Trifolium dubium Sibth., Lesser Trefoil

Account Summary

Growth form, recognition and preferred habitats

A frequent to common, small, creeping, ruderal, winter-annual with fibrous roots and stems varying from 3-30 cm in length, T. dubium is another of the diminutive, trefoil-leaved plants commonly used on 17th of March (St Patrick's Day) as the 'Shamrock'. This little nitrogen-fixing legume is typically found in short, disturbed grassland such as lawns, roadside verges and banks, waste ground and other dry or well-drained, open, sunny, mainly lowland sites including in quarries and on the tops of walls.

In freshly sown lawns, T. dubium can be a troublesome weed in the early days as it forms an interlacing mat of thin wiry stems prior to the grasses tillering up and forming a proper turf that will soon competitively exclude it. Having said this, T. dubium is renowned for its ability to tolerate mowing and trampling. Like its close relative, T. campestre (Hop Trefoil), the ecological established strategy of T. dubium is described as R/SR (Grime et al. 1988, 2007), meaning it is intermediate between a straight ruderal and a stress-tolerant ruderal, rather lacking in competitive ability, probably on account of the usually rather dry, low nutrient soil conditions it appears to mainly tolerate and frequent.

Despite its preference, or rather its tolerance for drier, sometimes droughted, infertile soils, Lesser Trefoil does also occur quite commonly in Fermanagh on damper, winter-wet, lakeshore grasslands, which may sometimes be moderately fertile. In any such pasture conditions, T. dubium is also remarkably tolerant of grazing and trampling pressures (Sinker et al. 1985).

In flower, T. dubium could be confused with Medicago lupulina (Black Medick), but vegetatively the latter has leaves that are usually densely hairy and apiculate, ie with a small, abrupt point in the notch at the leaf tip, whereas T. dubium has sparsely hairy to glabrous leaves that lack any apiculus (Rich & Jermy 1998).

T. dubium might possibly also be confused with T. campestre, but it is smaller in all its parts than the latter and has fewer flowers per head (only 10-15 in T. dubium compared with 20-40 in T. campestre). Also T. campestre is mainly coastal in NI and it is very rare in Fermanagh, while by comparison T. dubium is extremely common and widespread.

Clover evolution and variation

In a review of the question of pasture clover species origins, Abberton (2007) commented, "There appears to be no significant role for interspecific hybridization in the evolution of the genus." Interspecific hybridization in the genus Trifolium by conventional crossing techniques has been largely unsuccessful and there currently are no known clover hybrids anywhere in the flora of B & I (Stace et al. 2015). Post-zygotic barriers appear to be a primary cause of the reproductive isolation and these are associated with endosperm disintegration and consequent abnormal differentiation and starvation of the hybrid embryo. Evans (1976) postulated that the rarity of wide hybridization in the ten main Trifolium forage legume species may well be associated with their predominant adaptation to insect pollination, although the annual clovers at least are often capable of additional self-pollination. Ellison et al. (2006), in a comprehensive DNA based phylogenetic analysis, found only five or six instances of apparent hybrid speciation in fodder clover species.

Various chromosome counts have been obtained for T. dubium ranging from 2n=14, 16, to 28 or higher. Other work suggests Lesser Trefoil is an allotetraploid with a chromosome number 2n=32 (Sell & Murrell 2009). A similar chromosome count of 2n=30 is given in a study testing the idea that T. dubium arose from the crossing of two diploid clover species of similar world distribution, T. campestre (2n=14) and T. micranthum (Slender Trefoil) (2n=16), followed by chromosome doubling by means of unreduced gametes (Ansari et al. 2008). T. dubium is said by these workers to be morphologically intermediate between these two parent species.

The current author (RSF) could only access a summary of the Ansari et al. (2008) paper since it is protected by a paywall, but it appears to involve a number of the same research workers and follows similar genetic phylogeny analyses using evidence from DNA sequence analyses, molecular cytogenetics, interspecific hybridization and post-hybridization experiments as a subsequent freely available study on the origin of T. repens which indicated a hybrid origin for White Clover that involved multiple crossings of T. pallescens Schreb. (Pale Clover) and T. occidentale D.E. Coombe (Western Clover). These crosses were probably associated with the major species migrations southwards into refugia during past glacial climatic phases (Williams et al. 2012). It appears, therefore, from the two examples mentioned, that hybridization between clover species was possible and did occur some thousands of years ago, although this is no longer the case since barriers to embryo survival have subsequently arisen.

In B & I, T. dubium is sufficiently variable for two varieties to be recognised by Sell & Murrell (2009): var. microphyllum (Ser.) P.D. Sell is a dwarf form with stems up to 10 cm; spreading or prostrate leaves small, mostly under 5 mm; and inflorescence also small, up to 7 mm in diameter. The alternative form, var. dubium has stems up to 30(-45) cm, ascending or erect, leaves up to 12 mm, and inflorescence up to 10 mm in diameter.

Flowering reproduction

T. dubium seed germinates in the spring and the species flowers, often profusely, from May to October. The individual plant produces large numbers of axillary, subglobose, inflorescence heads, 5-10 mm in diameter, each usually containing 3-15 yellow pea flowers, the corolla 3-4 mm in length. After fertilization, either by insect-pollination (Fitter 1987) or, as is more usual in small clover species, self-pollination (Proctor & Yeo 1973), the corolla turns brown and the standard petal folds itself down to cover and partially conceal the developing legume fruit pod. The legume is 2.5-3 mm in length, ovoid in shape and it is usually single seeded (Clapham et al. 1987). Like related legumes, seeds develop hard coat dormancy and can survive soil burial for many years (Thompson et al. 1997).

Fermanagh occurrence

Lesser Trefoil has been recorded in 181 Fermanagh tetrads, 34.3% of those in the VC. As the tetrad map shows, it is widely scattered everywhere around Fermanagh except on high ground. Although most typical of open, disturbed sites on damp to dry, relatively infertile soils, T. dubium does also occur on limestone cliffs, as at Knockmore Hill and Trien Mountain and by tracks in lime-flushed areas of upland blanket bogland, eg at Black Bridge in the W of the VC.

British and Irish occurrence

T. dubium is common and widespread throughout B & I in suitable lowland grassy meadows, lawns and waysides, open waste ground and rocky sites. It is absent mainly from acid, peaty mountain sites in Scotland and is introduced and naturalised in Shetland (VC 118). The New Atlas hectad map shows there is no significant change in T. dubium presence in the 40 years between the two BSBI Atlases (D.A. Pearman, in: Preston et al. 2002).

European and world occurrence

Lesser Trefoil is widespread in Europe stretching from the Atlantic Isles eastwards to N Iran and northwards to S Scandinavia. It has been widely introduced with other agricultural clover and grass seed mixtures both in Europe and beyond to places as far apart as Iceland, the African Cape Province, S Australia, Tasmania, New Zealand and N America. Thanks to its long-lived seed, it is naturalised in some of its European and more outlying areas (Hultén & Fries 1986, Map 1241).

Threats

None.

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