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Pittosporum kirkii

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Pittosporum kirkii
A bokeh image of P. kirkii among with other trees and moss in the forest in the Coromandel Peninsula, in New Zealand's North Island.
P. kirkii in the Coromandel Peninsula

Declining (NZ TCS)[1]
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Clade: Asterids
Order: Apiales
Family: Pittosporaceae
Genus: Pittosporum
Species:
P. kirkii
Binomial name
Pittosporum kirkii

Pittosporum kirkii, commonly known as Kirk's kōhūhū or the thick-leaved kohukohu, is an evergreen perennial shrub in the family Pittosporaceae. It is endemic to New Zealand with a known restricted geographical range in the North Island, extending from the Northland Region to the northern reaches of the Whanganui River. It is also found on some of the North Island's offshore islands (such as Great Barrier Island).

P. kirkii was first described in 1869 by Joseph Dalton Hooker from material collected by Thomas Kirk. Hooker honoured Kirk by applying the specific epithet "kirkii" to the species. P. kirkii grows up to 5 metres (16 ft) in height and has leathery thick leaves that are widest towards the tip and with flattened hard capsules. Its leaves are 5–10 cm (2.0–3.9 in) long by 2–3 cm (0.79–1.18 in) wide. P. kirkii was assesed in 2023 by the New Zealand Threat Classification System as "At Risk – Declining", the decline of P. kirkii has been attributed to deforestation and the introduction of bushtail possums.

Description

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A bokeh image of the light Greenish-yellow leafage of a Kirk's kōhūhū (P. Kirkii) indvidual.
Leafage of Kirk's kōhūhū (P. Kirkii) in Waikato

Pittosporum kirkii is an evergreen perennial shrub in the family Pittosporaceae and is endemic to the northen half of the North Island (and some of its offshore islands) of New Zealand. P. kirkii is epiphytic and often grows on other trees. It has leathery thick leaves that are widest towards the tip and with flattened hard capsules. Its leaves are 5–10 cm (2.0–3.9 in) long by 2–3 cm (0.79–1.18 in) wide. Flowering occurs from October to December, P. kirkii produces yellowish-green coloured flowers, in a cluster at a tip of twigs. Fruiting occurs from January to May, it produces 2.5–4 cm (0.98–1.57 in) long fruit, and splitting into two showing the black seeds in yellow pith.[2]

P. kirkii grows to 1–5 m (3 ft 3 in – 16 ft 5 in) in height with the trunk up to 10 cm (3.9 in) in diameter. P. kirkii is commonly epiphytic; though, they can be observed growing by itself and rarely on rock surfaces.[3]

Its trunk holds densely arranged light green-brown coloured branches, with flexible stout reddish-to-purple branchlets. Its petioles are purple-reddish in colour and are 0.1–0.2 mm broad and short, 0.3–1.6 mm in length. The leaves are egg to oval shaped and are 4–12 cm (1.6–4.7 in) long and 0.7–3.2 cm (0.28–1.26 in) broad. Leaves are also generally coriaceous and are dark to pale green in colour.[4]

Its inflorescences (flower spikes) are pedicellate, complete and pentamerous (except for the gynoecia). Inflorescences have 1–10 flowers and are supported by 5–10 mm pedicels, subtended by numerous bud scales 1–2 mm long.[5]

Taxonomy

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P. kirkii was described by Joseph Dalton Hooker from material collected by Thomas Kirk, possibly from the Thames Goldfields, and published in 1869.[6][7] The initial brief description titled Pittosporum n. sp.? by Thomas Kirk was published in his paper on Great Barrier Island in 1868.[8]

Thomas Kirk provided brief descriptions for several unnamed species of the genus Pittosporum in his seminal paper on Great Barrier Island. Kirk's brief description of a new species "Pittosporum n. sp.?" was sent to Hooker along with herbarium specimens of the species in 1868. Hooker honoured Kirk by applying the specific epithet "kirkii". The holotype of P. kirkii is unclear as Kirk did not designate a holotype specimen, and while he mentions that P. kirkii was first discovered on Great Barrier Island, Kirk does not give the type locality in his description.[9] A 2001 thesis on phylogenetic research of P. kirkii has placed the species alongside its sister taxa P. dalli within the primary New Zealand Pittosporum clade, originating from all other New Zealand taxa approximately 13.5 million years ago. A second separate clade contains P. pimeleoides subspecies and P. cornifolium, which are believed to have originated from New Caledonia.[10]

Kirk's (1869) original species description reads:[6]

Pittosporum Kirkii, Hook. f., n. sp., A handsome, laxly-branched shrub, 3–15 feet high, branchlets stout, rigid, ascending; bark reddish-purple, leaves alternate, crowded or whorled, linear-obovate, acute or obtuse, 2-5 inches long, gradually narrowed into rather broad purple petioles, excessively coriaceous, glabrous, pale-green above, lighter below, midrib stout, prominent and cuiously flattened beneath; flowers terminal in 3-7 flowered umbels, peduncles rather stout, decurved; sepals broadly lanceolate with membranous margins; petals ligulate, bright yellowe, recurved; capsules erect, clustered, glabrous, elliptic, 1-1 1/2 inches long, obtuse, 2-valved, remarkably compressed, but the valves contract in a curious manner when the capsule bursts.

Etymology

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The etymology of the name of the genus derives from Latin, Pittosporum, translates to English as 'pitch seed'.[11] The specific epithet, kirkii, is named in honour of the New Zealand botanist, Thomas Kirk, and it is a Latinisation of his surname. P. kirkii is also commonly referred to as Kirk's kōhūhū and the thick-leaved kohukohu.[2][12]

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  • An image of the infloresence of P. kirkii that depicts its yellow colour with a light pink colour at its tip
    Infloresence
  • Capsules of P. kirkii with light green coloured leaves erected from the centre of the plant
    Capsules
  • P. kirkii growing on tāwari (Ixerba brexioides) and greenish moss with some orange moss can be seen growing in the background.
    P. kirkii on tāwari

Distribution

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Pittosporum kirkii is endemic to New Zealand and has a known restricted geographical range in the North Island, extending from Karikari Peninsula in the Northland Region and south of the northern reaches of the Whanganui River and west to Mount Taranaki. It has an estimated mean altitude of 50–1,116 m (164–3,661 ft) metres above sea level.[10][13] It is also found on some of the North Island's offshore islands such as Great Barrier Island and Little Barrier Island.[14] P. kirkii is naturally not present in the Taupō Volcanic Zone, where favourable environmental conditions are anticipated; though, the vegetation composition has been significantly impacted by the extensive ecological disturbance brought about by an 186 AD eruption in Taupō.[15] P. kirkii's natural habitat is in dense forest usually growing near or on other plants in suitable sites with limited browsing animals, it is can also be found on visable rock and rubble slopes.[2]

Ecology

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P. kirkii is likely entomophilous, because its flowers have unspecialised structures and are small in size. Flies likely dominant in the cross-pollination of its flowers, as they were observed on the leaves of individuals of P. kirkii when they were in full bloom. In lowland and mixed broadleaf forests, P. kirkii and other species of the genus Pittosporum have been observed to host the endemic fringed scale insect Kalasiris perforata (Maskell). The fruit of other endemic Pittosporum species, as well as new growth and flowers, are consumed by introduced brushtail possums (an invasive species in New Zealand).[16][17]

Conservation

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P. kirkii was assesed in 2023 by the New Zealand Threat Classification System as "At Risk – Declining" it is also considered naturally uncommon.[1][2] The decline of P. kirkii has been attributed to deforestation and the introduction of bushtail possums. It is possible that the habitat of P. kirkii was historically occupied and impacted by the deforestation of favoured "host trees" within the its distribution zone.[18] In long-term conservation monitoring strategies, P. kirkii is included as an "indicator species" for healthy old-growth forests, underscoring its significance. The gradual and continuous population decline of P. kirkii make conservation management crucial. An emerging threat to the northen populations of P. kirkii is kauri dieback.[18]

See also

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References

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Citations

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  1. ^ a b de Lange et al. 2009, pp. 16–17.
  2. ^ a b c d De Lange 2025.
  3. ^ Myron, Clarkson & Gemmill 2021, p. 3.
  4. ^ Myron, Clarkson & Gemmill 2021, p. 4.
  5. ^ Cooper 1956, pp. 163–164.
  6. ^ a b Kirk 1869.
  7. ^ Cheeseman 1914, p. 17.
  8. ^ Kirk 1868.
  9. ^ Myron, Clarkson & Gemmill 2021, p. 7–8.
  10. ^ a b Myron, Clarkson & Gemmill 2021, p. 8.
  11. ^ Orwin 2007.
  12. ^ Myron, Clarkson & Gemmill 2021, p. 7.
  13. ^ Dawson 1986; De Lange 2025.
  14. ^ Dawson 1986, p. 79.
  15. ^ Myron, Clarkson & Gemmill 2021, p. 13.
  16. ^ Myron, Clarkson & Gemmill 2021, p. 20.
  17. ^ Moinet et al. 2024, p. 1.
  18. ^ a b de Lange et al. 2009, p. 20.

Works cited

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Journals

Mscellaneous

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