Ecology of Dimerias

Dimerias in the juvenile stage

The Dimerias are primarily C₄ grasses, which exclusively prefer the natural tropical climate. Dimerias enjoy monsoon climate in general, and are localized along the Tropical wet region.

The sprouting, flowering and fruit-setting of annual species occur within a period of 4 months. In lateritic plains (Western Ghats), the Dimerias usually sprout during October and completely dry out by the end of February. The seeds that fall and are deposited in the crevices of laterite remain there for almost 10 months and on getting the post-monsoon rain the seeds germinate.

Dimerias show remarkable colour changes as they grow from the juvenile to the mature stage. In some areas the colour of the vegetation shows a gradual change from green to golden yellow, and from green to reddish brown in some other places. This is attributed to the formation of pigments called anthocyanins.

Phytogeography of Dimerias

'Sea of Dimeria' along the lowland region of Western Ghats

In Dimeriinae, about 90% of the known world species can be found in the following three areas, viz. Peninsular India, Sri Lanka and Southern China. Most of the species are having restricted distribution in one of these 3 regions.

The distribution pattern inferred from the literature and herbarium data indicates that subtribe Dimeriinae was naturally distributed in the southern erstwhile Gondwanaland region, an area composed of Madagascar, India, Sri Lanka, Australia and South East Asia, especially the Malesian region.

Three species are restricted to Madagascar, and all of them are endemic. This geographical isolation of Malagasy species from other species of Dimeria by approximately 2000 km, indicate an interesting direct phytogeographical link between Peninsular India and Madagascar. Some species are also found in volcanic soils of Mascarene Islands. This distribution supports the geological data which indicated that the archipelagic connections existed between the Indian plate, the Mascarene plateau, Madagascar and Africa until 75 million YBP!

History of Dimeria

H. M. S. Endeavour (courtesy: Wikipedia)

Dimeria has two tough racemes (in most of the species) of spikelets, from which the name of the genus is presumably derived. In Greek, dis means ‘double’ and, meros means ‘part’.

The genus Dimeria was created in 1810 by Robert Brown. (Type: D. acinaciformis R. Br. - an endemic species of Australia). The plant was collected by Joseph Banks (1743--1820) and Daniel Carl Solander (1736--1782) in 1770. They were passengers on Captain James Cook’s First voyage (1768--1771) in the H. M. S. Endeavour. During this trip, they explored many Asian and Australasian countries and deposited the massive plant collections from these regions at Banks Herbarium. The type specimen of the genus Dimeria R. Br. is now housed at British Museum, London (BM).

Dimerias: Habit & Habitat

The tufted habit of Dimerias (Annuals)

Most of the Dimerias (80 %) are annuals, the majority of which are laterite-loving, where as perennials prefer rocky crevices along mixed forest margins, rarely in high altitude meadows, or wet sandy plains along water courses. Monsoon plays a major role in the establishment and diversity of Dimeriinae in general. The humid tropical wet climate favours the successful growth, maximum diversity and endemism in Peninsular India.

• Annuals: Solitary (or few); Many & Tufted; Branched & Matforming
• Perennials: Tufted & rooting at lower nodes; Stoloniferous & creeping

Dimerias in Peninsular India

A 'Dimeria-field' (mature stage) in Northern Kerala

Dimerias dominates the herbaceous stratum of western peninsular regions of India during the period of retreat-monsoon (September–November). Members of Dimeriinae are particularly abundant in Western Peninsular India, which includes the Northern Western Ghats and its coastal zone and the Southern Western Ghats region.

The highest concentration of species is found in Southern Western Ghats. In fact 28 of the 42 Peninsular Indian taxa (including 7 varieties) can be found in this small region from the South of Goa, Karnataka, Kerala to Tamil Nadu states. 17 taxa are exclusively endemic to this region. The high degree of endemism in the Peninsular Indian region (especially in the Western Ghats), both within Dimeriinae and among its allies, suggests that the subtribe originated in this area. (excerpts from Ph. D. thesis).

Diversity of Dimerias

World Distribution

Dimeriinae Hack. is a little-known subtribe, coming under the tribe Andropogoneae Dumort. of subfamily Panicoideae. The subtribe is considered as an enigmatic group of the family Poaceae and occupies a somewhat anomalous position in the subtribal classification. It is strictly a palaeotropical subtribe, largely distributed in Tropical Asia and chiefly confined to the Indian peninsula.

Usually, Dimerias have two tough racemes (inflorescence) of spikelets, from which the name of the subtribe is presumably derived.

Reference format:

Kiranraj, M.S.(2010 onwards). Diversity of dimerias. DIMERIA: An on-line Newsletter from GRASSNEST

____________________________________________________

(Source: Kiranraj, M. S. 2008. Taxonomic revision of the subtribe Dimeriinae Hack. of Andropogoneae (Poaceae-Panicoideae) in Peninsular India. Ph.d. thesis. University of Calicut, India)