Mud, glorious mud

New Zealand’s complicated relationship with mangroves (9 minute read)

Among my more memorable school trips was to a mangrove forest on Auckland Harbour. There are two sensible ways to visit a mangrove forest. The easiest is to find one which has a boardwalk through it. I’ve encountered a few – there’s one in the Auckland suburb of Westmere and another near Waitangi in Northland. Whatever the level of the tide you can take a walk through the trees, experiencing the forest up close without getting wet or muddy. The other is to use a kayak at high tide, if you are brave and skilled enough to negotiate the tangle of branches.

My school trip, however, did not use a sensible method. We simply went for a walk. Or rather, we attempted to. Within a couple of steps of the high tide mark, we were ankle deep in mud. A couple more steps, and we were calf deep, then knee deep. At the surface, the mud was grey, but the layers beneath were black and smelled of rotten eggs. Sneakers and gumboots were sucked off by the sticky, clinging mud, and the owners had to retrieve them by putting their hands down the hole they left behind. The only sensible footwear turned out to be tightly laced tramping boots, which couldn’t be sucked off. They did, however, accumulate layer after layer of the mud, and it became progressively harder to walk as the boots got heavier and heavier.

At one point, one of my friends struck a particularly deep patch, and was stuck in the mud up to her thighs. Since I was one of those wearing the most suitable footwear, an ancient but sturdy pair of tramping boots, I squelched my way over to her and helped pull her out.

In the end, we travelled only a few metres into the forest before deciding to get out and go home. We did learn a little about mangroves – for example we learned that that sulphurous smell was a consequence of the mud being anaerobic or lacking in oxygen. We learned that mangroves cope with this by having special breathing roots, which poke up out of the mud. We spotted mud crab burrows, although we were making such a noise – imagine thirty sixteen-year-old girls in knee deep mud – that there was no chance of spotting a crab. But mostly, we learned that mangroves live in mud. Lots and lots of mud.

In New Zealand, mangroves are only found on the northern third of the North Island. Around Auckland most of the trees are in the range of 1-4 metres tall. In Northland, they can exceed 6 metres, while at their southern limit in the Waikato and Bay of Plenty they mostly grow as stunted shrubs only half a metre tall. Mangroves can only establish in protected areas such as estuaries and harbours, rather than on open coastlines exposed to constant wave action.

New Zealand’s mangroves, also known as manawa, all belong to a single species which is also found on the coasts of Australia, Asia, the Middle East and eastern Africa. It is one of just over fifty species of true mangrove. The species are not closely related to each other – they are defined as mangrove because of where and how they grow – in salty soil which is regularly submerged by the sea.

Mangrove forest at high tide (image credit: Melanie Newfield)

There are very few complex plants which grow in the sea. Green algae are plants (brown and red algae are not), but most green algae live as single cells or take simple forms, like the membranous sheets of sea lettuce. Complex plants, the kind with many different structures such as leaves, roots, flowers and seeds, evolved on land and mostly stay on land. Mangroves are among the few exceptions.

There are two main challenges that complex plants face in moving from the land to the sea. The first is the lack of oxygen in the mud of mangrove swamps. As my classmates and I observed on our ill-advised school trip, they cope with this with breathing roots which emerge above the mud. The second challenge is salt. Land plants need ways to manage salt when they live in high-salt environments. Mangroves have two ways of coping with this problem. Some types, including the mangroves in New Zealand, absorb salt into their cells and then excrete it in various ways, such as special pores on their leaves. Others are able to form a barrier which prevents the entry of salt in the first place.

In tropical regions, mangrove forests are important both for humans and the wider environment. They are highly productive environments, making them important carbon sinks, holding more carbon than the equivalent area of terrestrial forest. They protect coastlines from the impacts of storms and tsunamis. They provide habitat for numerous marine species, and they act as nurseries for the young of economically important fish and prawns.

When I was at school, scientists assumed that New Zealand mangroves played a similar role to tropical mangroves, and so I was taught how important they were. However, evidence now suggests that New Zealand mangroves are not as productive, nor as important as habitat for marine species, as tropical mangroves. They are still important though, and are more productive than many types of terrestrial forest in New Zealand. They do sequester carbon and protect coastlines from storms and erosion. But they don’t appear to be an important habitat for marine species, at least not in comparison with other estuary habitats.

In most of the world, mangrove forests are under threat. Mangrove trees are cut for firewood, and whole areas of forest are cleared to make way for coastal development, such as land reclamation or shrimp farms. Since 1980, the world has lost between 20 and 35% of its mangrove forests, and each year a further 1-8% of what remains is lost.

New Zealand has also lost mangrove forest to coastal development, but overall the situation in New Zealand is different from that seen elsewhere. In New Zealand, as well as some places in Australia, mangroves are expanding. Whether this is good or bad is the subject of considerable debate.

Experts consider it likely that significant areas of mangrove forest were lost due to coastal development prior to the 1960s, although the records are limited and it’s hard to be certain about how much we’ve lost. Since then, mangroves have been increasing. Estimates vary, and the rates are different in different locations, but the average rate appears to be around 4% per year.

To someone like me, who values native forest and grew up loving mangroves (despite the very muddy school trip), this doesn’t sound like any sort of problem. Aren’t mangroves just reclaiming areas that were lost? The evidence suggests that they are not. Some of the areas colonised by mangroves had previously been sand flats – non-muddy intertidal areas. Areas which formerly contained seagrass and shellfish beds have disappeared under mangroves. So have formerly sandy harbour beaches.

The expansion of mangroves into previously sandy areas has prompted heated debate among scientists, residents and local government.. Are mangroves causing other valued habitats in harbours to decline? Or is there something else going on?

The perception that mangrove expansion was causing areas to become more muddy, leading to the loss of beaches, shellfish beds and recreational access, led to calls for their removal. A number of groups were formed from the 1990s onwards with the aim of restoring estuaries by controlling mangroves. They have been removed from a number of areas, sometimes with consent from the local council, sometimes illegally. There have been some large-scale clearances covering tens of hectares, including some done as carefully monitored trials.

The results have been mixed. Some cleared areas have returned to a sandier condition following mangrove removal. Others have stayed muddy, and some have deteriorated ecologically, with algal blooms and decreased oxygen levels.

The relationship between mud and mangrove is rather like the proverbial chicken and egg. Is the mud there because of the mangroves, or the mangroves there because of the mud? The answer seems to be a combination of both. Mangrove forests do trap and stabilise sediment, gradually raising the level of the mud. But where mangroves are replacing shellfish beds and seagrass meadows, they are largely doing so because these areas have become increasingly muddy – making them more suitable for mangroves and less suitable for shellfish and seagrass.

The primary reason for mangrove expansion is not what is happening in the sea, but what is happening on land. More specifically, human activities on the land are affecting fresh water, which in turn affects estuaries. Deforestation and land disturbance for agriculture, forestry and urban development have led to massive increases in sediment entering our waterways, and much of this sediment has ended up deposited in estuaries. It is this increase in sediment entering estuaries which has increased the suitable area for mangroves. Another contributor to the increase of mangroves may be increased nutrient levels in estuaries – again the result of human activities on land.

All of this suggests that if we want to restore our estuaries to places with pleasant sandy beaches and productive shellfish beds, there’s little point on focusing on the mangroves. We have to look at the whole catchment surrounding each estuary, and try to stop the sediment and nutrients ending up in the water in the first place.

This is not to say that mangrove removal is always pointless or wrong. In some areas, particularly when it’s done using low impact methods such as seedling removal, it can help to return areas to sandier conditions. And mangroves are a species which is thriving in New Zealand. So removing limited areas of mangrove is not the same as cutting down a patch of native lowland forest, which is now a very rare environment.

But what about climate change? Has it had a part in mangrove expansion? And what will be the fate of mangroves in future years, with increased temperatures and the resulting sea level rise? Will they expand their distribution to the south with warmer temperatures? Will they be swamped by rising seas?

A warming climate is probably one of the factors which has contributed to mangrove expansion in New Zealand, and it will continue to do so. Mangroves in New Zealand are limited by cold temperatures, and not at all by heat – the New Zealand species of mangrove grows in much warmer climates than here. So, as the climate warms, it will only make the environment more suitable.

But exactly what sea level rise will do is unclear. In some areas, rising seas may increase erosion and lead to the loss of mangrove forests. In others, the rate of sediment accumulation may be high enough to keep pace with the rising sea, meaning mangroves will persist. As with mangrove removal and estuary restoration, each site is likely to be different. However, given the role of all mangroves in both sequestering carbon and protecting shorelines from erosion, mangroves are only likely to become more valuable, both in New Zealand and overseas.