorax. That such loud sounds can be made by an animal only a few centimetres long is indeed surprising.
The muscle-filled main body of the pincer opens a blunt, movable finger wide against a tensioned locking mechanism. When the internal trigger is released, the finger snaps shut against the fixed part with tremendous force and a startling sound. It is uncertain what function the snapping pincer has. Perhaps, as has been suggested for some overseas species, the sound is used to stun prey, which the shrimp can then drag into its muddy tunnel.
Wet mud and sand are easy to penetrate, and many animals seldom seen at low tide shuffle just below the surface as the water retreats. Some leave the mud pockmarked with holes, but uncovering the residents is always difficult. Frequently the holes are occupied by mud crabs, which will come out on to the surface as long as there is absolutely no movement within sight of the burrow. So if you squat quite motionless for a few minutes they will emerge all around you, allowing close inspection as they feed or spar with one another over territorial claims.
To observe some of the other inhabitants it is necessary to don snorkel and mask. Seemingly empty shells lying on the mud may be occupied by hermit crabs. These scuttle across the mud and will even climb the trunks of trees to find morsels of food amongst the oysters, mussels and modest barnacles which are filtering plankton from the water.
Small fish, frequently speckled triplefins, emerge from hiding too, hunting for tiny crustaceans: barnacle larvae, copepods and various juvenile shrimps.
Returning with the tide come skeins of adult translucent shrimps, Palaemon affinis, gliding effortlessly through the water. Around the mangroves they skip on tiptoe across the bottom or up and down the trunks and pneumatophores in search of small carrion or wounded encrusting animals—perhaps a barnacle or flea mussel with a broken shell, or even a dead fish stranded by the previous tide. With probing pincers they peck and pull at exposed flesh. tearing off small shreds and devouring them greedily.
The shrimps never have the chance to enjoy a leisurely lunch, for there are other scavengers equally sensitive to the smells of dead animal flesh in the water. The mud crab Helice crassa, which normally ekes out a living by sorting micro-organisms from surface sediments, and its grapsid relative the hairy-handed harbour crab, Hemigrapsus crenulatus, which feeds on the diatomaceous sludge that collects and grows in shallow depressions, are both partial to a meal of meat. Like the hermits, they feed messily, ripping off as much as they can in as short a time as possible, and stuffing the food into their mouths without ceremony before larger scavengers arrive on the scene.
Arriving more slowly, but at a good pace for a snail, are the harbour whelks, Cominella glandiformis. “Tasting” the water through long siphon tubes as they approach, they shoulder crabs and shrimps out of the way and begin feeding through their trunk-like probosces.
The litter of leaves, twigs, flowers and propagules that falls from the trees is like manna from heaven for the teeming hordes of deposit feeders that live on the muddy substrate.
This bountiful fare is § supplemented by run-off LL from the land: soil particles, decayed fragments of plants and animals, animal faeces and many small and micro-organisms that die in salt water. From the sea there is a twice-daily tidal supply of nutrients and marine plankton.
By comparison with the open sea, the waters of enclosed harbours are much more productive. On clear days the flats are warmed by the sun when the tide is out, and when it returns the water temperature is raised markedly as it flows over vast solar panels of hot mud. Algal growth is much greater in warm conditions, so much so that harbour waters occasionally have a distinctly green tinge, such is the abundance of phytoplankton.
The richness of the mangrove habitat is seen in the abundance and variety of animals that patrol these shores, or slither across the surface both at high and low tide.
When the tide is high as many as 30 different species of fish are known to frequent the forest or its fringes. About a dozen, mostly small fish, like triplefins, pipefish, yellow-eyed mullet (often known as sprats), piper, parore, the larger grey mullet and several flatfish (flounder, sole and dab), are more or less resident in the quieter parts of harbours where mangroves flourish.
Cruising in from outside to prey on some of these fish are kahawai, kingfish and jack mackerel. Snapper, trevally and eagle rays may accompany them to feed from the flats around the fringes. They uncover shallowly buried bivalve shellfish—mainly cockles and nut-shells—and have a keen eye for mud crabs and other small crustaceans caught out in the open.
Given the opportunity, yellow-eyed mullet will also take small live prey, but are able to make a meal from no more than the cloudy sediment in the water. It is estimated that a 150mm yellow-eyed mullet can strain out more than a kilogram of sediment per day.
Like most of the other fish residents of mangals, parore are scavengers for any worthwhile scraps, but they also browse algal growths from the trunks and lower branches of mangrove trees, leaving obvious tooth marks on the surfaces.
If they avoid being swallowed by kingfish and kahawai, some of the small fish may fall victim to coastal birds. At high tide several species of shags (pied, little, black and little black) hunt for long periods, repeatedly diving and remaining submerged for from 30 seconds to a full minute as they chase their quarry underwater.
Feeding from the water surface, especially on schools of small fish that are pursued from below by the likes of kahawai, are both red-billed and black-backed gulls and the largest of our terns, the Caspian, with its distinctive black cap and large scarlet bill. At low water all three are equally interested in scavenging for creatures stranded by the falling tide.
Patrolling the water’s edge as the tides rise and fall are a number of waders, mostly with long legs and long bills. Quite a rarity in New Zealand 40 years ago, an Australian immigrant, the white-faced heron, is today one of the waders most commonly seen among mangroves. Not to be confused with the now rarer reef heron that is more frequent where the ground is firmer or stony, the white-faced heron can be found at every stage of the tide, stalking prey along the water’s edge. It always walks into the current so that the little sediment that it stirs up with each careful step drifts away to the rear and does not obscure its view of prey.
At low tide, when not patrolling drainage channels, herons can also be seen taking measured steps across soft mudflats or amongst the fields of pneumatophores, standing motionless from time to time to trick crabs into believing there is no danger outside their burrows, then snapping them up as they emerge.
The steady increase in white-faced heron numbers has sadly been matched by an equivalent decrease in another wetland wader, the brown bittern, a slightly larger and rather shy bird. Probably more at home amongst raupo and rush meadow, it stalks prey in mangroves, too, but only in the more remote harbours of Northland and the eastern Coromandel Peninsula. Common earlier this century, the bittern’s demise is partly due to the loss of suitable habitat. So much of the salt marsh and rush meadow that forms a natural transition between mangroves and land vegetation has either been reclaimed or spoiled. Today, it is exciting to come upon even a solitary bittern.
Treading much the same ground that bitterns cover are two other reclusive waders, the marsh crake and banded rail. Crake mostly confine themselves to the thicker cover of rush and sedge and the tangle of salt marsh ribbonwood thickets that separate mangrove forest from the land vegetation, but where jointed rush and stunted mangroves mingle in remote locations, they can occasionally be seen stepping out amongst the trees. Banded rail are more regular visitors, and range much further out into the forest when the tide is low and the light subdued.
Many people do not associate pukeko with mangroves, but this handsome wader is a regular visitor to mature and extensive stands, especially where there is a good cover of litter on the ground amongst the trees. Sometimes they can be surprised well out from the high tide mark, searching amongst pneumatophore thickets for small animals, frequently in the company of other adults or with mature chicks in close attendance. Altogether more confident than most other birds, the pukeko will often not take flight, but just keep its distance, flicking its white tail as a warning to its companions to be on their guard, but continuing to search for crabs and other invertebrates.
One of the commonest crab catchers around mangrove shores is kotare, the New Zealand kingfisher.Though many spend time in farm paddocks, inland bush and swampy country, they can always be seen around harbour mudflats. There they frequently use mangrove trees as launch platforms from which they make swooping sorties across open ground between trees.
Sometimes the kingfisher will use one favourite perch and repeatedly fly out from it, swooping down to snatch up a mud crab that has strayed too far from its burrow and looping back to base with the quarry held firmly in its powerful bill.
Though the capture is blindingly fast, it is possible to discover what is being caught by keeping binoculars trained on the perch, for the mudflat plunderer with its turquoise cape, creamy yellow waistcoat and black highwayman’s face mask, always returns to exactly the same spot. Frequently the hapless victim can be seen clearly for a moment or two before the kingfisher bashes its bill on the branch several times to break open the crab’s carapace prior to swallowing it whole.
On wet mud around mangroves. one or more of four very common deposit feeders can normally be found: two snails and two crabs. Frequently, they are extremely numerous, with their cumulative body weight amounting to as much as two tonnes per hectare—a “stocking rate” twice that which is achievable with cattle on good pasture.
The mudflat horn shell Zeacumantus lutulentus drags its tall spired shell along behind as it ploughs a trail through soft, wet surface mud. These snails are often found in shallow depressions in densities of a thousand or more per square metre, repeatedly criss-crossing the surface with their furrows. As the puddles warm up on bright sunny days the growth of diatoms is so rapid that within an hour of the tide falling, rich greenish-brown scums are clearly visible covering the bottom. It is this diatomaceous broth that the horn shells devour. As they churn the surface layer into a slurry, it appears that by aeration and nutrient mixing they help to promote the growth of the microorganisms that they feed on.
The mud snail Amphibola crenata feeds only when the tide is out. When the tide returns it burrows down just far enough into the soft substrate to hide from predatory fish that cruise through the shallows. Unlike most marine snails, which respire by passing water across feathery gills, Amphibola is a pulmonate (like the common garden snail), which means that it breathes air into a modified lung.
When the tide is out, and other marine snails are resting, Amphibola feeds at such a rate that its faecal trail stretching out behind is a continuous string—occasionally coiled, showing that it is produced faster than the snail can crawl. Simple tests show that the sludge of sediment, rich with micro-organisms, that these snails slurp up from the surface passes right through the gut in about 20 minutes. This is surprising, for herbivores normally have very long guts and long digestion times to break down the complex cellulose molecules in plant matter.
Tests on the faecal strings suggest that they are little different from the adjacent substrate in calorific value or constituent organisms. It would appear that digestion may simply involve the crushing of diatom and bacteria cells between the hard sediment particles to release cell sap rich in soluble food substances that were already in a form suitable for absorption. The sap would contain a nutritious blend of amino acids and simple sugars recently produced by photosynthesis but yet to be converted into more complex carbohydrates. By ingesting such large amounts of surface sediment and processing it very rapidly, the mud snails may effectively be “tilling their gardens” of diatomaceous and filamentous algae, thereby promoting their growth.
While biologists have long marvelled at the intricate patterns of life within mangrove communities, the general public has been considerably less enthusiastic about these places. Indeed, after a century of being maligned, abused or, at best, ignored, mangroves are only now beginning to be appreciated, both in their own right as unique habitats, and also for the contributions they make to marine and terrestrial food chains.
Even so, they are still regarded as second class plant citizens, as was made clear earlier this year when locals of one Northland inlet started pulling out young mangrove trees, claiming that by doing so they were preventing the inlet from being choked by the “insidious mangrove weed”.
Only 25 years ago—considerably less in many areas—mangroves were considered to have no use whatsoever. Indeed, one former Northland Member of Parliament suggested that they all be cut down and turned into pulp for paper making, and the land “put to some useful purpose”.
Up until that time mangals had been used as rubbish dumps, filled in to shorten roadways around indented harbours, cleared and enclosed as sites for oxidation ponds and reclaimed to make playing fields and additional pastoral land.
The value of any woodland has traditionally been calculated by estimating either the standing crop of the timber on it or the potential productivity of the land if it were cleared and put into crops or pasture. However, we now recognise that forests have other values that are more difficult to calculate in dollar terms, but are no less important.
Among other things, they help regulate stormwater run-off, provide habitats for native animals (especially birds), make our countryside interesting to look at and provide city dwellers with important retreats from the concrete jungle.
Mangroves perform all of these forest functions, and they also make other valuable contributions. They act as nurseries for some coastal fish by providing shelter and a highly nutritious environment. They enrich coastal waters, because tidal flows carry micro-organisms and soluble or suspended organic material from the breakdown of detritus out of the inlets. They serve as breakwaters to reduce coastal wave erosion along the lower reaches of broad estuaries. (This is particularly important where low-lying land is developed for farming behind a high-tidal berm, as is found in the Firth of Thames.)
One major beneficiary of mangrove ecosystems is the New Zealand oyster farming industry. Both the native rock oyster, Saccostrea glomerata, and the introduced Pacific rock oyster, Crassostrea gigas, grow amongst mangroves. The lower limit of distribution of the mangrove and the upper limit of rock oysters overlap at about mid tide level, and the shellfish can be found growing on the trunks and pneumatophores wherever there are mature trees.
Rock oysters grow best on sheltered shores bathed in waters that are rich in the plankton that they filter out for food. They are fairly tolerant of silted water, but only grow vigorously where they are cemented clear above the substrate away from the thick soup of fine, unconsolidated sediment that lies on the muddy bottoms of quiet backwaters.
Most oyster farms are found among mangroved inlets, and, growing on racks, the shellfish are held well clear of the gill-choking bottom sediments.
As the Pacific rock oyster, the species which is cultured commercially, can tolerate longer submersion times than the slower-growing native species, the racks on which the shellfish are grown can be positioned harmoniously just outside and beyond the lower limits of mangroves. There, enriched by the nutrients flowing out of the mangrove ecosystem, rock oysters reach maturity in a year to eighteen months.
In the early years of European colonisation, settlers were too few in number and too preoccupied with survival to have had much impact on mangrove forests. Dumping of rubbish was not an issue (theirs was not a wasteful lifestyle, and what little was jettisoned was likely to be biodegradable), and most transport was by sea or natural inland waterways. There was thus little pressure on the landward margins of mangrove forests for roads.
During the middle period of European settlement, from about 1880 up to the First World War, road access remained poorly developed. Most long distance cartage was still by sea, often by shallow draft scows that could negotiate tidal inlets.
While coastal road reclamations were just beginning, other activities were almost certainly having a major effect. This was the period when most of the country’s kauri trees were logged, and the bush crushed and cleared to extract the timber.
The logs were usually shifted by water, often in destructive water-borne avalanches as huge dams holding back thousands of logs and millions of gallons of water were tripped to send them cascading down to the tidal inlets for rafting to the harbour mouth mills.
When the dams were tripped the force was often so great that the bush was stripped from the faces of the gullies the logs flowed down. Although records are scarce, it is certain that these massive “drives”would have destroyed large areas of mangrove forest, and run-off from the stripped land would have washed enormous sediment loads into the harbours.
Between the wars there was a rapid expansion of our road system. Most towns were coastal, or on major rivers, and it was natural to build the service roads along the easiest contours. This usually meant lowing the coastline along the indented margins of harbours and estuaries.
The construction of these roads, while not intruding much into mangrove forest, nevertheless caused a major change to our coastal natural history by cutting across salt meadow and rush marsh. This action interrupted the natural transition from low tidal eelgrass beds, through mid to high tidal mangroves, across flats dominated by glasswort and rushes and up into the fringing bush of flax, cabbage trees, manuka, salt marsh ribbon-wood and pohutukawa [See fold-out after page 48].
Good examples of this gradual natural transition are now hard to find because rush meadow cut off from the tide by road building became an easy target for drainage and conversion to pasture. In the north, where so much of the topography is steep and difficult, any flat land was highly attractive for farming. It was therefore only natural (or at least human) to “develop” land which was only covered with unpalatable (to stock) rushes, and had no obvious value.
Given the generous loans offered by successive governments for such reclamations, it is not surprising that so much of this important part of New Zealand’s natural heritage has disappeared.
Intrusion into mangroves proper started to occur when councils deemed it wise to straighten out the dangerous corners of coastal roads, and when debris from winter land slips was pushed or dumped off the seaward sides of roads—a practice that continues to this day. Later, with an ever-increasing desire to cut travelling times, and with access to more sophisticated roadmaking equipment, there was a leap forward in coastal road construction (a leap backwards for wetland ecology) with the engineering of many causeways and bridges that cut across large side-arms of harbours and estuaries.
Frequently, tidal flow through to the upstream areas of mangrove forest and rush meadow was restricted by bridges and culverts being made too small. One result was that sediment settled out of water above a causeway at a much faster rate than normal, smothering the mangrove pneumatophores and killing the trees.
The upgrading of roads has progressively seen the metalled surfaces sealed with bitumen, and the sickness and death of some adjacent fringing mangrove stands has been attributed to heavy oil seeping out from these seals, clogging the vital breathing snorkels and again suffocating the trees.
The roads in and out of coastal towns, meandering around the mangroved margins of harbours, gave easy access for dumping rubbish into backwaters that had no specified purpose and were not owned by anybody who could be identified. These quiet corners were altogether too handy for tipping refuse; out of sight of each township, but not too far for cartage.
We now appreciate that using these valuable wetland corners for rubbish tips was completely Mappropriate. In addition to spoiling aesthetically pleasing portions of coastline, which were frequently at the gateways to coastal towns for visitors and tourists, these tips also poisoned coastal wildlife when cocktails of toxic chemicals leached out from them. Remarkably, new tip sites in or beside mangroves were still being permitted in the late 1970s, and a few remain in use.
Where mangroves haven’t been bulldozed for the sake of roads or turned into rubbish tips, large acreages have been smothered to create cheap industrial land. While flat land at a good price may attract new industry to an area, there can be disastrous side effects. Without an area of spongy wetland to absorb stormwater run-off from surrounding hills during heavy rain, there is nothing to restrain the rush of water across reclaimed land and into the harbour. The result can be serious flooding.
Notwithstanding the sad history of human interference with mangroves, in recent years their lot has improved, or at least not worsened much. Small, illegal in-fillings and roadside dumpings continue to nibble away and despoil harbour edges, but substantial reclamations now need to be vital to national interests before they are consented to. Nearly all harbourside rubbish tips are now closed or sealed, and many farmers have been persuaded to fence off paddocks that abut mangrove forest.
Close to urban centres an upsurge in public interest in the quality of natural open space has seen the burgeoning of watchdog actions, with harbour protection societies working in tandem with larger conservation organisations to monitor both shoreline dumping and coastal developments.
Several boardwalks have been constructed across the mangroved arms of inlets. These have enabled the public to enter these unusual forests at all stages of the tide without getting wet or muddy, and without causing any damage to the forest. Recently an area of mangrove forest in the Waitemata Harbour around the Pollen Island shellbank has been given marine reserve status. Located close to Auckland City, this move is an acknowledgement of the growing esteem with which mangrove forests and associated salt meadow are held.
Elsewhere local councils and water catchment boards have become interested in trying to replant areas where mangrove forest has been damaged or killed by former deliberate or incidental abuse. This has not proved easy, and techniques for restoring these areas have recently become the focus of investigations being conducted by Auckland University’s Centre for Conservation Biology.
Perhaps the tide is turning for our forests of the sea . . .