Casuarina equisetifolia
Subspecies equisetifolia near Darwin
Conservation status
Least Concern  (IUCN 3.1)[1]
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Clade:	Rosids
Order:	Fagales
Family:	Casuarinaceae
Genus:	Casuarina
Species:	C. equisetifolia
Binomial name
Casuarina equisetifolia L.[2]
Subspecies
C. e. subsp. equisetifolia C. e. subsp. incana
Synonyms[2]
Casuarina equisetifolia var. typica Domin

Casuarina equisetifolia, commonly known as coastal she-oak, horsetail she-oak,^[3] ironwood,^[4] beach sheoak, beach casuarina, whistling tree^[5] or Australian pine^[6] is a species of flowering plant in the family Casuarinaceae and is native to Australia, New Guinea, Southeast Asia and India. It is a small to medium-sized, monoecious tree with scaly or furrowed bark on older specimens, drooping branchlets, the leaves reduced to scales in whorls of 7 or 8, the fruit 10–24 mm (0.4–0.9 in) long containing winged seeds (samaras) 6–8 mm (0.2–0.3 in) long.

Casuarina equisetifolia is an evergreen tree typically growing to a height of 6–12 m (20–39 ft), sometimes to 35 m (115 ft) tall. The bark of young specimens is smooth and greyish, older trees have scaly, greyish-brown to black bark.^[3][7]

The foliage consists of slender, drooping branchlets up to 30 cm (12 in) long, the leaves reduced to scale-like teeth 0.3–0.5 mm (0.01–0.02 in) long, arranged in whorls of 7 or 8 (occasionally 6) around the branchlets. The sections of branchlet between the leaf whorls (the "articles") are 5–13 mm (0.2–0.5 in) long and 0.5–1.0 mm (0.02–0.04 in) wide.^[3][5][7]

This species of Casuarina is monoecious with male and female flowers produced on the same tree, unlike most other species of its same genus which are dioecious.^[7] Its male and female inflorescences are both shaped like catkins.^[6] Its male flowers appear in simple spikes 0.7–4 cm (0.28–1.57 in) long in whorls of 7 to 11.5 per cm (per 0.4 in), with anthers are 0.6–0.8 mm (0.02–0.03 in) long, whereas its female flowers are 3–13 mm (0.1–0.5 in) long on short, sparsely hairy peduncles.^[3][5][7]

The fruit is an oval woody structure 10–24 mm (0.39–0.94 in) long and 9–13 mm (0.35–0.51 in) in diameter when mature, superficially resembling a conifer cone made up of numerous carpels. Each carpel contains a single small winged seed 6–8 mm (0.24–0.31 in) long.^[3][5][7]

The genus Casuarina was first formally described in 1759 by Carl Linnaeus in Amoenitates Academicae and the first species he described was Casuarina equisetifolia.^[8][9] The specific epithet equisetifolia means 'horsehair-leaved'.^[5]

In 1873, George Bentham described Casuarina equisetifolia var. equisetifolia and C. equisetifolia var. incana in Flora Australiensis,^[10] and in 1982, Lawrie Johnson changed the names to subspecies equisetifolia and incana respectively, in the Journal of the Adelaide Botanic Gardens:^[11]

• Casuarina equisetifolia L. subsp. equisetifolia,^[12] a tree 7–35 m (23–115 ft) high, the articles 5–8 mm (0.2–0.3 in) long and 0.5–0.7 mm (0.02–0.03 in) wide with 8 to 10 teeth 0.3–0.8 mm (0.01–0.03 in) long, the male spikes 7–40 mm (0.3–1.6 in) long, the cones 12–24 mm (0.5–0.9 in) long and 9–11 mm (0.35–0.43 in) wide on a peduncle 3–10 mm (0.1–0.4 in) long.^[13]
• Casuarina equisetifolia subsp. incana (Benth.) L.A.S.Johnson.^[14] a tree 6–12 m (20–39 ft) high, the articles 7–13 mm (0.3–0.5 in) long and 0.7–1.0 mm (0.03–0.04 in) wide with 8 to 10 teeth about 0.7 mm (0.03 in) long, the male spikes 12–25 mm (0.5–1.0 in) long, the cones 10–20 mm (0.4–0.8 in) long, 10–13 mm (0.4–0.5 in) wide and densely covered with white to rust-coloured hairs, on a peduncle 3–13 mm (0.1–0.5 in) long.^[3][15] The epithet incana means 'white' or 'hoary'.^[16]

There is some doubt as to whether Linnaeus' publication of C. equisetifolia is valid, since he based his description solely on Rumphius's description of Casuarina litorea in Herbarium Amboinense^[17] and there are no type specimens.^[18]

This species is sometimes given the common name "Australian pine" because it has features that seem superficially like those of a pine, but it is not a conifer.^[6]

Casuarina equisetifolia subsp. equisetifolia grows near the sea, behind beaches and near estuaries, sometimes on rocky headlands in Australia, New Guinea, Southeast Asia and India.^[2][5] The species is native to India and Bangladesh in South Asia; Myanmar, the Andaman Islands, Nicobar Islands, Vietnam, Thailand, Cambodia, Peninsular Malaysia, the Philippines, Borneo, Java and the Lesser Sunda Islands, islands in the South China Sea, Sulawesi and Sumatra in Southeast Asia; the Carolines, Fiji, the Marianas, the Marshall Islands, New Caledonia, Samoa, Solomon Islands, Tonga, Tuvalu and Vanuatu in the Pacific Ocean; New Guinea; and New South Wales, the Northern Territory, and Queensland in Australia.^[2][5][13]

Subspecies incana grows on rocky headlands near the coasts of eastern Queensland and New South Wales as far south as Laurieton.^{[3][5][15][19]}

Casuarina equisetifolia has been introduced to many other continents and islands.^[2] It is an invasive species in the United States, but biological control by insects, including by a Selitrichodes wasp and Carposinidae and Gelechiidae moths, has been effective.^[20][21][22] It is also regarded as being invasive in South Africa.^[23]

Casuarina equisetifolia is an actinorhizal plant able to fix atmospheric nitrogen^[24] through a symbiotic relationship with strains of Frankia, a actinomycete hosted in nodules on its roots.^[25]: 294

Other than ornamental purposes, Casuarina equisetifolia was explored for its potential in remediation of textile dye wastewater. The leaves were found to be useful as absorbent material for the removal of textile dyes, such as reactive orange 16,^[26] Rhodamine B,^[27] methylene blue, malachite green^[28] and methyl violet 2b.^[29] Similarly, the dried cone was also reported to be able to remove Rhodamine B,^[30] and methyl violet 2b.^[31] The bark was reported to able to remove methylene blue.^[32] Even the seed was found to be useful in dye removal of neutral red and malachite green.^[33] The carbon derived from the cones of C. equisetifolia was found to be a good absorbent for landfill leachate,^[34] while another laboratory also reported good absorbency for copper ions from aqueous solution.^[35] Its line planting in coastal areas has been known to help control the wind force. In countries like India, it has been known as a suitable species for wasteland development.^{[citation needed]}

In Singapore, there is a road named Tanjong Rhu Road because it once had many of these trees growing along the coast from Kallang to Rochor.^[36] In the island of Langkawi, Kedah, Malaysia, there is a sand spit in the mouth of the Ayer Hangat river in the Kilim Karst Geoforest Park about 20 km from the town of Kuah also named Tanjung Rhu, where a line of these trees may be seen.^[37] The town of Tanjung Aru in Sabah is so named because many specimens of this tree (aru) are found on its beach.^[38]

In the Philippines, the town of Agoo is named after the tree (known as agoho or aroo in the Ilocano language).^[39]

Casuarina equisetifolia exhibits a high degree of adaptability to different environmental conditions, including coastal habitats, sandy soils, and disturbed areas. This versatility allows it to thrive in a wide range of ecosystems, increasing its potential for invasiveness.^[40]

The species produces large quantities of winged seeds contained within cone-like structures. These seeds are easily dispersed by wind and water, facilitating the rapid spread of C. equisetifolia over long distances. This dispersal mechanism enables it to colonize new areas and outcompete native species.^[41]

Casuarina equisetifolia's lack of natural predators or pests contributes to its unchecked growth in many regions. While specific scientific references directly addressing this aspect might be limited, the absence of significant herbivory or predation on C. equisetifolia in non-native environments has been observed in ecological studies documenting its invasive behavior ^[42][43]

Human activities, such as urbanization, agriculture and landscaping, often contribute to the spread of C. equisetifolia. The species is frequently planted for erosion control, windbreaks, and ornamental purposes, inadvertently introducing it to new areas where it can become invasive.^[44]

• 
  Branchlets and male spike
• 
  Mature female cones
• 
  Samara
• 
  Male spikes

• Pinus kesiya, the Khasi or Benguet pine
• Allelopathy

Wikimedia Commons has media related to Casuarina equisetifolia.

• NT Flora: Casuarina equisetifolia. Northern Territory Government.