Marvelous mangroves

MARVELOUS MANGROVES

As a child growing up in Auckland, the summer months were filled with family picnics. We spread our rug on many parts of the North Island coastline. It was never a success, however, when the destination, after seemingly hours of driving, was a strip of water bordered by a patch of ‘smelly’ mangroves.

However, quite unexpectedly I began to see mangroves in a different light, but it took a few 2 ½ hour plane trips for their worth to be appreciated. The experience was linked to holidays to New Caledonia to visit family and friends on properties around the capital, Noumea and further up the coast.

This meant lots of driving, including negotiating narrow curving island roads. Depending on the tide and time of year, the water reached right up to road level, thickly edged with mangroves you could touch from the open car window. I thought how muddy and boggy and weed like, they looked and what a shame there weren't tall rustling tropical palms in their place, under-laid with welcoming white sand. But, that was before I discovered that some of our visits coincided with mud crab season and their preferred habitat is the soft silty mud of the mangrove swamps and riverbanks. With the mangroves, covering over 20,000 hectares of coastland and 79% of that running along the western coast that was our route, there was a lot to be grateful for. What’s more, our hosts were outstanding chefs who created numerous scrumptious crab dishes for us to enjoy.

The years passed and our move from the city to the country, encouraged a greater understanding and appreciation of the environment. Just as well, as the subject of my childhood love - hate relationship, became a part of my immediate environs. Our own farm had a westward track through the paddocks leading straight down to a healthy thick stretch of mangrove trees. The area is the upper Waitemata/Rangitopuni River, where sea meets river water. It’s quiet and protected and the mangroves thrive, sending up hundreds of breathing roots.

By now I knew that mangrove refers to trees that have adapted to live in the sea and the ones growing in New Zealand are the southernmost in the world, thought to have been around for 10,000 years. While the New Caledonian varieties included Rhizophora on the island’s outer fringe, Bruguiera gymnorrhiza on the more stable mudflats and Avicennia officinalis on more stony substrates, the NZ Mangrove, commonly known by Maori as ‘Manawa’ is Avicennia marina var resinfera. Resinfera means resin bearing and the misconception goes back to the time of Captain Cook’s explorations, when lumps of resin or kauri gum washed down from nearby kauri forests were found amongst the trees and were thought to have come from the Mangroves.

My research revealed that Mangals or Mangroves play a significant role in the ecological system in our harbours, from sheltering young fish and providing a refuge for birds, to stablising the land. Ours enjoyed sheltered shallow waters and limited wave action, plus a low level of salinity, ideal conditions for the variety. Our exposure to the coastline was about 250 to 300 metres, part flat and part cliff several metres high. They formed a natural barrier between the water and land, softening the line between the two where they met at ground level, while helping prevent erosion by acting as a breakwater at base of the raw bank where they grew at its feet. The help comes from the trees’ strong horizontal roots that form a grid going out many metres, from which they send up breathing roots called pneumatophores that operate above the mud surface and close up when the tide comes in.

Further north trees grow to nine metres or so but ours were a healthy three to four metres in height, about right for their more southerly location. The oval leaves have a leathery texture with a dark green glossy upper surface covered with hundreds of minute pores and a thin layer of densely matted, creamy white hairs on the undersurface. The waxy coating on the top helps withstand salt levels and low oxygen conditions, while the plant breaths through the minute glands on the underneath, which also help to expel excess salt.

We saw them loose more than half their leaves each year, which I understand helps get rid of salt build-up. The fall increases in summer to limit water loss through evaporation off the leaves. Small clusters of yellowy star-shaped flowers, 6-7 mm in diameter appeared late summer/early autumn. They give off a sweet fragrance that helps attract bees, flies and other insects.

The flowers produce seeds that sprout and grow while still attached to the parent plant. When big enough these green pods or porpagule fall off and begin to grow in the mud or float out to sea to carry on growing when they reach suitable land.

Knowing that they move silt from the murky river waters and absorb many of the contaminants brought down by the land, we did what we could to guard against contributing to the pollution. All waste was properly dealt with. There were no runoffs from septic tanks etc.

We were also well fenced against stock wandering into the area, except for the odd time our ‘cute baby lambs’, turned the corner into bulky adolescence and ran the wire strand fences or wriggled under improbably low wires to escape. The odd one headed off down the track but the twice daily count, soon indicated that numbers were short, then it was hide and seek till the wanderers could be rounded up and another fence repair completed. I’m pleased to say there were no deaths by drowning.

However there was the time when one of the horses got stuck and that was serious. The voice of a panicking daughter could be heard from the house tens of metres away. We were there smartly to help soothe the frightened animal (and it’s rider) till it was calm enough to a get a footing and be coaxed out.

There was also the time the family came visiting by boat. They timed their arrival with high tide when the channel was at its deepest. However, the trip took longer than predicted and they arrived on an outgoing tide. The boat stuck fast and it was a team effort, requiring all present to wad out to pull them in. It’s not easy going. You need to avoid stepping on the roots for your own sake and the tree’s. The trees breath through their roots and the sharp shell edges of Elminius modestus, the little black barnacles that attach themselves to the roots, are to be avoided. You could call it a natural thigh deep mud bath and I must admit the skin had a certain glow the next day!

I became fond of the gnarled twisted trunks and found out that the wood is tough with a plywood type structure. In some communities it’s used for fuel because it gets very hot, but it does have a pungent smell if burnt dry. The stems can be processed to make soap while the bark can be used for tanning. Some parts are also used for medicine. Maori used the tree lichen to make dye and the leaves to keep freshly caught fish cool.

We also knew that the roots, provided an ideal place for some young fish to grow initially and this seemed to add truth to the insistence by our neighbours that they had chased the odd flounder ashore. For us, fishing of any sort was a failure.
Bearing in mind that pukeko or swamp hen, use a range of habitats including mangroves to roost at night, I was sure this was a contributing factor to the large numbers of handsome pukeko that came calling (not always welcome in quantity, as they dug up the newly planted globe artichokes). In general the bird life that visited the farm was prolific. We had kingfisher, which are also associated with mangrove areas as well as rosellas, ducks, grey herons, fantails, quail, Moreporks, seagulls, blackbirds, swallows, thrush, sparrows, magpies, finches, pheasants and even kookaburras.

There was always plenty to see on the shoreline. It was a great place to wander to at sunset and sit on a fallen log to watch and listen. As the light faded, if the tide was right, the depth of sound invaded your senses – soft plopping noises made by the host of local inhabitants, in particular snapping shrimps, plus the buzz of flying insects, bird movements and song of dozens of cicadas. Then as night fell the call of Morepork was added.

There are some areas in New Zealand, where locals are concerned with the creeping spread of mangroves brought about by climatic changes (it’s getting warmer) and man-induced changes (e.g. sawdust runoff from milled forests) and there is talk of keeping this spread in check.

However, there are few plants that would survive in the harsh conditions they have adapted to and also play such an important part in the ecosystems of our harbours. We grew to appreciate their proximity and although they didn’t encourage diving or sunbathing, they stabilized the coast and encouraged sea and bird life, giving us plenty to see and hear always.


©2009 Linda Donald
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Appeared in Lifestyle Farmer
Words 1549