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.
Description
Habit
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]
Foliage
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]
Flowers
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]
Fruit
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]
Taxonomy and naming
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]
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]
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 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]
Culture
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.[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]
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]
^ abcde"Casuarina equisetifolia". Australian Biological Resources Study, Department of Agriculture, Water and the Environment: Canberra. Retrieved 29 April 2023.
^ ab"Casuarina equisetifolia subsp. incana". Australian Biological Resources Study, Department of Agriculture, Water and the Environment: Canberra. Retrieved 29 April 2023.
^William T. Stearn (1992). Botanical Latin. History, grammar, syntax, terminology and vocabulary (4th ed.). Portland, Oregon: Timber Press. p. 431.
^Rumphius, Georg E. (1743). Herbarium Amboinense. Vol. 3. Amsterdam. p. 86. Retrieved 29 April 2023.
^Wilson, Karen L.; Johnson, Lawrence A.S. (1989). Flora of Australia(PDF). Canberra: Australian Government Publishing Service. p. 201. Retrieved 29 April 2023.
^"Casuarina equisetifolia"(PDF). World Agroforestry (Centre for International Forestry Research). Retrieved 29 April 2023.
^Bharti, Vikash; Shahi, Amrita; Geed, Sachin; Kureel, M.K. (2017). "Biodegradation of reactive orange 16 dye in the packed bed bioreactor using seeds of Ashoka and Casuarina as packing media". Indian Journal of Biotechnology. 16: 216–221.
