A small, summer-deciduous perennial, growing from a `corm`. It prefers skeletal acidic soils that are moist or flooded in winter but experience summer drought. A poor competitor, it is most frequent on pans of bare soil over S.-facing rock outcrops, but also grows on erosion pans and footpaths. It sometimes occurs in short turf, and in dune-slacks in Guernsey. Lowland.
The distribution of this species is stable. A few sites have been lost due to a lack of grazing, to fire and to conversion to cultivation, but it has colonised areas from which turf has been stripped.
Light (Ellenberg): 8
Moisture (Ellenberg): 7
Reaction (Ellenberg): 5
Nitrogen (Ellenberg): 1
Salt Tolerance (Ellenberg): 0
January Mean Temperature (Celsius): 6.6
July Mean Temperature (Celsius): 16.2
Annual Precipitation (mm): 829
Height (cm): 4
Perennation - primary
Life Form - primary
Clonality - primary
Count of 10km squares in Great Britain: 3
Count of 10km squares in Ireland: 0
Count of 10km squares in the Channel Isles: 5
RDB Species Accounts
Isoetes histrix Bory (Isoetaceae)
Status in Britain: LOWER RISK - Near Threatened.
Status in Europe: Not threatened. Endemic.
I. histrix occurs in Britain only on the Lizard peninsula. Here it favoured in three habitats: the coast path and other paths where trampling is not too severe; coastal erosion pans, with fragments of serpentine and skeletal soil, which are wet and preferably flooded in winter but dry in summer; and principally pans of bare soil on rock outcrops, mainly with a southerly aspect. On the last, conditions of winter wet and summer drought are essential, as is rough grazing. In each of these habitats there is much bare soil, but I. histrix will survive in short turf at up to 80% cover, together with Allium schoenoprasum, Herniaria ciliolata, Juncus capitatus, Lotus subbiflorus, Minuartia verna, Scilla autumnalis, S. verna, Trifolium bocconei and T. strictum. It is predominantly a coastal plant and, apart from four sites at Black Head, near Coverack, all are on the western, more exposed side of the Lizard. A few populations occur inland on Goonhilly Down and elsewhere. All known sites are on serpentine, with the exception of one on schist and one on the junction of schist and serpentine.
This species is a small, rosette-forming perennial, growing from an underground 'corm'. It reproduces by spores of two types, which are produced in sac-like sporangia more or less embedded in the leaf (sporophyll) bases, and ripening in April and May. Some sporangia contain megaspores (up to 480 µm diameter) which, on germination, form a 'prothallus' tissue which bears the female organs (archegonia). Other leaves bear similar sporangia containing dust-like microspores (35-40 µm diameter); these spores produce male gametes which fertilise the archegonia of the germinating megaspores. As spores are formed below the soil surface, their distribution depends on disturbance of the substrate. Spores can be long-lived; they have germinated in wetted soil samples which had been kept dry for 34 years (D.A.Coombe, pers. comm.). Leaves emerge with autumn rains, typically in late September and October, and wither in late May or early June.
I. histrix was not discovered in Britain until 1919, and not confirmed until 1937. Prior to this it had been known, however, in Guernsey and Alderney. A comprehensive survey on the Lizard in 1982 showed it to be present in 27 sites in sixteen 1 km squares (within two hectads), with a total population of 100,000 plants (Frost, et al. 1982). Since then, an additional small site has been found in a third hectad. Almost all populations are on NNRs, SSSIs or National Trust land, and most of its recorded sites are still extant. The few losses have been caused by fire, cultivation (two sites at Caerthillian with about 200 plants were ploughed before 1982), and by a lack of grazing.
The reproduction process of I. histrix is assisted by grazing and disturbance. Grazing keeps the turf short, and trampling (by people and animals) spreads the spores or exposes them to the wind. The bulk of the population in 1982 (though only a small percentage of the total sites) was on areas dependent on rough grazing by livestock. Rough grazing has declined everywhere on the Lizard in the past twenty years, and it is significant that in the 1982 survey, four out of the five extinct populations were on sites where grazing had ceased. However, tourist pressure is at its heaviest from June to September, the dormant period of I. histrix, and the consequent trampling and scuffing of turf probably compensate to some extent for the lack of grazing.
I. histrix is principally a plant of Mediterranean coasts, where it is present in habitats which are wet in winter but dry out completely in summer. It also occurs along the Atlantic coast, reaching its northern limit in Britain.
A. J. Byfield and D. A. Pearman
Atlas text references
1984. Flora dels Països Catalans, I. Introducció. Licopodiàcies-Capparàcies.
Jalas & Suominen (1972)
1978. Ferns and their allies. The Island of Mull: a survey of its flora and environment. :12.1-12.7.
1997. The ferns of Britain and Ireland, edn 2.
1999. British Red Data Books. 1. Vascular plants, edn 3.