Urtica ferox

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Urtica ferox
Leaves and spines of U. ferox
Leaves and spines of U. ferox
Not Threatened
Not Threatened (NZ TCS)
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Embryophytes
Clade: Tracheophytes
Clade: Spermatophytes
Clade: Angiosperms
Clade: Eudicots
Clade: Rosids
Order: Rosales
Family: Urticaceae
Genus: Urtica
Species:
U. ferox
Binomial name
Urtica ferox

Urtica ferox, commonly known as tree nettle and ongaonga, is a species of nettle endemic to New Zealand. A member of the Urticaceae, it is covered in spines or trichomes, which contain numerous toxins that are poisonous to humans. It is a large woody shrub that can grow to a height of 3 metres (10 feet), with the base of the stem reaching 12 cm (5 in) in thickness. Its large spines can cause painful stings that can last several days or weeks; there are two recorded human deaths from contact with the plant. U. ferox is found throughout New Zealand.

The plant was first described by German naturalist Georg Forster in 1786. It is pollinated by the wind, and its seeds are later dispersed by gravity or the wind. Several butterfly and moth species can be hosted on the plant, and it is the favoured source of food for the larvae of the New Zealand red admiral (Vanessa gonerilla) butterfly. It has some traditional and medicinal uses for the indigenous Māori people, despite it being a plant usually avoided by people. Its 2023 assessment in the New Zealand Threat Classification System was "Not Threatened".

Description

Close-up photo of U. ferox needles along the leaf mid-vein
Needles along the leaf mid-vein
Close-up photo of buds and developing flowers
Buds and developing flowers

Urtica ferox (ongaonga) is a dioecious shrub or small tree in the family Urticaceae. It can grow to a height of 3 metres (10 feet) with the base of the stem or trunk reaching 12 centimetres (5 in) in thickness.[1][2] The pale green leaves are very thin like a membrane, and the surface of the leaf, stems and stalks are covered in stiff trichomes (stinging hairs) that can grow up to 6 mm (0.24 in) long. These spines are prominent along the salient mid-vein and leaf margin. The leaves range from 3–5 cm (1.2–2.0 in) in width and 8–12 cm (3.1–4.7 in) in length. They are oppositely arranged.[3][4][5] Juvenile leaves can reach up to 15 cm (5.9 in) long.[4] The leaf shape is triangular, ovate-triangular, to lanceolate-triangular with coarsely serrated leaf margins.[3][4][6]

Flowering occurs from November to March. Pollen grains are collected by the densely packed stigmas on the flowers. The fruit, which are found in achenes, mature in January, each containing a single seed, 1.5 mm long, which takes one month to germinate.[7][1][8] The flowers are produced in panicles, which can reach 8 cm (3.1 in) long.[6][2] Burrows (1996) found that U. ferox is capable of building up large seed banks in the soil which may survive for several years, like other species in the genus Urtica which also show seed bank dormancy.[9] U. ferox has a diploid chromosome count of 48.[1]

Taxonomy

Urtica ferox was first described by German naturalist Georg Forster in 1786.[10][11] There are 69 species of the Urtica genus currently accepted by the Plants of the World Online taxonomic database. These species are found throughout the entire world.[12] There are six species native to New Zealand.[13] U. ferox is unique among the New Zealand Urtica species due to it being tall and woody; it is not closely related to the other New Zealand members of the genus Urtica.[14] Grosse‐Veldmann et al. (2016) constructed a phylogenetic tree of the genus Urtica based on gender characteristics and genetic sequencing.[15] They found that the South African species U. lobulata is the sister species to U. ferox.[16][14] It has been presumed that the distinctive morphological evolution and toxicity of U. ferox is a result of the ecological influence of browsing by the now-extinct moa.[14][17]

Etymology

The etymology of the genus name Urtica is derived from the Latin word for stinging nettles, which comes from urere, meaning 'to burn'. The specific epithet ferox means 'ferocious', 'savage', or 'wild'.[18] The species is commonly known as tree nettle and ongaonga.[1] In Māori, the word ongaonga is used to describe anything 'annoying' or 'irritating'. Cognates found across different Polynesian languages, including 'hongohongo' and 'okaoka', are words typically used to describe stinging insects.[18] U. ferox has several recorded Māori names, including: houhi, ongaonga, okaoka, puruhi, taraonga, and taraongaonga.[19]

Ecology

A yellow-orange butterfly, called New Zealand yellow admiral, visiting U. ferox.
A yellow admiral (Vanessa itea) visiting a U. ferox bush.
A female New Zealand red admiral butterfly visiting ongaonga, a plant with many spines.
A female New Zealand red admiral (Vanessa gonerilla) laying eggs on a large U. ferox bush.

Urtica ferox seeds are dispersed by gravity and the wind.[20] Like all Urtica species, U. ferox is pollinated by the wind,[21][22] although bees occasionally visit the flowers to collect nectar.[5][23] Urtica ferox can be semideciduous or entirely deciduous in some southern localities in winter, an uncommon characteristic in New Zealand's native plants, only found in about 5% of the country's native woody species.[20][24][25]

The plant is a favoured host of the larvae of the endemic New Zealand red admiral (Vanessa gonerilla).[26] The female butterflies lay their eggs on the new growth of U. ferox leaves, which they are essentially immune to. The caterpillar larvae hatch and then spend up to six weeks feeding on those leaves. During this vulnerable stage, the caterpillars use the spiny leaves for protection, wrapping the leaves around themselves similar to a blanket or building a small tent to hide from potential predators such as birds and insects. Populations of V. gonerilla have decreased, possibly in part because of people treating U. ferox as a weed and removing it.[27]

The yellow admiral (V. itea) butterfly can also be present on the plant.[28] Several other moth caterpillar species feed on the plant, including Mnesictena flavidalis, Diarsia intermixta, Meterana inchoata, Pasiphila urticae, Epichorista crypsidora and two species from the genus Pseudocoremia and also Udea.[5][28] A rare moth species, Pseudocoremia flava, uses U. ferox as a host plant.[29] The beetle Oemona hirta can also be found on the plant.[30] A species of gall mite, Vittacus mansoni, is hosted on U. ferox.[31] Rust fungi from the genus Puccinia can also be present on the plant.[32]

Deer, sheep, and goats will eat U. ferox leaves despite their stinging spines. The fruit are sometimes eaten by common brushtail possums.[21][5] The stinging spines have however killed cattle, dogs, and horses in the past.[5][18][6]

Toxins

The toxins present in the trichomes contain the neurotransmitters histamine, serotonin and acetylcholine.[33][34] These neurotransmitters have also been found in U. dioica and U. urens.[35] A 2022 study in the Journal of Biological Chemistry found that two peptides, Δ-Uf1a and β/δ-Uf2a, are likely responsible for the painful sensations. Δ-Uf1a is suggested to create pain by disturbing cell membranes and β/δ-Uf2a the same by modulating voltage-gated sodium channels. Stings have a painful reaction which causes hives, numbness, and itchiness. In severe cases, it can also cause ataxia, blurred vision, confusion, polyneuropathy, respiratory distress, and hypersalivation. These sting reactions can last for days to weeks.[33][36] The reaction can rarely be fatal,[37] with two recorded human deaths from contact with U. ferox. In 1961, a young hunter died five hours after walking through a dense patch in the Ruahine Ranges.[38][1] In 2002, a young man's death in the Ruahine Ranges was retrospectively attributed to U. ferox by a pathologist.[39]

Distribution

A large U. ferox bush next to a stream
Urtica ferox growing next to a stream near Porirua

Urtica ferox is endemic to New Zealand. The plant is found throughout the North and South Islands.[1] The botanist Thomas Cheeseman believed that Otago was the species' southern limit.[40] However, a more recent revision of New Zealand flora suggests that it occurs south to Stewart Island.[41] It is especially common near Wellington and Christchurch.[5] The botanist Lucy Moore noted it is uncommon or not found north of Auckland.[42] The 2023 assessment of U. ferox in the New Zealand Threat Classification System was "Not Threatened".[1]

Habitat

Urtica ferox is typically found in coastal to lowland environments.[1] It is common in forest margins and shrublands. It occurs from sea level to 800–1,000 m (2,600–3,300 ft) above sea level at maximum elevation.[5][38] It is found on various soil types, but it prefers fertile soils, particularly with basaltic rock as a substrate. It can be found on such basaltic formations well inland, for example, in the Canterbury Region on the Harper Hills near Glentunnel, where it is uncommon. It occurs on alluvial soils along the valley floor of Taramakau Valley of Westland, within Arthurs Pass National Park.[43] It can also be found growing on the bases of fallen trees and stream-beds.[43][44]

Uses

The stinging hairs of plants have been of interest to humans for centuries.[45] Urtica ferox is a plant usually avoided by people, but it did have some traditional and medicinal uses for the indigenous Māori people.[43][46] In Māori mythology, the plant was placed in forests to prevent people moving freely and to irritate people. The Polynesian explorer, Kupe, was said to have placed "several obstacles", including this plant, to hinder pursuers whose wives he had stolen.[34] In traditional medicinal practices, the bark of U. ferox, and the leaves of kawakawa (Piper excelsum), boiled together, were used to treat skin conditions.[18] The plant was also a traditional Māori food source. The inner stems were sometimes consumed after the leaves and outer bark had been removed. The thin film that makes up the inner bark was also eaten raw and is said to have a sweet taste.[27][19]

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