British distribution: Widespread but local and decreasing, very rare or extinct in much of lowland England.
L. clavatum is commonly taken as the 'typical' clubmoss as described in textbooks.
It shows an alternation of independent generations. The gametophyte is an underground, small, saucer-shaped structure that grows saprotrophically in partnership with mycorrhizal fungi. In this species the gametophyte may persist for several seasons. As is the definition of a gametophyte, it bears antheridia and archegonia, which in turn produce the gametes which produce sporophytes after sexual fusion.
The 'dominant' generation is the sporophyte, the terrestrial, photosynthetic generation that produces spores within sporangia after meiosis. In Lycopodium (as the genus is now recognised in a restricted sense), the sporangia are clustered into cone-like strobili (singular: strobilus), in which each sporangium is protected by a leaf-like sporophyll. In L. clavatum the strobili are normally in pairs, on a well-developed stalk (peduncle).
The vegetative plant consists of a branching (fundamentally dichotomous), open, rooting, mat-like growth, sometimes forming large patches. The stems are 'leafy', bearing narrow, simple microphylls.
The genus Lycopodium and its near relatives represent one of the most ancient classes of vascular plants, the Lycopodiopsida, which first evolved some 380 million years ago. Although modern species are all herbaceous, members of this class included giant trees in the Carboniferous period, notably Lepidodendron of the order Lepidodendrales. Lycopodium clavatum can be considered as a survival from the earliest of vascular land-plants.
There are two other British species of Lycopodium as now understood :- L. annotinum, a nationally scarce species almost confined to the Scottish Highlands, and L. lagopus, a largely subarctic species previously overlooked in Britain but now known at high altitude (above 850 metres) also in the Scottish Highlands (Rumsey, 2007). The latter closely resembles occasional variants of L. clavatum with single strobili, but the peduncle is very short or lacking and it differs in characters of the microphylls.
The few other British species of clubmoss are placed in separate genera: Huperzia, Lycopodiella, Diphasiastrum and the more distantly related Selaginella.
L. clavatum is a plant of heathy grassland and moorland and, through loss of lowland habitat, is now primarily a species of higher altitudes. It grows in open grassland, on thin soils and banks, and amongst heather, but it is sensitive to 'muirburn' and so absent from regularly burnt grouse moors or confined to the damper hollows. It tolerates late snow-lie.
Though now rare in lowland areas it can be a colonist of old mine workings and other disturbed sites on acid soils. It is also sometimes abundant in the dune-heath stage of succession on base-poor sand-dunes.