Casuarina equisetifolia

Casuarina equisetifolia is a monoecious tree that typically grows to a height of 6–12 m (20–39 ft), sometimes to 35 m (115 ft), and has bark that is smooth and greyish on young specimens, scaly or furrowed on older trees. The branchlets are drooping, up to 300 mm (12 in) long, the leaves reduced to scale-like teeth about 0.7 mm (0.028 in) long, arranged in whorls of 7 or 8 around the branchlets. The sections of branchlet between the leaf whorls (the "articles") are 5–13 mm (0.20–0.51 in) long and 0.5–1.0 mm (0.020–0.039 in) wide. Male flowers are arranged in spikes 7–40 mm (0.28–1.57 in) long in whorls of 7 to 11.5 per centimetre (per 0.39 in.) the anthers 0.6–0.8 mm (0.024–0.031 in) long. The female cones are on a peduncle 3–13 mm (0.12–0.51 in) long and sparsely covered with soft or woolly hairs. Mature cones are 10–24 mm (0.39–0.94 in) long and 9–13 mm (0.35–0.51 in) in diameter, the samaras 6–8 mm (0.24–0.31 in) long.^[3][5][6]

Common names for the tree include Australian pine, horsetair tree, ironwood, beach sheoak; although it has features that seem superficially like a conifer, it is not a pine.^[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. incana and C. equisetifolia var. equisetifolia 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 (0.20–0.31 in) long and 0.5–0.7 mm (0.020–0.028 in) with 8 to 10 teeth 0.3–0.8 mm (0.012–0.031 in) long, the male spikes 7–40 mm (0.28–1.57 in) long, the cones 12–24 mm (0.47–0.94 in) long and 9–11 mm (0.35–0.43 in) wide on a peduncle 3–10 mm (0.12–0.39 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 (0.28–0.51 in) long and 0.7–1.0 mm (0.028–0.039 in) with 8 to 10 teeth about 0.7 mm (0.028 in) long, the male spikes 12–25 mm (0.47–0.98 in) long, the cones 10–20 mm (0.39–0.79 in) long, 10–13 mm (0.39–0.51 in) wide and densely covered with white to rust-coloured hairs, on a peduncle 3–13 mm (0.12–0.51 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]

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]

Other than ornamental purposes, Casuarina was also explored in for its potential in remediation of textile dye wastewater. Casuarina leaves were found to be useful as absorbent material for the removal of textile dyes such as reactive orange 16^[24] Rhodamine B,^[25] methylene blue, malachite green^[26] and methyl violet 2b.^[27] Similarly the Casuarina dried cone was also reported to be able to remove Rhodamine B,^[28] and methyl violet 2b.^[29] The Casuarina bark was reported to able to remove methylene blue.^[30] Even the Casuarina seed was also found to be useful in dye removal of neutral red and malachite green.^[31] The carbon derived from the cones of Casuarina was found to be good absorbent for the landfill leachate,^[32] while another laboratory also reported good absorbent for copper ions from aqueous solution.^[33]

Names of places

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.^[34] 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 these trees line here.^[35] The town of Tanjung Aru in Sabah is also named because a lot of this tree (aru) is found in its beach.^[36]

In the Philippines, the town of Agoo is named after the tree (known as agoho or aroo in the Ilocano language).^[37]

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.^[38]

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 Casuarina equisetifolia over long distances. This dispersal mechanism enables it to colonize new areas and outcompete native species.^[39]

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 Casuarina equisetifolia in non-native environments has been observed in ecological studies documenting its invasive behavior ^[40][41]

Human activities such as urbanization, agriculture, and landscaping often contribute to the spread of Casuarina equisetifolia. The species is frequently planted for erosion control, windbreaks, and ornamental purposes, inadvertently introducing it to new areas where it can become invasive.^[42]

• 
  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.