Casuarina is a genus of flowering trees in the family Casuarinaceae (order Fagales), commonly called she-oaks, Australian pines, or native pines. The genus was first formally described in 1759 by Carl Linnaeus and currently comprises around 14 accepted species, native to Australia, the Indian subcontinent, Southeast Asia, islands of the western Pacific Ocean, and eastern Africa.
Despite their common names, she-oaks are not true oaks or pines but flowering plants (angiosperms) that have evolved a remarkable pine-like appearance. Their most distinctive feature is a system of slender, pendulous, photosynthetic branchlets that function as leaves; the true leaves are reduced to tiny scales arranged in whorls of 5 to 20 around each jointed branchlet segment. Deep furrows at each whorl conceal the stomates, an adaptation to reduce water loss. The bark is fissured or scaly, ranging from greyish-brown to black.
Most species are dioecious (separate male and female trees), though the widespread coastal she-oak, Casuarina equisetifolia, is monoecious. Male flowers are borne in slender spikes along the branchlets; female flowers appear in spikes on short lateral branches. After fertilisation, female spikes develop into woody, cone-like structures whose thin bracteoles extend well beyond the cone body and enclose grey or yellowish-brown winged seeds (samaras).
The genus was significantly reorganised in 1982 when Lawrence Johnson segregated two new genera — Allocasuarina (now encompassing most Australian species) and Gymnostoma (Pacific island species) — from Casuarina, restricting the name to species native to Australia, South and Southeast Asia, and the Pacific.
Several species, particularly C. equisetifolia, C. cunninghamiana, and C. glauca, have become widely naturalised outside their native range and are classified as invasive in the United States, especially in southern Florida, as well as in Bermuda and other countries.
Etymology
The genus name Casuarina derives from the Malay word kasuari, meaning cassowary, reflecting the striking resemblance between the drooping, feather-like branchlets of these trees and the shaggy plumage of the cassowary bird. The common name "she-oak" refers to the oak-like grain of the timber in some species.
Distribution
Casuarina species are native to Australia, the Indian subcontinent, Southeast Asia, islands of the western Pacific Ocean, and eastern Africa. Several species — notably C. equisetifolia, C. cunninghamiana, and C. glauca — have become naturalized across tropical and subtropical regions worldwide, including Argentina, Brazil, Cuba, Egypt, Israel, Kenya, Mauritius, Mexico, South Africa, and the United States. In the US, they are particularly invasive in southern Florida, where populations of C. equisetifolia nearly quadrupled between 1993 and 2005.
Ecology
She-oaks support a range of specialist herbivorous insects. Seed-feeding wasps of the genus Bootanelleus and gall-inducing wasps (Selitrichodes spp.) are associated with the branchlets and cones. Moth caterpillars in the family Oecophoridae — including Zauclophora pelodes and Cryptophasa irrorata — feed on leaves, tie leaves, or bore in branches and stems. Cone-feeding weevils (genus Haplonyx) and sap-sucking jumping plant lice (Casuarinicola, Triozidae) also exploit the genus. The gall midge Ophelmodiplosis clavata induces galls on branchlet tips.
Taxonomy Notes
Casuarina was first formally described by Carl Linnaeus in 1759 in Amoenitates Academicae, with C. equisetifolia as the type species. In 1982 Lawrence Johnson substantially reorganised the family Casuarinaceae in the Journal of the Adelaide Botanic Gardens, raising Allocasuarina (for most Australian endemic species) and Gymnostoma (for Pacific island species) as separate genera and transferring over 45 species out of Casuarina. The genus now contains approximately 14 species accepted by Plants of the World Online, placed in the order Fagales alongside oaks, beeches, and birches.
