When one thinks of the Australian vegetation, one might think of towering eucalypts or hardy acacias. One might contemplate unwelcoming spinifex or vibrant grevilleas. But perhaps few groups of plants are so distinctively Australian as the Casuarinaceae, the casuarinas or she-oaks. Members of this family are also found in south-east Asia and the Pacific Islands, but it is in Australia that they reach their highest diversity.
Casuarinas are also unmistakable. They are flowering plants, but they are wind-pollinated and the flowers are highly reduced, being borne in small clusters or spikes. The clusters of fruits, when mature, look more like a miniature pine cone than anything else. The trees that bear these cones also look a bit like pines themselves, with their narrow photosynthetic branches (cladodes) bearing a superficial resemblance to pine needles. The leaves proper are reduced to tiny teeth arranged around nodes or joints on the branches. The outer layer of the cladodes is composed of a thick cortex which together with the needle-like morphology helps resist desiccation. The name of the family refers to the resemblance of their branches to the hair-like feathers of a cassowary Casuarius. Casuarinas are so distinct from other flowering plants that their affinities were long uncertain, though more recent studies have suggested a relationship to other wind-pollinated trees in families such as the Betulaceae (Steane et al. 2003).
In line with their drought-resistant mien, casuarinas are most often found growing in arid and/or coastal regions. The most widespread species, the beach she-oak Casuarina equisetifolia, is found along coastlines from the Bay of Bengal to Polynesia. Their persistance in harsh conditions is also assisted by the presence of nodules on their roots containing bacteria of the genus Frankia, that function like the Rhizobium in root nodules on legumes to fix nitrogen from the atmosphere. Casuarinas also resemble pine trees in forming a mat around their base of fallen cladodes that restricts the growth of competing vegetation.
Until relatively recently, casuarinas were all classed in a single genus but most authors now recognise four genera in the family. The most distinctive, whose position as sister to the remaining genera is confirmed by molecular analyses (Steane et al. 2003), is Gymnostoma, which contains eighteen species found from south-east Asia to Queensland and Fiji. Whereas other genera of Casuarinaceae have the stomata on the cladodes hidden within deep longitudinal grooves, Gymnostoma has much shallower grooves on the cladodes and the stomata more or less exposed. As such, it is less resistant to desiccation than the other genera. Gymnostoma has four of these grooves on each cladode, corresponding to four leaf-teeth around each node, so the cladodes also tend to have a squarish cross-section.
The second-most divergent genus, Ceuthostoma, contains just two species found from Palawan and Borneo to New Guinea. Ceuthostoma resembles Gymnostoma in having four teeth around each node, but resembles the remaining two genera, Casuarina and Allocasuarina, in having the stomata hidden within deep grooves. In Casuarina and Allocasuarina, the number of teeth around each node is generally increased (up to twenty in Casuarina), meaning that the cladodes are more rounded than square. As noted by Steane et al. (2003), rounder cladodes with more grooves mean that the opening of each groove is narrower, further improving desiccation resistance. Casuarina and Allocasuarina are most readily distinguished by the appearance of their seeds, which are paler and dull in Casuarina but dark brown or black and shiny in Allocasuarina. Allocasuarina is the most diverse genus of the family, with over fifty species endemic to Australia. Casuarina contains fewer species but is more widespread. Steane et al.'s molecular analysis suggested a division within Casuarina between two main clades, one of which was restricted to Australia while the other was primarily composed of Indomalesian species (as well as C. equisetifolia which, as noted above, is found damn near everywhere).
Fossils of Casuarinaceae date back to the Palaeocene epoch, and indicate that the family was more widespread in the past with species known from the Eocene of South America and the Miocene of New Zealand. Casuarinaceae-like pollen is also known from the Palaeogene of southern Africa and Antarctica. The South American species have been assigned to the living genus Gymnostoma; the New Zealand species, though originally assigned to Casuarina, is probably also more closely related to Gymnostoma (Zamaloa et al. 2006). Though dominant in the modern flora, the drought-resistant clade of the other three genera is probably of more recent origin, and has probably only ever been unique to the Australasian region.
And I've just realised that I haven't answered the question in the title to this post. As I noted above, an alternate vernacular name for these trees to 'casuarina' is 'she-oak'. I used to wonder why this should be, seeing as casuarinas look about as unlike oaks as you might care to imagine. A good summary of the solution can be found in this newspaper column from the Western Mail of 1914. Though some have suggested that 'she-oak' may be a corruption of an Aboriginal word (despite no such word having been put on record), the more simple explanation is that even if the tree itself doesn't look like an oak, the wood that comes out of it does.
Steane, D. A., K. L. Wilson & R. S. Hill. 2003. Using matK sequence data to unravel the phylogeny of Casuarinaceae. Molecular Phylogenetics and Evolution 28: 47–59.
Zamaloa, M. del C., M. A. Gandolfo, C. C. González, E. J. Romero, N. R. Cúneo & Peter Wilf. 2006. Casuarinaceae from the Eocene of Patagonia, Argentina. International Journal of Plant Sciences 167 (6): 1279–1289.