A mound of rushes
Published 19 September 2022
Numerous plants found growing in frigid or alpine habitats have adopted a cushion growth habit. They grow in compact mounds with leaves packed tightly together, resisting heat loss or damage from the elements. Among the adopters of this habit are the tuftrushes of the genus Oreobolus.
Oreobolus is a genus of over fifteen species of small Cyperaceae found in bogs and other damp habitats on mountains of South America, Australasia and south-east Asia. Species grow in compact cushions or mats from erect or ascending rhizomes (Seberg 1988). The base of each leaf forms a sheath surrounding the rhizome. The main blade of the leaf is separated from this basal sheath by a narrowed section referred to as a ‘pseudopetiole’. Silica bodies are present within epidermal cells. Inflorescences are produced terminally and comprise one or more spikelets, subtended by bracts. After flowering, lateral shoots are produced just below the inflorescence, supported by adventitious roots. As a result, the cushion grows upwards and outwards. However, basal rhizomes wither over time so mature clumps tend to develop into rings around a dead centre.
In most species of Oreobolus, each spikelet of the inflorescence bears a single bisexual flower. A single species found in Tasmania, O. oligocephalus, bears multi-flowered spikelets in sub-capitate inflorescences. As a result, it was regarded as forming its own distinct genus Schoenoides by Seberg (1988). However, a more recent analysis of phylogenetic relationships within the genus by Chacón et al. (2006) did not support its separation. Flowers bear two alternating whorls of tepals that are usually scale-like except in the Bornean O. ambiguous in which they are bristle-like. The fruit is a pyriform achene.
Phylogenetic studies of Oreobolus have tended to focus on biogeography. Chacón et al. (2006) identified the South American species as forming a derived clade nested within the Indo-Australasian taxa. Their results suggested that the genus first dispersed to southern South America from Australia, subsequently spreading northwards. A single species, O. furcatus, has an intriguing distribution in the eastern Pacific, being known from the Hawaiian islands and Tahiti. It appears to have reached these islands from Australasia rather than South America, but how it got there (and how it dispersed between the two parts of its range) remains a mystery.
Chacón, J., S. Madriñán, M. W. Chase & J. J. Bruhl. 2006. Molecular phylogenetics of Oreobolus (Cyperaceae) and the origin and diversification of the American species. Taxon 55 (2): 359–366.
Seberg, O. 1988. Taxonomy, phylogeny, and biogeography of the genus Oreobolus R.Br. (Cyperaceae), with comments on the biogeography of the South Pacific continents. Botanical Journal of the Linnean Society 96: 119–195.