![]() ![]() ![]() In the Far North, the annual litter drop is about five to six tonnes per hectare. In summer, when the water loss through evaporation would be greatest, the mangrove increases its leaf drop by 10 times. Mangroves lose about 60 percent of their leaves in a year. Many plants use this as a way of isolating and getting rid of unwanted chemicals in old leaves. Along with the salty residues of splash and spray, this is easily rinsed away by rain or the rising tide.Īnother way of getting rid of salt is by shedding leaves. These are the openings of salt-secreting glands which get rid of extra salt by exuding a brine that is more concentrated than full strength seawater. Under a microscope, hundreds of tiny pores can be seen on the upper surface of a mangrove leaf. The leaves of the mangrove also help the plant regulate its salt content by being able to secrete salt. Mangroves can survive in such a salty environment because the salt water in its sap stops water loss from the plant tissues. Mangrove leaves have several adaptations for salty living. This is much higher than land plants which have sap concentrations of 0.2 grams per litre, but still much lower than seawater. The cell sap concentration of the New Zealand mangrove is about 2 grams per litre. They use a number of clever mechanisms to regulate the amount of salt in their sap, so they can survive in seawater.įull strength seawater has a salt concentration of about 34 parts per thousand, or 34 grams per litre. Mangroves stand in saltwater all the time, and this is the reason for some of the plant's greatest adaptations. The warmer the climate, the bigger they grow. Mangroves hate frost, which is why they like the warm, Northland climate so much. They can tolerate being completely submerged in seawater but need to be uncovered for at least half of each tidal period so that the trees can absorb oxygen. Mangroves need to grow in quiet waters such as inside harbours and estuaries, because exposed areas would either uproot them or carry away the silt in which they would take root. Worldwide there are 23 genera from eight different families that have species described as mangroves. The word "mangrove'' refers to all types of trees that have adapted to living in the sea. In Australia it is called Avicennia marina while in New Zealand it is called Avicennia resinifera. However, in the southern parts of Australia and the northern part of New Zealand, where the climate is cooler, only one species of mangrove exists - although they have different names. There are 50 species in South-East Asia, with more than 30 in northern parts of Australia. Mangroves mostly grow in warmer climates, reaching their greatest size and diversity in the tropics. Sometimes they are found in Aotea and Kawhia harbours a little further south. On the east coast they extend naturally as far south as Ohiwa Harbour in the Bay of Plenty, and on the west coast Raglan Harbour is generally regarded as their southern limit. The mangrove trees of New Zealand only grow in the top half of the North Island of New Zealand. The proposed plan sets rules regarding mangrove pruning and removal, which vary according to the size of the mangroves and where they are. The council's Proposed Regional Plan sets out rules and guidelines for the region's "coastal marine area", which is the area from mean high water springs to the 12 nautical mile (22.2km) limit of New Zealand's territorial sea. Sustaining a beautiful coastline for future generations takes planning. Council's policy on mangrovesĮxtensive areas of mangrove forest and saltmarsh in harbours on both coasts provide a haven for young fish species and feeding and roosting areas for coastal birds. However, it is now known that mangroves play an important part in the ecosystems of our harbours - sheltering young fish, stabilising land and forming a buffer zone to absorb floodwaters. There are not many other flowering trees that could survive in these conditions, yet the mangrove has adapted so well that it has formed dense forests in sheltered harbours in Northland. It has adapted to living in the harshest of conditions - a dunking in salt water twice a day when the tide comes in and heavy, stinky mud with no oxygen for its roots. The mangrove tree is one of the marvels of our Northland harbours. ![]()
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