Mangroves: to clear or not to clear?

Mangroves: to clear or not to clear?

NIWA scientists have written a guide for managing mangroves, prompted by a desire for people to learn more about mangrove ecosystems, and what happens when they are removed.

The guide will be formally launched at the Manukau Harbour Symposium in Auckland today.
NIWA marine ecologist Dr Carolyn Lundquist says the aim of the manual is to provide guidance for managing mangrove expansion while maintaining the ecological functioning of estuaries and harbours.

In New Zealand mangroves grow on coastal mud flats from Kawhia north. As mangroves have been expanding rapidly in recent decades, they are often targeted for mass removal in attempts to return the harbour seascapes to what they were in past. Reasons for removal include improving recreation values and access to the ocean, improving views, returning habitats to firm sand flats, and improving drainage or flood protection.

Dr Lundquist says that until recently little was known about the effect of mass removal on the environment or even if it worked.

NIWA has surveyed more than 40 areas where mangroves have been removed and found that the practice often does not result in a return of sand flats, and that many removals have had detrimental effects on the local ecosystem.

“In the past, a decision to remove mangroves was not always informed by the best science. In this guide, we have compiled what we hope will be useful information for community groups, councils and individuals.”

Opinions on mangroves are often divided, ranging from those wanting them protected and those seeking to eradicate them. Dr Lundquist says she is not taking sides in what is often a fraught debate, but would prefer people are properly informed on whatever action they decide to take.

The guide explains that mangroves act as a buffer zone from waves and storms and protection from erosion of the coastline. They also store carbon and nutrients, and support a diversity of animal life which feed off the plants’ organic matter.

However, decisions made on land, such as increased conversion of land for agriculture, forestry or urban use, have huge downstream implications for estuaries where mangroves grow.

“What we do on land has a huge impact on sediments which enter estuaries and raise the height of tidal flats, increasing the area that mangroves can colonise.”

Dr Lundquist says the best way to manage the expansion of mangroves over the long term is to limit the amount of sediment reaching the coastline from the surrounding land.

Areas where mangroves have been removed also need regular maintenance to keep mangrove seeds from re-establishing – an annual cost that can range from $1000 to $5000 per hectare for seedling removal and disposal. This is a cost that is often overlooked, Dr Lundquist says.

The guide discusses a range of management strategies that address mangrove expansion, and also advises on where removal is unlikely to achieve the desired outcome or could be very costly to maintain.

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