Cenchrus pseudotriticoides (Poaceae: Panicoideae), a resilient pyrophyte grass from Central Madagascar

A new combination is made placing Pennisetum pseudotriticoides in the genus Cenchrus. This species is common and often dominant in the southern part of central Madagascar, associated with inundation and fire, and withstanding high grazing. A description, a detailed plate, a map, and list of known specimens are provided.


Introduction
Endemic lineages provide a window into local evolutionary history. The comparatively high endemicity of Poaceae in Madagascar indicates an ancient origin of its grass flora, while the endemic C 4 lineages serve as evidence of pre-human open canopy habitats Hackel et al. 2018). Open grasslands of central Madagascar have likely been shaped by the same fire and grazing disturbances as the African savannas (Solofondranohatra et al. 2020) but remain designated as secondary ecosystems in the latest vegetation classification by Gautier et al. (2018). Naming, classifying and characterising the common grasses of central Madagascar is important to reconstruct ecosystem history, plan conservation, and build species knowledge to support local livelihoods.
The traditionally delimited genera Cenchrus L. and Pennisetum Rich. are both ecologically significant across the tropics, characterised by bristles surrounding and subtending each spikelet, the whole structure deciduous as a unit (Clayton & Renvoize 1986). It has become increasingly clear that traditionally delimited Cenchrus originated from within Pennisetum, and both are now placed in the expanded genus Cenchrus in the Poaceae subfamily Panicoideae, tribe Paniceae, subtribe Cenchrinae (Chemisquy et al. 2010;Veldkamp 2014;Kellogg 2015;Soreng et al. 2017). Ten species of the united genus Cenchrus are listed for Madagascar by Bosser (1969), not including the cultivated crop pearl millet Cenchrus americanus (L.) Morrone (formerly Pennisetum glaucum (L.) R.Br.). All these species have names available under the genus Cenchrus except the sole endemic species, which is the subject of this study.
Cenchrus pseudotriticoides was originally named Pennisetum triticoides by Baker (1885). Camus (1947) published the replacement name P. pseudotriticoides as part of her grass inventory of the central Malagasy "prairies", followed by a more extensive description (Camus 1950) where the species was characterised as "covering hard surfaces, withstanding bush fires, but very bad fodder". The Malagasy P. pseudotriticoides was placed in synonymy under P. hohenackeri Hochst. ex Steud. by Clayton (Clayton & Renvoize 1982) but later accepted as a distinct species by Clayton et al. (2016). It differs from P. hohenackeri (now known as Cenchrus hohenackeri (Hochst. ex Steud.) Morrone) by its smaller height of 50 -80 cm (vs 60 -200 cm), more narrow panicles, few involucral bristles (vs numerous involucral bristles) subtending each spikelet, upper glume with absent to obscure veins (vs distinct primary and lateral veins), and a cartilaginous (vs membranous) upper lemma.
Today Cenchrus pseudotriticoides is a common roadside and village grass of south-central Madagascar. Recent ecological surveys by Solofondranohatra et al. (2020) found that its occurrence is associated with frequent fire, although it also resists grazing, becoming abundant in heavily grazed areas. The origin of the C. pseudotriticoides lineage is estimated to be around 1.8 -2.5 Ma (Hackel et al. 2018) but is likely to be more recent since the most likely close relative C. hohenackeri was not included in the analysis. Here this species is assigned a name in the genus Cenchrus, and a summary of the data available on this species is presented. All specimens cited were seen by the author.

USES.
Effective for preventing soil erosion and limiting runoff due to its powerful roots. Poor fodder. (Bosser 1969). NOTES. Dominant on some sandy alluvia and sandy ridges on the banks of rivers, can withstand temporary submergence (Bosser 1969). Covers vast areas (fide Perrier de la Bâthie 10875). The lectotype is selected due to its superior quality material and annotations on the sheet.