Trifolium repens L., White Clover
Account Summary
Native, very common, widespread and locally abundant. Eurosiberian boreo-temperate, but very widely naturalised and now circumpolar.
1881; Stewart, S.A.; Co Fermanagh.
Throughout the year, peaking in May.
Growth form and preferred habitats
Very common, extremely variable, locally abundant and geographically widespread, White clover is one of the compound, three-leaflet species which in Ireland is regarded as St Patrick's Holy Trinity teaching aid and Irish emblem, the Shamrock. It is a stoloniferous perennial of indeterminate height, rooting at the nodes of procumbent, much-branched creeping stems, up to 50 cm long. Most of the wiry, fibrous roots are shallow, although it has a taproot that can reach depths of 60 cm. Plants are ± hairless (glabrous or glabrescent), with trifoliate leaves on erect petioles usually 7 cm or longer, leaflets with a whitish or reddish angled band towards their base. The inflorescence is a globular head of 20-40 white or pinkish, sweetly scented pea flowers on a peduncle that usually is longer than the leaf petioles (Burdon 1983).
The most important pasture nitrogen-fixing legume in many parts of the temperate zone, native forms and cultivar escapees of T. repens are abundant in lowland, moist, open, roadside grass verges, rough grassy waste ground and other ruderal and wayside habitats. It is rarely a pioneer colonist as it has relatively high nutrient requirements. At the coast, it colonises damp pasture areas and dune slacks in fixed and late stage sand dunes, despite having a low tolerance of sodium chloride (sea salt) in the soil: it is seldom abundant on dunes and absent from salt marshes for this reason (Burdon 1983). Preferring flat or gently sloping, moist, fertile ground, T. repens avoids only the wettest and most acid soils, but does occur to a lesser extent on grassy heaths and upland moors. Like other legumes, it also abhors shade and is almost totally absent from woods, scrub and tall vegetation of any kind (Grime et al. 1988, 2009). T. repens is also intolerant of prolonged drought and severe frost, the latter potentially most damaging to stolons of clones with long internodes that root less frequently (Ronningen 1949, quoted in Turkington & Burdon 1983).
The creeping shoots of T. repens enable it to colonise vegetation gaps in meadows and pastures, especially those involving Lolium perenne (Perennial Rye-grass), with which clover is very commonly sown. As an efficient nitrogen fixer, T. repens holds a key position in the economy of many agricultural grasslands and may provide a driving force for the cyclic changes occurring between it and associated grass species in many grassland communities (Turkington & Harper 1979 a, b).
Grazing, mowing and trampling pressure
The effect of grazing on T. repens abundance in pastures is determined by its frequency, timing and intensity. In general, frequent and intense grazing encourages the growth of T. repens and tends to lead to the development of simple pastures entirely dominated by T. repens and Lolium perenne. Lighter, less intensive grazing pressure allows the entry or survival of taller grasses such as Dactylis glomerata (Cock's-foot) and Holcus lanatus (Yorkshire-fog), and this is associated with a decline in T. repens abundance.
Differing seasonal growth rhythms of clover and grass species are also significant in terms of the effect of the timing and duration of grazing pressure and the effect observed on species composition of the sward. This is especially the case when selective grazing by sheep is involved. Close, heavy grazing of a T. repens/Lolium perenne sward in March to May led to a substantial increase in clover as the Rye-grass was grazed at a critical growth period. A slightly later and lighter grazing regime beginning in mid-April resulted in a clover decline as L. perenne was able to maintain its vigour and remain dominant (Jones 1933).
A similar pattern of botanical changes in clover-grass mixtures occurs with variations in timing and frequency of mowing. T. repens/L. perenne swards cut three times per year quickly become grass dominated, while swards cut six times per year become clover dominated (Kishi 1973, 1974).
White Clover is one of the most trampling-resistant common pasture legumes known. Again, as with grazing pressure, heavy trampling leads to a simplification of the species mixture present and the predominance of the familiar T. repens and L. perenne partnership (Brown & Evans 1973).
Flowering reproduction
T. repens reproduces both sexually and asexually.
Reproduction by seed allows the rapid colonisation of newly available habitats at a distance from the existing breeding population, while vegetative reproduction is important in maintaining individual genetic individuals (genets) in the dynamically changing environment of long-term pastures. Disturbance of the existing field vegetation appears necessary to create a gap allowing seed to germinate or a vegetative genet to colonise the ground. Seedlings of T. repens are very infrequently observed in undisturbed pastures and even more rarely are seen to develop into established plants (Burdon 1983). However, this is almost certainly true of very many other pasture and meadow species.
Flowering takes place from June to September peaking in July, the small, honey-like scented pea flowers with corollas 7-10 mm long, offer pollen and nectar to attract pollinators. They are visited by flies, solitary-, honey- and bumble-bees, particularly the last two mentioned (Proctor & Yeo 1973; Burdon 1983). Nectar is secreted between the base of the stamen tube and the ovary and is accessible to insects with quite a short proboscis, although they have to use their weight to open the flower by separating the wing and keel petals. Other insects frequently bite and puncture the base of the flower and steal the nectar without effecting pollination.
Plants may flower in their first year of growth and continue to flower yearly thereafter. In established native populations, however, the entire sexual reproductive effort of the population may be concentrated in a small number of the genets present on a site. Burdon (1980) found that of 50 clones collected from such a population, 20% produced 81% of the flowers, 40% produced no flowers at all and a further 10% produced only a few flowers.
The flowers are basically self-incompatible, although very rarely a minute amount of self-pollinated seed may be produced and very rarely a few highly self-fertile plants have been found. After fertilisation, the flowers wither and droop, becoming reflexed. Seed matures about 28 days after pollination. The resultant oblong legume pod develops, 4-5 mm long, constricted between the 2-6 seeds (usually just two seeds) and it protrudes from the calyx tube while still surrounded by the dry, withered corolla (Burdon 1983). The pod is indehiscent, but it eventually releases the seeds, although no special dispersal mechanism exists for them. By the time the seeds are released, hard seed dormancy has developed due to the outer seed coat becoming impermeable. Most seed is probably incidentally spread by the movement of grazing and other passing animals, some being ingested and carried undamaged in the gut of cattle, deer and horses (Ridley 1930). Seed may also be transported with dried animal fodder such as hay. Hard seeds can survive soil burial for more than five years and possibly up to 80 years, although the density of viable seed in soil is often low. Germination occurs in spring and early summer (Burdon 1983).
Hybrids with other Trifolium species do not occur naturally, although with great difficulty they have been artificially created (Burdon 1983).
Vegetative reproduction and genet longevity
Vegetative reproduction by the lateral extension of stolons is very important in T. repens. The frequency of new stolon branch initiation by lateral nodes varies with the genotype and through the growing season. It is always at a maximum in May and minimum in October and is strongly governed by temperature. Under competitive field conditions, an annual extension of around 18 cm seems usual for stolon growth, but this is very variable with respect to local environment and the particular genotype (Burdon 1983).
The profuse vegetative growth and frequent intermingling of clones of varying genome makes any estimation of individual genet longevity very difficult. Potentially the lifespan of any genet is indefinite, particularly after the eventual break up of the original individual into a number of genetically identical ramets. For instance, in old dune slacks Harberd (1963) estimated that a number of White Clover clones were 20 years old, while a particularly fragmented clone had a minimum age of 60 years and a possible maximum age in excess of a century. This contrasts with a typical plant of the less persistent Ladino crop variety that behaves as a winter annual: germinating in the autumn, flowering vigorously in the spring of the following year before dying (Burdon 1983).
Fermanagh occurrence
Together with T. pratense (Red Clover) this is one of the two most common and the most widespread legumes in Fermanagh and indeed throughout B & I. Locally, T. repens is slightly the more frequent of these two clovers, although T. pratense is slightly more widespread – ie the latter is present in 89% of the Fermanagh tetrads, while T. repens is recorded in a mere 86.4% of them (462 tetrads)!
Unlike tap-rooted T. pratense, T. repens also grows quite frequently on upland moorland and thus it is likely that surviving, potentially untainted native populations of White Clover are only or chiefly found in remote sites. In Fermanagh, these include higher altitudes on Cuilcagh and Belmore and on obviously completely unsown ground, eg many wet or unapproachable lake shores.
Variation
A highly variable species with considerable differences within and between populations over a wide range of morphological characters, over the years T. repens has presented many difficulties in taxonomy and nomenclature. Genetically, T. repens is so extremely variable that it has been described as, "the Drosophila of plant ecology" by Turkington & Burdon (1983). T. repens is an allopolyploid species with chromosome number 2n=32.
Tutin et al. (1968) in Flora Europaea 2 and Burdon (1983) recognised six subspecies, of which only subsp. repens is considered native in Britain, the other five subspecies being chiefly found in the mountains of S Europe. When it comes to varieties, some older ones claimed as native in Britain (Erith 1924), such as var. rubescens Ser. ex DC. (with pink flowers) and var. sylvestre Alef. (white flowers), do not appear in any standard B & I Floras and fail to equate with other taxa listed by Sell & Murrell (2009). In their critical Flora of B & I, the latter recognise four T. repens varieties: var. carneum Gray, corolla slightly or deeply tinged pink, a native found on heaths and by the sea; var. townsendii Beeby, corolla purple, a rare form from the Isles of Scilly; var. repens, inflorescence up to 25 mm, corolla white, a widespread mix of native wild and escaped cultivated forms; and var. grandiflorum Peterm., inflorescence 30-35 mm, corolla white, formerly grown for hay, now planted with wild flower seed.
Cultivation and agricultural benefits
Its vigorous growth, efficient nitrogen-fixing ability over a very wide range of ecological conditions, reliable long-season availability and nutritious quality as animal fodder, all taken together, have made T. repens by far the most important cultivated pasture legume in these islands. The beneficial characteristics of T. repens include the very high quality of herbage protein and mineral content and its high levels of acceptability and digestibility by grazing animals. These characteristic benefits are retained throughout the growing season significantly better than all grasses, and also better than other tap-rooted legumes including T. pratense (Red Clover) and Lucerne (Medicago sativa subsp. sativa) (Williams 1970). As forage for animals, T. repens compares favourably with grass, being higher in protein and certain key minerals and low in structural fibre. Unlike grass it maintains a high level of digestibility as it matures (Thompson 1984). Since it is easily and quickly digested and is very palatable, stock animals preferentially seek it out and eat a lot of it. However, if the proportion of White Clover present in clover/grass ley herbage exceeds 50%, there is a danger of cattle developing bloat, as the rapid fermentation that takes place in normal ruminant digestion may release dangerous amounts of gas, which the animal cannot get rid of quickly enough (Davies 1992).
In the past, White Clover growth patterns and the nature of the interaction with grass have tended to cause significant seasonal variation of clover content in swards – from as little as 5% in the spring up to 60% in summer – but clover breeding is producing varieties that are more compatible with modern rye-grasses and have more even seasonal growth curves. Accumulated experience and scientific evidence indicate that the optimum balance is achieved with a White Clover content of 30–35% of the total annual dry matter yield of the sward.
The most notable agricultural advantage of White Clover is its contribution to the nitrogen balance of soils. Agronomists reckon T. repens can contribute roughly the same annual amount of nitrogen element (150-200 kg N/ha) to pasture soils as the national average application of fertiliser N to sheep-grazed pastures in the UK. Clearly this is dependent on the clover content of the sward however, which can vary greatly (Davies 1992).
Since about the 16th century onwards White Clover has been and remains extremely commonly sown in pasture mixtures with grasses. Over 400 metric tonnes of White Clover seed are sown annually in the UK (DEFRA Seed traders annual return – year ended 30 June 2002). Unfortunately up-to-date comparison figures are not available, since some years ago DEFRA stopped collecting annual measurements of agricultural seed use. Around 75 different cultivars are listed for Europe (Burdon 1983).
Crossbreeding and its effects
T. repens is mainly outbreeding, so hybridization and introgression have taken place between the old, presumably native forms and the multitude of introduced cultivated varieties, crosses very probably commonly occurring in areas of overlap. As a result we may never know the distribution of the 'unpolluted' native species. Cultivated strains tend to be larger in all their parts and individual White Clover plants persist for only one or two years. These clover varieties and differing forms can seed themselves profusely however, even when growing in heavily grazed or disturbed ground and the resultant seeds become added to the long-persistent fraction of the soil seed bank present in improved pastures and meadows (Duke 1981; Burdon 1983).
The seed of T. repens is small and light and may be transported both by wind (perhaps insignificant) and internally by grazing animals which is probably highly significant (Ridley 1930). Thus any surviving native strains of clover would need to be extremely remote from farmland to avoid interbreeding or competing with the widely used cultivated forms.
There is an enormous scientific literature dealing with many aspects of T. repens, especially on the agricultural properties of the species since it offers unrivalled flexibility in its utilisation as a quality animal fodder (Turkington & Burdon 1983). The latest breeding development in White Clover has been the introduction of a hybrid variety between T. repens and Caucasian clover (Trifolium ambiguum), which is a drought tolerant, rhizomatous species. This new variety has both stoloniferous and rhizomatous root structures that aid grazing tolerance, drought tolerance and winter hardiness (Marshall et al. 2004).
British and Irish occurrence
T. repens is almost ubiquitous throughout both B & I, but as noted above, distinguishing native populations as opposed to cultivated forms and crosses between the two in the field is very difficult or almost impossible (New Atlas).
European and world occurrence
The postulated centre of origin of T. repens s.l. is in the Mediterranean mountains of Europe from where it has spread across the whole continent up to about 71°N and it now occupies around 97% of European territory, although in warmer southern regions, including the Alps and the Mediterranean basin it is confined to upland pastures and meadows. It is also considered native in NW Africa and N & W Asia and in phytogeographic terms is considered Eurosiberian boreo-temperate (Davies 1992). However, it has been spread almost worldwide by its use from the 17th century onwards in agricultural grass seed mixtures. T. repens s.l. has become partly naturalised in most parts of the world, especially in N America and thus can now be recognised as circumpolar boreo-temperate (Hultén & Fries 1986, Map 1235; Preston & Hill 1997).
Threats
Hybridization between native and cultivated populations will be increasingly likely, but selection pressure in non-agricultural sites will continue to favour persistence, with the ability to colonise relatively fertile, rather drier soils than the agricultural norm.