Perhaps our most unappreciated native tree could be one of the most important; learn what makes our mangroves so special.
Saltwater Specialists
Most plants can’t live in salt water but mangroves have made a niche for themselves with the use of some clever adaptations to survive being covered by salt water twice a day. They have a thick waxy layer on their leaves that makes them waterproof. The hairy underside of the leaves helps to reduce water loss. They shed their leaves at a rapid rate as a way to extrude salt and their root system includes snorkels which help them to breathe in the dense mud.
Habitat Creators
The settled waters around mangrove forests are great spaces for aquatic species to breed in and the branches above create a habitat for nesting birds . In urban environments without forests, mangroves provide an ecological corridor for birds to travel through. Many birds utilise mangroves e.g. greenfinches which feed on their seeds, swamp harriers (kahu) lay in wait for prey on the high tide and white-faced herons and banded rails use them for cover until the tide recedes and they then move out to forage on the mudflats. Mangroves shed their leaves often which adds a lot of organic matter into the ecosystem, this supports invertebrates which in turn feed larger animals.
Climate Change Fighters
Mangrove forests are considered very good carbon sinks; the rate of carbon sequestration is estimated at up to 100 times faster in coastal vegetation than on land. Carbon stored by coastal vegetation can be locked away for millennia if undisturbed. New Zealand’s mangrove swamps are estimated to store a total of about 120 tonnes of carbon per hectare.
Mangrove roots and the sediment built up by them stabilise coastlines making them less susceptible to erosion. They also perform an important buffer to wave action which helps protect the coastline from the impacts of sea level rise.
Sedimentation and Mangroves
Across the world mangroves are in decline, but New Zealand's mangroves are rapidly expanding at an average rate of 4% per year since the 1940s, enabled by elevated levels of sediment due to land-use changes.
Mangroves are often hated for the false belief that they turn beaches into mud but research has shown that mangroves thrive where there is already sediment and tests have proven removing mangroves does not reduce the accumulation of mud (sediment). In the large majority of estuaries where research has been carried out, sedimentation rates are now 2-5mm per year and this will continue with deforestation, construction and other activities that disrupt the land.
Mangroves are taking the opportunity provided to them by our disruption of the land, turning something bad: sedimentation, into something good: an amazing functioning ecosystem. So don’t hate on mangroves; they are doing incredible things.