''Metrosideros robusta'', commonly known as the northern rātā, is a forest tree endemic to New Zealand. It grows up to or taller, and usually begins its life as a hemiepiphyte high in the branches of a mature forest tree; over centuries the young tree sends descending and girdling roots down and around the trunk of its host, eventually forming a massive, frequently hollow pseudotrunk composed of fused roots. In

disturbed ground In ecology, a disturbance is a change in environmental conditions that causes a pronounced change in an ecosystem. Disturbances often act quickly and with great effect, to alter the physical structure or arrangement of biotic and abiotic element ...

, or where there are gaps in the forest cover, northern rātā will grow on the ground with a normal but short trunk.

Distribution

Northern rātā is found in the

North Island The North Island ( , 'the fish of Māui', historically New Ulster) is one of the two main islands of New Zealand, islands of New Zealand, separated from the larger but less populous South Island by Cook Strait. With an area of , it is the List ...

from Te Paki in the north to

Wellington Wellington is the capital city of New Zealand. It is located at the south-western tip of the North Island, between Cook Strait and the Remutaka Range. Wellington is the third-largest city in New Zealand (second largest in the North Island ...

in the south. Formerly widespread, it is now uncommon over large parts of its former range, and is apparently absent from all but the south-east of Hawke's Bay. In the

South Island The South Island ( , 'the waters of Pounamu, Greenstone') is the largest of the three major islands of New Zealand by surface area, the others being the smaller but more populous North Island and Stewart Island. It is bordered to the north by ...

, northern rātā is common from Nelson to

Greymouth Greymouth () (Māori language, Māori: ''Māwhera'') is the largest town in the West Coast, New Zealand, West Coast List of regions in New Zealand, region in the South Island of New Zealand, and the seat of the Grey District Council. The populat ...

and Hokitika. It reaches its southern limit near Lake Mahinapua at 42°4′ South latitude. The natural habitat is forest along the coasts and in the lowlands. In some parts of its range northern rātā occurs in montane forest. Formerly, with rimu (''

Dacrydium cupressinum ''Dacrydium cupressinum'', commonly known as rimu, is a species of tree in the family Podocarpaceae. It is a dioecious evergreen conifer, reaching heights of up to , and can have a stout trunk up to in diameter. It is endemic to New Zealand ...

'') it was a dominant tree in a forest type known as rimu/rātā forest. In 2024 an unusual 105 feet (32 meters) tall northern rātā near Karamea on the west coast of South Island, nicknamed the "walking tree" because of its pair of leg-like trunks resembling legs walking across a field, won the New Zealand Arboricultural Association's (NZ Arb)'s Tree of the Year Award with 42% of the public vote.

Description

Northern rātā is a massive tree, easily distinguished from other ''Metrosideros'' species by its small, leathery, dark green leaves that are long, and have a distinct notch at the tip. Young growth is generally pink and covered in fine rust-coloured hairs that are gradually shed as the foliage ages but tend to persist at the midrib and in the vicinity of the leaf base. The flowers, borne in sprays on the tips of branches, are a mass of dark scarlet stamens. Flowering peaks between November and January, and seeds take a year or slightly more to ripen. The bark is usually brown or grey-brown and rather corky and provides an ideal stratum for the roots of epiphytic plants such as '' Astelia'' species and '' Freycinetia banksii'' (kiekie). The wood is reddish brown, and the manner of its growth results in a twisted grain.

Hemi-epiphytic growth habit

Northern rātā often begins life as hemi-epiphyte, and the resultant tree has a hollow trunk up to 4 m in diameter made up of interlocking roots that enclose the space left by the former host tree. The host tree of epiphytic northern rātā is usually

rimu ''Dacrydium cupressinum'', commonly known as rimu, is a species of tree in the family Podocarpaceae. It is a dioecious evergreen conifer, reaching heights of up to , and can have a stout trunk (botany), trunk up to in diameter. It is endemis ...

. In former times, the tree was described as a 'strangler', however it may be that rātā can only establish in trees that are already in decline. Northern rātā usually occurs in hardwood, podocarp, and southern beech forests. It is often associated with such species as rewarewa, tawa, hīnau,

kānuka ''Kunzea ericoides'', commonly known as kānuka or white tea-tree, is a tree or shrub in the myrtle Family (biology), family, Myrtaceae and is endemic to New Zealand. It has white or pink flowers similar to those of ''Leptospermum'' and from its ...

kahikatea ''Dacrycarpus dacrydioides'', commonly known as kahikatea (from Māori language, Māori) and white pine, is a Pinophyta, coniferous tree endemism, endemic to New Zealand. A Podocarpaceae, podocarp, it is New Zealand's tallest tree, gaining hei ...

, kāmahi, kohekohe, pukatea and māhoe.

Conservation

The greatest threat to northern rata is browsing by introduced possums (''Trichosurus vulpecula''), which cause severe damage by eating the leaves, buds, flowers and young shoots of the tree. In severe cases this can lead to the death of the tree within two years. In some districts, this has resulted in the local loss of the species. The situation of the species has been improved through the efforts of Project Crimson. Hybridisation with

pōhutukawa Pōhutukawa (''Metrosideros excelsa''), also known as the New Zealand Christmas tree, or iron tree, is a coastal evergreen tree in the Myrtus, myrtle family, Myrtaceae, that produces a brilliant display of red (or occasionally orange, yellow o ...

is another threat to the survival of northern rātā. Pōhutukawa has naturalised in areas south of its natural range, and poses a problem in localities where the northern or southern rātā species are present. Other threats include cutting for firewood, and forest clearance for roading or construction.

Cultivation

Northern rātā may be propagated readily from fresh seed. Viability of the seed drops quickly after a few weeks or if it is allowed to dry out. Cuttings do not strike readily, but better results can be obtained from using water shoots. At least three

cultivar A cultivar is a kind of Horticulture, cultivated plant that people have selected for desired phenotypic trait, traits and which retains those traits when Plant propagation, propagated. Methods used to propagate cultivars include division, root a ...

s of northern rātā have been released. There are also known cultivars of hybrids between northern rātā and pōhutukawa (known as ''Metrosideros ×sub-tomentosa''), and between northern rātā and southern rātā. Known cultivars include: ∞ Hybrid of northern rātā and southern rātā
¤ Hybrid of northern rātā and pōhutukawa

References