A lot more known about man­groves

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the Whanga­mata Har­bour, and to date that process has con­sumed over a decade in time and in ex­cess of $1.5M in costs,” Mrs Goudie said.

The bill aims to em­power Thames–coromandel Dis­trict Coun­cil and Hau­raki Dis­trict Coun­cil to get the job of man­grove re­moval done more ef­fi­ciently.

A coun­cil state­ment said prior to the 1960s har­bours were clear of man­groves. How­ever the man­grove is a na­tive species that sci­en­tists say has been around for thou­sands of years.

The rate of ex­pan­sion has, how­ever, grown dra­mat­i­cally since that time.

Man­groves cover more than 26,000ha of New Zealand’s north­ern coast, equiv­a­lent in size to Welling­ton. The species is ex­pand­ing its cover by an av­er­age of four per cent a year and episodic waves of ex­pan­sion oc­cur in the Firth of Thames about ev­ery 10 years.

NIWA sci­en­tist An­drew Swales has ex­am­ined the pat­tern of man­grove coloni­sa­tion in the Firth by tak­ing sed­i­ment cores to 2m depth and dat­ing the age of the sed­i­ment be­fore and af­ter man­groves have colonised.

It’s work that will be of in­ter­est to de­bates at po­lit­i­cal level about whether man­groves ac­tu­ally speed up the silt­ing of har­bours that were once sandy. The sci­en­tists’ work con­firms that na­tive for­est clear­ances and changes in land-use are to blame – in­fill­ing es­tu­ar­ies with sed­i­ment, with man­groves merely tak­ing hold af­ter­ward.

“Ul­ti­mately man­groves are just mak­ing use of this in­crease in sed­i­ment to our es­tu­ar­ies,” says Richard Bul­mer, another NIWA sci­en­tist work­ing in the field.

In fact, be­tween 1940 and 2014, sed­i­ment has rapidly in­filled our es­tu­ar­ies, co­in­cid­ing with in­creases in man­groves ex­pand­ing from 2300ha to 10,500ha in the Auck­land re­gion.

Sim­i­lar sed­i­ment in­fill­ing and as­so­ci­ated man­grove ex­pan­sion has oc­curred on the Coromandel, in har­bours such as Whanga­mata, Tairua and Whangapoua.

One thing the sci­en­tists want to know is how this may be im­pact­ing the tiny an­i­mal com­mu­ni­ties that dwell there, and whether re­moval of man­groves makes a dif­fer­ence to them.

“When man­groves were cleared in Tau­ranga, we didn’t see any ev­i­dence of change to the neigh­bour­ing shell­fish beds,” says NIWA sci­en­tist Dr Carolyn Lundquist.

“But at the same time, we also didn’t see any new shell­fish beds re­colonis­ing where man­groves had been re­moved. Rather we of­ten see de­creases in the shell­fish be­cause of the sed­i­ment in­puts, so these beds may be al­ready gone by the time man­groves come in.”

One im­pact that’s hard to ig­nore in sev­eral clear­ance sites is the stink. That is the smell of anoxic sed­i­ment, and the re­al­ity is that it gen­er­ally sticks around rather than re­turn­ing to sand­flats. Dr Lundquist says a lot has been learned and newer meth­ods of man­grove re­moval are bet­ter at avoid­ing lon­glast­ing odour. There’s also less dam­age to the an­i­mals in the sed­i­ment.

“In Whanga­mata, four or five years af­ter clear­ances, worms, cock­les and other bi­valves were still more com­pa­ra­ble to an in­tact man­grove area rather than a sand­flat, though these an­i­mal com­mu­ni­ties are chang­ing grad­u­ally.

“But the re­al­ity is you don’t get the pris­tine sand­flat back in most places where man­groves have been re­moved. We’ve learnt a lot and we know there are places where man­grove re­moval is likely to be suc­cess­ful, whereas for other places you wouldn’t be­cause it would be a poor choice as it is un­likely to re­turn to a sandier state.”

While the rate of ex­pan­sion is alarm­ing, man­groves and the mud they in­habit may be na­ture’s way of fix­ing things. New Zealand sci­en­tists want to know what role our own coastal habi­tats — man­grove, coastal marsh, sea­grass or mud­flats — might play in stor­ing green­house gases. Even the mud, it seems, is of value.

Dr Lundquist and her team have re­cently com­pleted a pilot study to test the plant’s po­ten­tial for re­duc­ing the im­pact of cli­mate change. Over­seas stud­ies in­di­cate trop­i­cal man­grove forests can take up car­bon diox­ide 100 times faster than land-based forests. Man­groves store car­bon in the branches and other woody veg­e­ta­tion, but up to half the car­bon in man­grove forests is stored in the sed­i­ments.

“We went to 40 places where man­groves had been re­moved,” Richard Bul­mer says. “Even 16 years af­ter re­moval, we’re still get­ting high rates of car­bon diox­ide be­ing re­leased from the sed­i­ment, while you have at the same time lost all of the car­bon that is stored and ac­cu­mu­lated within the man­grove trees, plus you’re not get­ting car­bon stored in the trees ei­ther.”

A study funded by NASA in Florida says the spread of man­grove trees could hold the key to pro­tect­ing shore­lines as cli­mate change im­pacts are felt.

“Man­grove habi­tats pro­vide a stag­ger­ing 800 per cent more coastal pro­tec­tion than salt marshes (and mil­lions of dol­lars more value than man­made bar­ri­ers),” says the study pub­lished in Hy­dro­bi­olo­gia, the In­ter­na­tional Jour­nal of Aquatic Sci­ence. In the US, wet­lands pro­vide an es­ti­mated $23.2 bil­lion per year of pro­tec­tion against eco­nomic losses as well as deaths in ma­jor storm events.

Ecol­o­gist Meg Graeme thinks man­groves are un­der­rated as a cli­mate change tool and de­serve to be seen as a quin­tes­sen­tial part of New Zealand’s nat­u­ral char­ac­ter .

“A favourite pas­time of mine is kayak­ing around man­grov­e­lined creeks with yel­low-eyed mul­let, parore and floun­der swim­ming ahead. Apart from im­por­tant car­bon ab­sorp­tion and food web roles they play, man­groves ac­cu­mu­late and sta­bilise sed­i­ment to pro­tect against ris­ing sea lev­els.

“Man­groves pro­vide vi­tal habi­tat for shore birds, fish fol­low the tides to feed among the man­groves, and banded rail are de­pen­dent on them. We have a lot of lessons that can in­form that TCDC bill,” Dr Lundquist says.

‘We’ve learnt a lot and we know there are places where man­grove re­moval is likely to be suc­cess­ful, whereas for other places you wouldn’t be­cause it would be a poor choice as it is un­likely to re­turn to a sandier state. DR CAROLYN LUNDQUIST

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