Myrtaceae in New Zealand

Importance | Description | Species and genera in NZ | Myrtle rust | Threatened status

Overview

Myrtaceae, or the myrtle family, is a family of flowering plants placed within the order Myrtales. It is a large, pantropical family with about 140 genera and 5,500 species, with centres of diversity in South America, Southeast Asia and south-western and eastern Australia. The main Old World genus is Syzygium. In the Neotropics the genera with the most species are Eugenia, Myrcia, and Psidium.

Importance

Eucalyptus is economically the most important genus due to its timber. The family also contributes to the spice trade, for example, allspice (Pimenta dioica), cloves (Syzygium aromaticum), and several genera, including Eucalyptus and Leptospermum, produce natural oils with medical uses. The family includes several edible fruits: feijoa (Acca sellowiana), guava (Psidium guajava), and several species of Syzygium.

In New Zealand, several indigenous species of Myrtaceae are utilised to produce honey, with mānuka (Leptospermum scoparium) honey being especially valuable. Popular native trees and shrubs, grown for their ornamental qualities, include selections of mānuka (Leptospermum scoparium), pōhutukawa (Metrosideros excelsa), and ramarama hybrids (Lophomyrtus ×ralphii).

Notable exotic members cultivated in New Zealand include eucalypts (Angophora, Corymbia, and Eucalyptus spp.), grown for their forestry and amenity uses, and feijoa (Acca sellowiana), grown for their edible fruit. Australian bottlebrush (Callistemon and Melaleuca) and tea-tree (Leptospermum) species and cultivars are also popular as ornamental trees and shrubs.

Description

All species are woody, and are large to small trees, sometimes shrubs, rarely vines (e.g. several New Zealand Metrosideros species), with bark that is rough and fibrous, hard or stringy, or smooth and peeling, mottled, and often coloured (e.g. Eucalyptus).

The leaves of Myrtaceae are simple, opposite or alternate, rarely whorled, and always evergreen. The leaf margins are generally entire (i.e. without a toothed margin). The leaf surfaces usually have glandular, oil-filled dots containing essential oils, and so they often have a strong citrus or camphor (eucalypt) smell when crushed. Intramarginal veins are usually present and often conspicuous.

The flowers are usually in clusters or rarely solitary, and may be either terminal or lateral on branches. A few species produce flowers from the main stems and branches (cauliflorous). The flowers are regular (actinomorphic) and bisexual. They usually have a base number of five petals, although several genera have four petals, or they are minute, absent or fused into a cap (operculum), which drops off as the flower opens. In a few genera the petals may be brightly coloured.

There are four or five sepals, usually free or fused into a cap (operculum), or sometimes reduced.

The stamens are usually very conspicuous, mostly white but sometimes yellow, orange, pink or red. They are numerous, rarely 10 or fewer, and sometimes occur in bundles, in which case they are usually opposite the petals. The ovary is usually inferior (i.e. the seed-bearing part is located below the attachment of other floral parts), (1–)2–6(–12)-locular, with a single style and stigma.

The fruit is either a fleshy berry or a dry capsule. Fleshy fruits have 1–10(–22) seeds per fruit, and dry fruits contain many (more than 10) seeds per fruit.

Species and genera in New Zealand

New Zealand is represented by six indigenous genera of Myrtaceae: Kunzea, Leptospermum, Lophomyrtus, Metrosideros, Neomyrtus, and Syzygium.

  1. Kunzea contains more than 50 species endemic to Australasia. The New Zealand K. ericoides complex has recently been revised and expanded to include 10 species.
  2. Leptospermum contains about 87 species, mostly Australian, but extending to Malesia and New Zealand. Leptospermum morrisonii ‘Copper Sheen’ is the most cultivated Australian Leptospermum in New Zealand. The indigenous New Zealand species, L. scoparium (mānuka), is very variable, with two varieties currently accepted. More than 150 ornamental cultivars have been selected from this species.
  3. Lophomyrtus has two species (L. bullata and L. obcordata), both endemic to New Zealand. Many ornamental cultivars of hybrid origin have been selected.
  4. Metrosideros contains more than 50 species of trees, shrubs and vines, mostly found in the Pacific region. New Zealand is well represented by having 12 endemic species. Many ornamental cultivars have been selected, particularly from M. excelsa.
  5. Neomyrtus is a New Zealand endemic genus and is also monotypic, being represented by only one species, N. pedunculata.
  6. Syzygium contains more than 1,200 species in Africa, Asia, Malesia, Australasia, New Caledonia and the Pacific Islands. New Zealand has one endemic species, S. maire, and several introduced species, including S. australe and S. smithii which are becoming environmental weeds.

This key includes 19 exotic genera of Myrtaceae. All are cultivated and several have naturalised.

  1. Acca contains three South American species, represented in New Zealand by A. sellowiana, which produces the well-known fruit feijoa. A range of cultivars have been selected for their edible qualities, fruit size and ripening time.
  2. Agonis contains four species. All are endemic to Western Australia.
  3. Angophora contains 10 species related to Eucalyptus and endemic to eastern Australia.
  4. Astartea contains 22 species endemic to south-western Western Australia.
  5. Backhousia contains 13 or more species endemic to Australia.
  6. Callistemon contains about 50 species endemic to Australia. Some botanists do not recognise Callistemon, placing species in Melaleuca instead. In this key we follow the Australian Plant Census (APC) by recognising Melaleuca and Callistemon as separate genera.
  7. Chamelaucium contains more than 10 described species and many undescribed taxa. All are shrubs endemic to southwestern Western Australia.
  8. Corymbia contains more than 100 species, mainly in Australia but also extending into New Guinea. Until 1990, Corymbia was included in the genus Eucalyptus and there remains debate among botanists as to whether separating them is valid. Corymbia is currently an accepted name in the Australian Plant Census (APC).
  9. Eucalyptus contains nearly 800 species, most of which are endemic to Australia, with a few species extending to parts of Malesia and as far north as the Philippines.
  10. Lophostemon contains four species indigenous to Australia, with one species extending to New Guinea.
  11. Luma has two species indigenous to Chile and Argentina. Only L. apiculata is known to be present in New Zealand.
  12. Melaleuca contains about 230 species, centred in Australia but extending to Asia, Malesia, and New Caledonia.
  13. Myrtus contains three species, as currently recognised. Numerous names were proposed in the genus, but almost all have been transferred to other genera or regarded as synonyms. Only Myrtus communis is widely cultivated.
  14. Psidium contains about 100 species indigenous to tropical America.
  15. Syncarpia contains three species endemic to Queensland and New South Wales in Australia. Only S. glomulifera is known to be present in New Zealand.
  16. Taxandria contains 11 species endemic to Western Australia. Four species are recorded in New Zealand, with T. juniperina the most common and occasionally naturalising.
  17. Thryptomene contains about 47 species endemic to Australia. Several species and cultivars have been grown in New Zealand, but none are common.
  18. Tristaniopsis contains about 40 species in Southeast Asia, New Guinea, New Caledonia, and Australia. Tristaniopsis laurina is the main species known to be present in New Zealand.
  19. Ugni contains four species indigenous to southern Mexico and South America. Only U. molinae is widely cultivated.

Several other exotic genera of Myrtaceae are present in New Zealand, but these are quite rare and not included in this key.

Myrtle rust

Myrtle rust is a fungal disease caused by Austropuccinia psidii and affects members of the Myrtaceae family. Myrtle rust originated in South America and has spread across the world at an increasing rate since the 1930s. It is now found in more than 20 countries. Across the Pacific Ocean it arrived in Hawaii in 2005, Australia in 2010, New Caledonia in 2013 and New Zealand in 2017.

This rust is a threat to New Zealand’s many ecologically, culturally and socially high-value and iconic indigenous species, such as pōhutukawa (Metrosideros excelsa), rātā (Metrosideros spp.), and mānuka (Leptospermum scoparium), and their associated flora, fauna, ecosystems, and ecosystem services. Depending on their susceptibility to myrtle rust, this disease is a potential threat to the mānuka (Leptospermum scoparium) honey industry, and to forestry, amenity and commercially important exotic species such as the eucalypts (Eucalyptus spp.) and feijoa (Acca sellowiana).

Threatened status

Because of concerns over the arrival of myrtle rust to New Zealand, the threat status of all indigenous Myrtaceae was raised in rank in 2018. Threat classifications are stated in the NZPCN website and follow the latest New Zealand Threat Classification System.