Ophioglossum lusitanicum

Ophioglossum lusitanicum L. (Ophioglossaceae)      

Least adder's-tongue fern

Status in Britain: VULNERABLE. WCA Schedule 8.

Status in Europe: Not threatened.

In Britain, O. lusitanicum occurs only on St Agnes in the Isles of Scilly. It was discovered there in 1950, since which time many botanists have searched unsuccessfully for it on the other islands and on the British mainland. The plant is confined to heathland in the southern part of St Agnes, growing in discrete patches in open acid grassland. It usually occurs in thin turf on peaty soil overlying granite, but also occasionally in patches of bare peat or amongst Polytrichum species. Associated species include Aira praecox, Anagallis arvensis, Armeria maritima, Danthonia decumbens, Festuca rubra, Plantago coronopus, P. lanceolata, Radiola linoides and Sedum anglicum.

In a wet warm autumn, fronds may appear as early as August, but they normally appear in October. They grow throughout the winter, turning yellow and disintegrating in spring. The plant is easily overlooked since the fronds are extremely small, the exposed tips usually less than 1.5 cm long. It can be mistaken for O. azoricum which shares many sites, but the fronds of O. lusitanicum are often still green to some extent when O. azoricum starts to emerge in spring. As a rule of thumb, if the fully grown plant is the same size as or smaller than the average rabbit dropping, there is the strong possibility that it is O. lusitanicum! A useful diagnostic feature is that O. lusitanicum often has fewer than six sporangial slits on the fertile frond.

None of its colonies covers more than a few square metres, and the number of fronds in a colony varies from only one or two, to several hundreds. Most plants comprise 1-3 sterile fronds arising from very short erect underground stems with long spreading roots that may bear more such stems, so that dense patches emerge from the moss or turf around the margins of large granite slabs. The total population is estimated to be fewer than 2,000 plants but in some years only about half that number are found (estimates assume that plants have on average two sterile fronds). There is usually a total of only a few hundred fertile fronds in a particular season. Grazing and other damage may soon reduce numbers, and it is not unusual to find colonies at some sites with only one or two fertile fronds, and often none at all. It is not known whether the population has increased recently or is stable, as most of the colonies have been found only in the last ten years. Furthermore, the number of fronds produced each year at individual sites can be extremely variable. In both O. lusitanicum and O. azoricum, the rootstock seems to remain healthy for several seasons underground, without producing any fronds above ground. Coombe (1992) has observed that in cultivation it is extremely prolific, is fertile throughout the year, and can recover from prolonged (eighteen month) desiccation.

Because the turf is very thin, colonies are vulnerable to erosion and trampling, especially since several colonies grow in the short grassland favoured by holidaymakers. Fronds are brittle and easily broken off, and at least one colony has been severely damaged by trampling. Light trampling on level ground is, however, less damaging than on slopes. With all the colonies restricted to such a small area they could also be vulnerable to uncontrolled burning, especially when fires penetrate the peat. Rabbit grazing helps to maintain the open grassy sward which favours O. lusitanicum. 

O. lusitanicum occurs in the Channel Islands, around the western and southern coasts of Europe from France and Iberia eastwards to Greece, on most of the Mediterranean islands, in Turkey, and in the Canary Islands and the Azores.

R. E. Parslow