December - Pōhutukawa - Native Plant of the Month
Our national Christmas tree is an icon that needs no introduction. Pōhutukawa are one of our most recognised and loved native trees.
Pōhutukawa belong to the Metrosideros genus. Plants in this genus are instantly recognisable for their dark green oval leaves and fluffy red or white flowers with masses of protruding stamens. In New Zealand most of our native Metrosideros are rātā, many of which are climbing vines. The name Metrosideros means iron heart and refers to the extremely hard and dense wood that many plants of this genus have - particularly larger species like pōhutukawa and southern rātā. Metrosideros species are distributed around the Pacific, including Hawai’i, the Solomon Islands and Tahiti. Aotearoa is considered a centre for diversification of this genus and a paper published in 2003 found genetic evidence that Metrosideros species in the north Pacific have evolved from our very own pōhutukawa.
Here in the northern parts of New Zealand we tend to generously consider pōhutukawa a national treasure, but ‘New Zealand’s’ Christmas tree does not actually brighten beaches across our whole country - pōhutukawa are only native to the north of the North Island. The natural range of pōhutukawa is from the top of the North Island down to Mt Taranaki in the west and Gisborne in the East, along the coast and on the edges of some of our larger lakes - although it has been suggested that the pōhutukawa growing around the shores of the Rotorua lakes and lake Taupō may have been deliberately planted by Māori. Pōhutukawa are not an inland tree - they are frost sensitive and well adapted to the warmer coastal climate. Despite this they have long been cultivated and planted south of their natural range and it is easy to find pōhutukawa growing in Wellington, Nelson, Christchurch or Dunedin. Trees planted in cooler areas rarely set seed and so are less able to naturally regenerate. Pōhutukawa have also been planted overseas, including in Portugal and California, where they are sometimes considered a weed!
Pōhutukawa are natural beach bums. They love to be beside the sea as much as we do. They have a suite of adaptations that enable them to thrive in the coastal habitat and overcome challenges that keep some of our other tree species from embracing the coasts. Salt spray coming up off the sea is air that is laden with moisture and small salt crystals. If you have worn sunglasses on a west coast surf beach you may have noticed that they become coated in an almost greasy film - this is from the salt spray. Salt is toxic to most plants and living in an environment laden with salty air can be a challenge. Salt crystals are sharp and can tear into soft leaf tissue in high winds. This is why most coastal plants have thick, leathery leaves. Pōhutukawa leaves are no exception. New leaf buds on pōhutukawa are protected by leaf scales, which fall away as they emerge.
Leaf scales are not seen on many of New Zealand’s native plants - they are typically an adaptation to protect the growing tips of deciduous trees through cold winters and are seen in many northern hemisphere trees such as oaks. Our pōhutukawa have adapted these leaf scales to protect their growing tips not from snow and frost, but from salt spray. Karo, another coastal species, has also made this adaptation. Pōhutukawa leaves are tough and leathery on top, but have a soft silvery underside. This comes from a fine coating of pale hairs, called a tomentum, which helps to regulate humidity near the leaves’ stomata - where air, carbon dioxide and oxygen are taken in and expelled as part of plants’ respiration and photosynthesis processes. Coastal wind and high salt saturation in the air can create very dry conditions for pōhutukawa to survive in and their tomentum helps overcome this challenge.
The most distinctive feature of pōhutukawa is their crimson flowers. Their brilliant floral displays, which coincide with the beginning of our summer holiday period, mean that many of us have warm associations with pōhutukawa and happy times at the beach. Pōhutukawa flowers are typical of the Myrtle family to which they belong. Instead of showy petals they have a dense brush of stamens with a single pistil rising from the centre. The shallow, bowl-like base of the flower is filled with nectar. Red flowers have typically evolved to attract bird pollinators and pōhutukawa flowers are definitely a hit with tūī. They sport golden crowns of pollen from ducking their heads into flowers to feed on the nectar below. Tūī aren’t the only birds that pollinate pōhutukawa, you might also see kākā, sparrows, silvereyes/tauhou and bellbirds/korimako dipping into the blooms. Because of their open form and prolific nectar, pōhutukawa flowers are also visited by a wide suite of non-bird pollinators including honey bees, native bees and geckos.
While most of us think of pōhutukawa as having red flowers, if you look around carefully you will find more variety. Different trees have slightly different shades of red, ranging from deep dark blood reds, through to orangey tones. Occasionally trees will grow with pink, yellow or even white flowers. Commercial cultivars of these unusually coloured trees are sold in some nurseries, but in our eyes you can’t beat the classic red flowers. You may have noticed that pōhutukawa are not flowering very profusely this year. This is because pōhutukawa are a mast seeding species. This is a habit that many of our native plants have where they synchronise mass flowering some years, followed by other years with little (or even no) flowering. Mast years are thought to be cued by weather conditions and help to prevent insects or other herbivores damaging too many flowers, fruit or seeds to ensure good survival. Last year was a mast year for pōhutukawa, but this year is not. In non-mast years trees tend to spend more energy growing new leaves and shoots so you might notice more silvery new growth on trees.
Pink, crimson and yellow pōhutukawa flowers
You may have heard the saying that pōhutukawa flowering earlier in the year means that it will be a hot summer to come. Pōhutukawa blooming is used as part of maramataka - the Maori calendar. This observational way of understanding the patterns of the natural world helps guide different activities to best work with the changing seasons. A gnarled pōhutukawa growing from a cliff below Cape Reinga at the top of the country is also significant in traditional Māori beliefs as the place where the spirits of the dead begin their journey back to ancestral Hawaiki. Another ancient pōhutukawa growing at Kāwhia was crowned as the inaugural Tree of the Year in June. This tree was where the Tainui canoe was pulled up and secured at the end of its journey to Aotearoa.
Pōhutukawa have an unusual form, growing as wide or even wider than they are tall at maturity. Their large, twisting multiple trunks are very distinctive and are often amazing play spaces for children. Pōhutukawa wood is not just hard, it is also flexible, allowing the trees to cope with strong coastal winds, as does their iconic spreading shape. The twisting branch shapes of the trees made them sought after by boat builders in the 19th and early 20th centuries. Bent branches - called knees and elbows, were used to build curved boat frames and this use meant that many pōhutukawa were lost near boat building areas. Rangitoto bach owners reportedly protected their pohutukawa from poachers looking for knees to cut. The tough, durable timber was also used by Māori for a range of uses, including paddles and implements.
As well as having knees and elbows, pōhutukawa sometimes also have beards. These are made of aerial roots and can form huge masses hanging down from branches. Aerial roots help the trees absorb moisture from the air and collect any rain or condensing mist that drips down branches and trunks. If you look closely at old pōhutukawa trees you will often see that their limbs appear woven with large aerial roots that wrap around trunks and run down to the ground. When aerial roots meet the ground they will thicken, become woody and begin acting like a regular root. This can help large branches anchor to the ground as they sprawl out away from the base of the tree. This ability to gradually collapse outwards and root down makes it very hard to age extremely old pōhutukawa, which can appear like twisting groves rather than single trees.
Pōhutukawa have an amazing ability to grow in cracks in rocks or between paving stones. They are well adapted to cliff-side life. This makes them fantastic pioneers and they are usually one of the first trees to colonise new land. Rangitoto is a beautiful example of this - pohutukawa grow straight out of the rough scoria surface of the volcanic cone. The thread-like seeds, about the size and shape of a baby’s eyelash, are dispersed by the wind and easily lodge into small gaps. Rangitoto is an interesting ecosystem as both pōhutukawa and northern rātā colonised the rocky slopes and a hybrid forest has developed. Hybrids between pōhutukawa and Northern rātā are quite common and nursery-grown hybrids are often planted out in civic spaces. These are usually sold under names like ‘pōhutukawa Mistral’ and combine qualities of the two parent species.
Pōhutukawa are hardy, long-lived trees that can survive the extremes of a coastal habitat, but they are still vulnerable to other threats. In fact, they are listed as threatened - nationally vulnerable under DoC’s threat assessment schedule. There are three reasons for this: habitat loss, possum browsing and myrtle rust. Coastal areas are very attractive to people and are often where we want to build baches or have recreation areas. This has meant that large areas of coastal pōhutukawa forest have been cleared and many more trees continue to be removed. Pōhutukawa and rātā are very attractive food sources for possums and these pests will browse trees to death if uncontrolled. Project Crimson has been working for several decades to respond to this threat and protect our pōhutukawa from possums. Myrtle rust arrived in New Zealand in 2017 and has spread rapidly. Among the plants that it can infect are pōhutukawa. This fungal pathogen infects tender new growth and can kill seedlings quickly. As myrtle rust is increasing in our communities we are seeing more and more pōhutukawa infected. Reducing exotic host species (like lilly pilly) in our environments is an important way to reduce the disease pressure on pōhutukawa and other native species that are impacted by myrtle rust so that we can enjoy these fantastic trees for many generations to come.
Click here for more information about how to recognise and what to do if you find myrtle rust.