Triodia

Triodia pungens, copyright Mark Marathon.

Belongs within: Chloridoideae.

Triodia, spinifex, is a genus of grasses with rigid, spine-like leaves. Species of this genus are dominant in many regions of arid Australia. The genus Plectrachne has historically been recognised for species with long glumes and deeply three-lobed, awned lemmas, but is no longer regarded as distinct.

The most Australian of plants
Published 4 December 2018

Imagine yourself standing in a remote corner of northern Australia. Before you stretches an expanse of rolling hills, extending as far as the eye can see. The hills are covered with a carpet of green. You step forward, eager to explore these open fields. But as you approach them, everything changes. What appeared to be a uniform carpet is actually dense tussocks, each separated by an underlay of bare gravel. And instead of soft, yielding blades, the tussocks offer you nothing but resin and hate. Welcome to spinifex country.

Grassland dominated by Triodia pungens (bright green) and T. basedowii (grey-green), copyright Hesperian.

The spinifexes of the genus Triodia are a uniquely Australian group of plants. Some North American grasses have been assigned to this genus in the past but have since been moved elsewhere. There is also a widespread genus of coastal grasses that formally goes by the name of Spinifex but that is something different again. In many parts of arid Australia (and arid Australia equals most of Australia), spinifexes are the dominant form of plant life. As noted above, they grow in tight tussocks that may reach remarkable sizes and densities: clumps of the largest species may reach 2.5 metres in height and six metres in diameter (Lazarides 1997). Not uncommonly, these largest patches will be ring-shaped due to the centre dying off while growth continues around the edges. The leaf blades are long, needle-shaped, woody and rigid. Speaking from experience, the sharp tips of these blades will break off all too easily, embedding themselves in the flesh of any passers by. And some idea of their rigidity will also be conveyed by the fact that, in the growth season, it was not uncommon to discover macabre shish kebabs made from jumping grasshoppers that had had the misfortune to land on the end of one.

Mature stand of Triodia irritans, showing the tendency of hummocks to grow into circles as the centre dies off. Copyright ANBG photo M. Fagg.

Nearly 70 species are currently recognised within the genus, often differing in their preferred microhabitat. One of the most common species, Triodia basedowii, extends its range across almost the entirety of the continent between 18 and 30 degrees South and west of the Great Dividing Range. This species has a preference for sandplains and dunefields. Other species are far more localised. Barrett & Barrett (2011) described two new species found in association with sandstone cliff faces in the Ragged Range in Western Australia. Triodia barbata was found only in a thin band along the top of the cliff faces and may have had a population of only about 300 individuals. The more abundant (but still not widespread) T. cremnophila was found only on the vertical faces of the cliffs themselves. However, it must be noted that large gaps may exist in our knowledge of the ranges of Triodia species because of the remoteness and difficulty of getting to many of the regions in which they are found (seriously, if you’ve never been to central Australia yourself, it is difficult to appreciate just how much Absolutely Nothing there is there). Triodia mollis is known from two widely separated regions in northern Western Australia and Queensland with no confirmed records as yet from the entire expanse of the Northern Territory in between.

Preferred habitat of Triodia cremnophila, from Barrett & Barrett (2011). Yes, it only grows on the cliff face. Yes, someone presumably went down the cliff face to get specimens.

Being as woody and harsh as it is, it should come as no surprise that relatively few animals are capable of eating spinifex. Many Australian termites, such as the endemic genus Drepanotermes, are spinifex specialists; workers of Drepanotermes may be seen leaving their nest at night to collect pieces of spinifex blades and carry them back. Pastoralists may refer to ‘hard’ and ‘soft’ spinifex varieties but the difference is one of degree only; even the ‘soft’ spinifexes (usually the resin-producing species) are pretty damn hard by the standards of any other grass. Livestock are sometimes grazed on spinifex when bettter options are unavailable, in which case patches of spinifex may be burnt off to encourage the production of younger, more palatable growth (spinifex burns exceedingly well but also grows back readily from the remnant rootstock). The resin from spinifex also has a history of being used by indigenous Australians as an adhesive when making tools. For the most part, though, the main value of spinifex remains in its role as the dominant vegetation and habitat for the areas where it is found.

Passing awns
Published 13 May 2023

The dryland spinifex-grasses are a uniquely Australian affair, creating a landscape unlike any found elsewhere. Their classification, however, has not been free of contention. And one of the biggest questions about placing the spinifexes has been what to make of Plectrachne.

Feathertop spinifex Triodia schinzii, copyright Mark Marathon.

The genus Plectrachne was first established by J. Th. Henrard in 1929 for one Australia’s most widespread and abundant spinifexes, P. schinzii. Subsequent authors expanded the genus until it covered some 18 species. Plectrachne was distinguished from the more diverse genus Triodia by two features. Firstly, the glumes (the pair of bracts at the base of a spikelet or flower cluster) were much longer than the individual florets. And secondly, the lemmas (the larger of the bracts subtending individual florets) were deeply divided into three lobes with each lobe tipped by a long needle-like awn (Lazarides 1997).

Individual spikelet of Triodia schinzii, showing the long-spined lemmas, copyright J. Mant.

The problem with these distinctions is that, as time went by, the dividing line became less and less clear. Several species were assigned to Plectrachne in which the glumes were shorter than the florets. Several more in both Plectrachne and Triodia had variable-length glumes that might be shorter or longer depending on happenstance. Numerous species of Triodia had lemmas ending in teeth or lobes that differed from Plectrachne only in lacking awns. In the species Plectrachne bynoei, the division between the lemma lobes was relatively shallow and the awns fairly short. Because of this gradation in characters, Lazarides (1997) eventually just merged the two genera together, placing all the Australian spinifexes in the single genus Triodia.

This approach was reinforced by later molecular phylogenetic analysis which found the species of ‘Plectrachne’ to be scattered among Triodia rather than forming a single clade (Crisp et al. 2015). Both ‘Triodia’ and ‘Plectrachne’ included both resinous and non-resinous species, reflecting a failure of spikelet structure to correlate with growth habit. For now, Australian plant taxonomy has the spinifexes standing together.

Systematics of Triodia

Characters (from Cheeseman 1906): Perennial grasses, of very various habit. Leaves narrow, rigid. Spikelets 2- to many-flowered, arranged in a lax or narrow panicle; rhachilla disarticulating above the two outer glumes and between the flowering glumes. Two outer glumes longer or shorter than the flowering glumes, somewhat rigid, empty, keeled, acute, awnless. Flowering glumes more or less imbricated, rounded on the back at the base, coriaceous or chartaceous, often hairy on the margins and callus, 3-nerved, 3-lobed or 3-toothed at the apex, the lobes equal or the central one produced into a short awn or mucro. Palea broad, thin, with 2 almost marginal keels. Lodicules 2. Stamens 3. Styles short, distinct; stigmas plumose. Grain usually compressed on the back, free within the flowering glume and palea.

<==Triodia Brown 1810 (see below for synonymy)CM15
`--T. subg. RhombolytrumC06
|--T. australis Petrie 1890C06
| |--T. a. var. australisC06
| `--T. a. var. mucronulata Hack. in Cheeseman 1906C06
|--T. exigua Kirk 1882C06
`--T. pumila Hack. in Cheeseman 1906 [=Atropis pumila Kirk 1882]C06

Triodia incertae sedis:
*T. pungens Brown 1810CM15 (see below for synonymy)
‘Plectrachne’ rigidissima (Pilger) Hubb. 1939 [=Triraphis rigidissima]G52
‘Plectrachne’ schinzii Henr. 1929G52
T. acuminataS06
T. acutispiculaLK14
T. aeriaLK14
T. angusta Burbidge 1946G52
T. basedowii Pritzel 1918G52
T. bitexturaLK14
T. brizioides Burbidge 1940G52
‘Plectrachne’ bromoides (Muell.) Hubb. 1939 [=Triraphis bromoides]G52
T. burbidgeanaLK14
T. bynoeiLK14 [=Plectrachne bynoei Hubb. 1941G52]
T. caelestialisLK14
T. claytoniiLK14
T. cunninghamii Benth. 1878G52
‘Plectrachne’ danthonioides (Muell.) Hubb. 1939 [=Triraphis danthonioides]G52
T. decumbensC06
‘Plectrachne’ desertorum Hubb. 1941G52
‘Plectrachne’ dielsii Hubb. 1941G52
‘Plectrachne’ drummondii Hubb. 1941G52
T. epactiaLK14
T. fitzgeraldii Burbidge 1946G52
T. gracilis (Lazarides) Crisp & Mant in Crisp, Mant et al. 2015 [=Symplectrodia gracilis Lazarides 1985]CM15
T. intermedia Cheel 1919G52
T. irritans Br. 1810 [=Festuca irritans; incl. T. arista]G52
T. lanigera Domin 1912G52
T. lanosa (Lazarides) Crisp & Mant in Crisp, Mant et al. 2015 [=*Symplectrodia lanosa Lazarides 1985]CM15
T. longiceps Black 1930G52
T. longilobaLK14
T. melvilleiB05 [=Plectrachne melvillei Hubb. 1941G52]
T. microstachya [=Festuca microstachya]B78
T. mitchelliiB00
T. paraguayensisS06
T. plurinervataKM08
T. procera Br. 1810 [=T. pungens var. procera]G52
T. racemigera Gardner 1952G52
T. rigidissimaG04
T. scariosaCWB06
T. schinziiB05
T. secunda Burbidge 1946G52
T. stenostachyaLK14
T. stipoides (Jacobs) Crisp & Mant in Crisp, Mant et al. 2015 [=*Monodia stipoides Jacobs 1985]CM15
T. triticoides Gardner 1952G52
T. wiseana Gardn. 1942G52
|--T. w. var. wiseanaG52
`--T. w. var. brevifoliaG52

Triodia Brown 1810 [incl. Monodia Jacobs 1985 non Breton & Faurel 1970, Plectrachne Henrard 1929, Symplectrodia Lazarides 1985; Triodiinae]CM15

*Triodia pungens Brown 1810CM15 [=Plectrachne pungens (Br.) Hubb. 1939G52, Triraphis pungensG52; incl. Triraphis dianthaB78, Festuca viscidaG52, Triodia viscidaG52]

*Type species of generic name indicated

References

Barrett, R. L., & M. D. Barrett. 2011. Two new species of Triodia (Poaceae: Triodieae) from the Kimberley region of Western Australia. Telopea 13 (1–2): 57–67.

[B05] Beard, J. S. 2005. Drainage evolution in the Lake Disappointment Catchment, Western Australia—a discussion. Journal of the Royal Society of Western Australia 88 (2): 57–64.

[B78] Bentham, G. 1878. Flora Australiensis: A description of the plants of the Australian Territory vol. 7. Roxburghiaceae to Filices. L. Reeve & Co.: London.

[B00] Braby, M. F. 2000. Butterflies of Australia: their identification, biology and distribution vol. 1. CSIRO Publishing: Collingwood (Victoria).

[C06] Cheeseman, T. F. 1906. Manual of the New Zealand Flora. John Mackay, Government Printer: Wellington.

[CWB06] Craig, M. D., P. C. Withers & S. D. Bradshaw. 2006. Patterns of diet and microhabitat use by four species of sympatric Ctenotus lizards: do they reveal foraging specialisation? Journal of the Royal Society of Western Australia 89 (1): 1–5.

[CM15] Crisp, M. D., J. Mant, A. Toon & L. G. Cook. 2015. Australian spinifex grasses: new names in Triodia for Monodia and Symplectrodia. Phytotaxa 230 (3): 293–296.

[G52] Gardner, C. A. 1952. Flora of Western Australia vol. 1 pt 1. Gramineae. William H. Wyatt: Perth.

[G04] Gibson, N. 2004. Flora and vegetation of the Eastern Goldfields Ranges: part 6. Mt Manning Range. Journal of the Royal Society of Western Australia 87 (2): 35–47.

[KM08] Keighery, G. J., & W. Muir. 2008. Vegetation and vascular flora of Faure Island, Shark Bay, Western Australia. Records of the Western Australian Museum Supplement 75: 11–19.

Lazarides, M. 1997. A revision of Triodia including Plectrachne (Poaceae, Eragrostideae, Triodiinae). Australian Systematic Botany 10: 381–489.

[LK14] Lyons, M. N., G. J. Keighery, L. A. Gibson & T. Handasyde. 2014. Flora and vegetation communities of selected islands off the Kimberley coast of Western Australia. Records of the Western Australian Museum Supplement 81: 205–244.

[S06] Stuckert, T. 1906. Segunda contribución al conocimiento de las gramináceas Argentinas. Anales del Museo Nacional de Buenos Aires, serie 3, 6: 409–555.

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