Metrosideros robusta

Metrosideros robusta, the northern rātā, is a forest tree endemic to New Zealand. It grows up to 25 metres (82 ft) 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, or where there are gaps in the forest cover, northern rātā will grow on the ground with a normal but short trunk.

Northern rātā
A northern rātā towers above the forest at Kaitoke Regional Park, Wellington.
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Clade: Rosids
Order: Myrtales
Family: Myrtaceae
Genus: Metrosideros
Species:
M. robusta
Binomial name
Metrosideros robusta

Distribution

Northern rātā is found in the North Island from Te Paki in the north to Wellington 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 Hawkes Bay. In the South Island, northern rātā is common from Nelson to Greymouth 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) it was a dominant tree in a forest type known as rimu/rātā forest.

Description
The leaves of northern rātā have a distinct notch at the tip.

Northern rātā is a massive tree, easily distinguished from other Metrosideros species by its small, leathery, dark green leaves that are 25–50mm long by 15–25mm wide, 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.

Rātā flowers at Mt Maungatautari

The host tree of epiphytic northern rātā is usually rimu. 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, kahikatea, 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 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 cultivars of northern rātā have been released.[1] There are also known cultivars of hybrids between northern rātā and pōhutukawa (known as Metrosideros ×sub-tomentosa), and between northern rātā and southern rātā.[2] Known cultivars include:

Cultivar name Year introduced Flower colour Introduced by Notes
M. robusta ‘Kawa Copper’ [1] 2001 Dull orange-red Deane Keir Coppery red-coloured new growth.
M. robusta ‘Krinkley’ [1] 1992 Dull orange-red - Variegated leaves with a characteristic twist on the leaves. Also marketed as M. robusta ‘Tane’s Gold’ by Duncan & Davies in 1996 and M. robusta ‘Twistie’.
M. robusta ‘Rangitoto Island’ [3] - Crimson Graeme Platt Sourced from Rangitoto Island.
M. ×sub-tomentosa ‘Hauparapara’¤ [2] Late 1980s Dull orange-red Graeme Platt Sourced from Hauparapara River in the Bay of Plenty.
M. ×sub-tomentosa ‘Mistral’¤ [2] - Deep scarlet Graeme Platt Sourced from east of Coromandel township on the Coromandel Peninsula.
Metrosideros ‘Sentinel Flame’∞ [2] 2007 Bright red[4] Graeme Platt / Vibrant Earth nursery Sourced from either Great Barrier Island or North-West Nelson.

∞ Hybrid of northern rātā and southern rātā
¤ Hybrid of northern rātā and pōhutukawa

See also
Wikimedia Commons has media related to Metrosideros robusta.

References
  1. "Metrosideros in cultivation: Rātā and other species" (PDF). New Zealand Garden Journal. 13 (2): 11. 2010.
  2. "Metrosideros in cultivation: Pōhutukawa" (PDF). New Zealand Garden Journal. 13 (1): 10–22. 2010. Retrieved 2015-05-25.
  3. "Metrosideros robusta 'Rangitoto Island.'". Vibrant Earth. Vibrant Earth. Archived from the original on 23 January 2015. Retrieved 16 June 2015.
  4. "Metrosideros umb x rob 'Sentinel Flame'". Vibrant Earth. Vibrant Earth. Archived from the original on 23 January 2015. Retrieved 16 June 2015.
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