Trifolium squamosum
Trifolium squamosum, usually called sea clover in English, is an annual herb in the pea family that is found close to the Atlantic and Mediterranean shores of Europe. It grows on patches of bare ground in saltmarsh, grassland or open woodland up to about 1,000 m towards the southern edge of its range. It is palatable to livestock but has no economic uses.
Description
Sea clover is an annual herb which tends to form patches up to about 1 m in diameter by 20-30 cm high, in coastal grassland. It has fibrous roots and numerous ascending to erect branched stems which are hairy when young, soon becoming glabrous.The leaves are arranged alternately along the stems, with a prominent green stipule up to 20 mm long at the base of each petiole. The leaves, like those of other clovers, are divided into three segments but are distinctive in being narrower than in most other species, being approximately oblong in shape, ca. 20 mm x 8 mm, with a small point at the tip. The leaves have translucent veins and are covered with appressed soft, simple hairs.
Flowering occurs in early summer, April to July in Britain. The oval flowerheads are terminal, subtended by an opposite pair of leaves, and either stalkless or on a short peduncle up to 0.5 cm long. Each head contains 20-30 small flowers consisting of a green calyx and pink corolla. The calyx can be hairy or glabrous, with a 5 mm long whitish tube with 10 prominent veins, ending in 5 green triangular lobes, the lower one being considerably longer than the upper four. The corolla is about 7 mm long, made up of 5 petals in the characteristic pea-flower shape with a hooded standard, two wings and the two lower petals fused to form the keel. There are 10 stamens bearing yellow anthers and one style with a brown stigma.
After flowering, the calyx teeth curve outwards, giving the fruiting heads a starry appearance. The fruit is an obovoid legume about 2.5 mm long containing round seeds about 2 mm in diameter.
Taxonomy
Sea clover was first recognised as a new species by Thomas Johnson, who found it in a saltmarsh at Dartford on 10th June 1633 and named it Trifolium stellatum glabrum, or 'smooth starrie-headed trefoile', in his revised edition of Gerard's Herball that same year.Linnaeus was impressed by the work of the early British botanists, especially Dillenius, who had produced, in his opinion, the most perfect Flora in his edition of John Ray's Synopsis methodica stirpium Britannicarum in 1724, so in 1759 he published a list of the plants of 'Anglia' in his book Amoenitates Academicae. This included, from Dillenius's Flora, Johnson's smooth starrie clover, which he renamed Trifolium squamosum. This name takes precedence because it is a properly formulated binomial, unlike Johnson's earlier polynomial. The epithet squamosum means 'scaly', but Linnaeus provided no explanation.
William Hudson was probably unaware of when he called sea clover Trifolium maritimum, or Trifolium spicis villosis globosis, calycibus patulis aequalibus, caule adscendente foliolis cuneiformibus hirsutis in his Flora Anglica in 1762. As this was three years after Linnaeus's list, it stands only as the first synonym, of which there are now many.
Trifolium squamosum has a chromosome number of 2n = 16. It is not known to hybridise with any other plants and it has no currently accepted subspecies.
Distribution and status
The native range of sea clover is along the Atlantic coasts of Europe, from Britain to Spain and in the western Mediterranean, including North Africa, becoming rare beyond Greece and into the Black Sea. It is generally coastal but extends inland in parts of France, Spain and Greece. It is considered an introduction further north, in Belgium, Germany and the former Czechoslovakia, and it has been established in some remote parts of the world, including California and southern Australia.Its global threat status has not been evaluated, but in England and Wales it is classified as LC on the grounds that it is not declining, although its range has decreased dramatically since the 1930s and losses are continuing. One author has calculated that it has declined by 49% in less than a century. Previously, it was considered 'nationally scarce' on the basis that there were only 60 hectads for it in the 1990s. A study of clovers in the Mediterranean classified it is very common in Greece, common in Portugal, Spain, France and Italy, and rare in Croatia.
Where it occurs in England and Wales it is classified as an axiophyte, which means it is of nature conservation importance.
Habitat and ecology
In Britain, sea clover grows mostly in coastal areas, typically in grassy upper saltmarsh habitats. Because it has an erect habit, it is not tolerant of heavy grazing or mowing and, because it is an annual, it tends to grow on patches of bare ground. This makes it difficult to describe a vegetation community: most of its sites are probably in upper saltmarsh communities such as sea rush swards, sea couch saltmarsh or tall fescue grassland but it seems to occur in disturbed areas which are not typical of the habitat. It is considered a therophyte, which means that it grows quickly to exploit its niche, and produces its seeds early in the year.The most likely place to find it in modern Britain is behind sea defences, particularly in the estuaries of the Thames and Severn. Here, it often grows along the side of the access track that often traces the landward edge of the sea wall. The disturbance associated with roads and tracks also accounts for its presence in inland sites, although it does not tend to persist.
Further south sea clover is much less restricted to coastal areas. In Greece, for instance, it grows in fields, grasslands and forests up to 1,000 m above sea level.
Its Ellenberg-type indicator values are L=9, F=6, R=6, N=7 and S=2, which show that it has a preference for bright sunlight, slightly damp conditions, neutral pH, low fertility and some salinity.
A few insects pests have been identified in Europe, but none in Britain yet:
- the weevil Protapion difforme feeds on this plant;
- the larvae of the beetle Bruchidius pygmaeus bore into the seeds;
- the scale insect Pseudococcus aridorum lives on the stems;