They are evergreen shrubs and trees growing to {{convert|35|m|ft|abbr=on}} tall. The slender, green to grey-green twigs bearing minute scale-leaves in whorls of 5–20. The apetalous flowers are produced in small catkin-like inflorescences. Most species are dioecious, but a few are monoecious. The fruit is a woody, oval structure superficially resembling a conifer cone, made up of numerous carpels, each containing a single seed with a small wing.^[1][2] The generic name is derived from the Malay word for the cassowary, kasuari, alluding to the similarities between the bird's feathers and the plant's foliage,^[3] though the tree is called rhu in current standard Malay.

Wilson and Johnson^[4] distinguish the two very closely related genera, Casuarina and Allocasuarina on the basis of:

• Casuarina: the mature samaras being grey or yellow-brown, and dull; cone bracteoles thinly woody, prominent, extending well beyond cone body, with no dorsal protuberance;
  
• Allocasuarina: the mature samaras being red-brown to black, and shiny; cone bracteoles thickly woody and convex, mostly extending only slightly beyond cone body, and usually with a separate angular, divided or spiny dorsal protuberance.

Ecology

Casuarina species are a food source of the larvae of hepialid moths; members of the genus Aenetus, including A. lewinii and A. splendens, burrow horizontally into the trunk then vertically down. Endoclita malabaricus also feeds on Casuarina. The noctuid turnip moth is also recorded feeding on Casuarina.

Selected species

• Casuarina cristata Miq. (Northeastern Australia: Queensland, New South Wales).
• Casuarina cunninghamiana Miq. – River Sheoak (Northern and eastern Australia: Northern Territory to New South Wales)
• Casuarina equisetifolia L. – Australian Pine, Beach Sheoak, Common Ironwood (Northern Australia, southeastern Asia, doubtfully native to Madagascar), ঝাউ in বাংলা (Bengali), ಗಾಳಿ ಮರ (GaaLi mara) in Kannada.
• Casuarina glauca Sieber ex Spreng. Gray Sheoak, Longleaf Ironwood, Saltmarsh Ironwood, Swamp Oak (New South Wales)
• Casuarina grandis L.A.S.Johnson (New Guinea)
• Casuarina junghuhniana Miq. (Indonesia)
• Casuarina obesa Miq. (Southern Australia: southwestern Western Australia, New South Wales [one site, now extinct], Victoria)
• Casuarina oligodon L.A.S.Johnson (New Guinea)
• Casuarina pauper F.Muell. ex L.A.S.Johnson (hinterland of Australia)

Sources:^[1][6][7]

Formerly placed here

• Allocasuarina acuaria (F. Muell.) L.A.S.Johnson (as C. acuaria F.Muell.)
• Allocasuarina acutivalvis (F. Muell.) L.A.S.Johnson (as C. acutivalvis F.Muell.)
• Allocasuarina campestris (Diels) L.A.S.Johnson (as C. campestris Diels)
• Allocasuarina corniculata (F.Muell.) L.A.S.Johnson (as C. corniculata F.Muell.)
• Allocasuarina decussata (Benth.) L.A.S.Johnson (as C. decussata Benth.)
• Allocasuarina distyla (Vent.) L.A.S.Johnson (as C. distyla Vent.)
• Allocasuarina drummondiana (Miq.) L.A.S.Johnson (as C. drummondiana Miq.)
• Allocasuarina fibrosa (C.A.Gardner) L.A.S.Johnson (as C. fibrosa C.A.Gardner)
• Allocasuarina fraseriana (Miq.) L.A.S.Johnson (as C. fraseriana Miq.)
• Allocasuarina grevilleoides (Diels) L.A.S.Johnson (as C. grevilleoides Diels)
• Allocasuarina helmsii (Ewart & Gordon) L.A.S.Johnson (as C. helmsii Ewart & Gordon)
• Allocasuarina huegeliana (Miq.) L.A.S.Johnson (as C. huegeliana Miq.)
• Allocasuarina humilis (Otto & A. Dietr.) L.A.S.Johnson (as C. humilis Otto & A.Dietr.)
• Allocasuarina inophloia (F. Muell. & F. M. Bailey) L.A.S.Johnson (as C. inophloia F.Muell. & F.M.Bailey)
• Allocasuarina lehmanniana subsp. lehmanniana (as C. baxteriana Miq. or C. lehmanniana Miq.)
• Allocasuarina littoralis (Salisb.) L.A.S.Johnson (as C. littoralis Salisb. or C. suberosa Otto & A.Dietr.)
• Allocasuarina luehmannii (R.T.Baker) L.A.S.Johnson (as C. luehmannii R.T.Baker)
• Allocasuarina muelleriana (Miq.) L.A.S.Johnson (as C. muelleriana Miq.)
• Allocasuarina nana (Sieber ex Spreng.) L.A.S.Johnson (as C. nana Sieber ex Spreng.)
• Allocasuarina paludosa (Sieber ex Spreng.) L.A.S.Johnson (as C. paludosa Sieber ex Spreng.)
• Allocasuarina pusilla (Macklin) L.A.S.Johnson (as C. pusilla Macklin)
• Allocasuarina thuyoides (Miq.) L.A.S.Johnson (as C. thuyoides Miq.)
• Allocasuarina torulosa (Aiton) L.A.S.Johnson (as C. tenuissima Sieber ex Spreng. or C. torulosa Aiton)
• Allocasuarina trichodon (Miq.) L.A.S.Johnson (as C. trichodon Miq.)
• Allocasuarina verticillata (Lam.) L.A.S.Johnson (as C. quadrivalvis Labill., C. stricta Aiton or C. verticillata Lam.)
• Gymnostoma deplancheanum (Miq.) L.A.S.Johnson (as C. deplancheana Miq.)
• Gymnostoma nodiflorum (Thunb.) L.A.S.Johnson (as C. angulata J.Poiss. or C. nodiflora Thunb.)
• Gymnostoma papuanum (S. Moore) L.A.S.Johnson (as C. papuana S.Moore)
• Gymnostoma rumphianum (Miq.) L.A.S.Johnson (as C. rumphiana Miq.)
• Gymnostoma sumatranum (Jungh. ex de Vriese) L.A.S.Johnson (as C. sumatrana Jungh. ex de Vriese)^[6]

Cultivation and uses

Commonly known as the she-oak, sheoak, ironwood, or beefwood, casuarinas are commonly grown in tropical and subtropical areas throughout the world. The tree has delicate, slender terminal branches, and leaves that are no more than scales, making the tree look more like a wispy conifer. The plants are very tolerant of windswept locations, and are widely planted as windbreaks, although usually not in agricultural situations.

C. equisetifolia is a common tropical seashore tree known as Common Ironwood, Beefwood, Bull-oak, or Whistling-pine, and is often planted as a windbreak. The wood of this tree is used commercially for shingles or fencing, and is said to make excellent, hot burning firewood.

C. oligodon has been planted in New Guinea in an ancient (more than 3,000 years) silviculture by highland gardeners practicing an intensive traditional permaculture. The wood of this tree is used for building-timber, furniture and tools, and makes excellent firewood. The tree's root nodules are known to fix nitrogen, and it{{clarify|date=May 2017}} is traditionally prized for its ability to increase the soil's fertility.{{citation needed|date=May 2017}} Its abundant leaf-fall is high in nitrogen and traditionally prized for mulch.{{citation needed|date=May 2017}}

The resin exuded from some casuarinas is edible and was a food source for Aboriginal people.

Gardeners in Bermuda can appreciate that all parts of the casuarina tree (needles, sawdust, bark, and prepared mulch) can be useful in lowering the soil pH,{{citation needed|date=May 2017}} as Bermuda's topography consists of naturally high pH limestone rock, and perhaps only a foot of red or sandy soil. Lowering soil pH makes the soil more acidic, which can help Bermuda's biodiversity by growing acid-loving plants such as blueberries, blackberries, azaleas, rhododendrons, and such plants that may be seen in similar hardiness zones as Bermuda (USDA 9-11), such as plants found in New Zealand and Florida, which naturally have a lower pH soil than Bermuda.

Invasive species

C. equisetifolia is widespread in the Hawaiian Islands where it grows both on the seashore in dry, salty, calcareous soils and up in the mountains in high rainfall areas on volcanic soils. It is also an introduced, invasive plant in Bermuda,^[12] where it was introduced to replace the Juniperus bermudiana windbreaks killed by juniper blight in the 1940s. Now the ironwoods are growing on cliffs and sandy slopes, competing with surrounding plants; they also erode the cliffs by splitting them apart via root penetration.

References