Out on a limb: the precarious pōhutukawa

Natalie Spyksma discusses the prevalence of pōhutukawa on Tiritiri Matangi, and how their future cannot be taken for granted with the risk of myrtle rust.

exact replicas of the parent tree, a form of cloning if you like, which appear as named varieties in nurseries and garden centres. These trees avoid the juvenile stage and flower almost immediately but are not usually the type used in revegetation projects, such as on Tiritiri Matangi, where locally-sourced seed from strong, healthy trees is the preference.

Variation

Pōhutukawa seedlings vary hugely. This is a consequence of the plant’s reproductive strategy, which focuses on multi-staged flowering. This helps to ensure continued gene pool diversity.

Flowers are pollinated by nectar-feeding birds (tūī, korimako and hihi on Tiritiri Matangi), bees, bats and geckos. They also have a short hermaphroditic stage during which flowers can self-pollinate (inbreed).

Seldom do the flowers appear all at once on either an individual tree or across the population. This provides multiple opportunity for pollination from different sources, especially in the face of adverse weather conditions.

Seed is dispersed by the wind through March and April and can travel vast distances, providing a useful mechanism for gene flow between populations. Seeds struggle to establish amongst grass or other vegetation, making bare earth, exposed coastal slip faces and rocky crevices ideal spots for germination to occur.

Nothing symbolises a northern New Zealand Christmas more distinctly than the crimson red flowers of the pōhutukawa tree (Metrosideros excelsa). These coastal stalwarts naturally inhabit shorelines and hang precariously from cliffs north of Taranaki and Gisborne, but have now been planted much further afield where frosts aren’t severe.

Pōhutukawa flower sporadically from December to late January, with the peak flowering period a short 10-day window either side of Christmas. The timing can vary from year to year and is generally a reflection of the previous growing season, particularly when bud initiation is taking place in the autumn.

As the ferry pulled away from Tiritiri Matangi a few days before Christmas, only a hint of red was revealed across the hillside, rather than the usual characteristic blaze.

Indeed, a late and poor flowering occurred on the Island during the 2022/2023 season.

Thousands of pōhutukawa seedlings were planted on Tiritiri Matangi between 1984 and 1994 to join the few mature specimens already standing. One of those on the Kawerau Track has since been carbon dated at 800–1000 years old. It is a sprawling monster!

There is now a literal forest of pōhutukawa in places on the Island and some trees are being removed to create light wells to allow other species to germinate, improving diversity. However, initially, they were quick to establish and provided shade for other species at the beginning of their lives.

Aside from seedlings, another way to grow pōhutukawa is from cuttings. Propagation material is ideally selected from mature healthy specimens that flower spectacularly and have good form. This method produces

This results in the wide variation of characteristics with which we are familiar. Some flower more prolifically than others, some don’t flower at all and the shades of crimson red vary. Natural selection will weed out the weakest along the way and, interestingly, this is more probable amongst the self-pollinated.

The support system

As well as producing basal roots, pōhutukawa can grow aerial (adventitious) roots from their trunk or branches, as needed. These help the trees to colonise rocky or poor soils where little else can survive. Development is mostly stunted on rocky terrain but under optimum conditions a mature specimen can form a wide-spreading tree 25m tall and 35m wide.

In order to support such a mass, pōhutukawa wood is extremely dense and strong (Metrosideros means hard heart wood). Trees will branch from a young age, eventually forming huge horizontal limbs that not only withstand wind damage and drought but spread and balance the weight of the crown as widely as possible across the unstable terrain they often inhabit. In turn, the crown protects the roots from the elements.

Families and threats

Being a member of the Myrtle family (Myrtaceae), sadly pōhutukawa are now coming under threat from myrtle rust, a disease thought to have arrived recently on the wind from Australia. The rust generally attacks and distorts new growth, buds, flowers and fruit and is listed as a biosecurity threat.

Possums, goats, trampling stock, habitat loss and fire are also huge threats to pōhutukawa. A grass fire or beach bonfire at their base can kill a mature tree.

Although there are only two endemic species of pōhutukawa in New Zealand, M. excelsa and M. kermadecensis (a smallergrowing variety from the Kermadec Islands), there are many other Metrosideros scattered throughout the Pacific, plenty of which now grow here in our gardens.

I remember a wise man once asking me if I was worried about planting the introduced cultivar, M. springfire, prized for its mass of spring flowers, in case it crossed with our pōhutukawa. I also remember replying, “No, they don’t flower at the same time, so it shouldn’t be an issue.” How wrong I was! This year I observed exactly that happening right outside our window. A huge old specimen flowered unusually early in November and neighbourhood springfires extended their flowering season right into early December. Tūī flew happily between them. Food for thought!