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Taxonomy
Scleranthus annuus L., Annual Knawel, is a widely accepted species, but its limits are constantly under review. Stace (1997) includes within it S. polycarpos L., as S. annuus L. ssp. polycarpos Bonnier & Layens.
It is an annual or biennial plant with inconspicuous green flowers. The size of the achene and the shape of the flower distinguish the two subspecies.
Chromosome No.: 2n = 44.
Distribution
It is widespread thoughout Britain, but absent from the very north and the Outer Hebrides. In Ireland it is rare but scattered throughout.
The ssp. polycarpos is recorded mainly in the Breckland, although there are a few records from other parts of the country. Lusby (in Preston et al. 2002) suggests that it is under-recorded.
The species occurs throughout Europe except in the extreme north and is introduced in North and South America, Australia and doubtless elsewhere (Tutin et al. 1993, GBIF.)
Its maximum recorded altitude is given by Dickie (1860) as 1200ft (365m) in Aberdeenshire, although the precise locality is not known.
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Status
Origin: native. The first record is considered to have been by Thomas Johnson in 1629, at Quex in Kent (Hanbury & Marshall 1899), although Clarke (1900) points out that there may have been some confusion between this species and Alchemilla arvensis.
Rarity: rare in Ireland, but not in Britain.
Threat: It has recently been added to the British Red List as ‘Endangered’ owing to an apparently significant decline (Cheffings & Farrell, 2005). In Ireland, the decline seems even more severe.
Conservation: most counties count it as an axiophyte, either of species-rich arable fields or of heaths and upland grassland.
Ecology
It tends to occur in two distinct habitats - arable fields and dry, heathy grassland. The latter would be more obviously a natural habitat for it, and Hanbury & Marshall (op. cit.), for instance, considered plants in such situations (which they called S. biennis) to be ‘the original form’, whereas ‘the commoner plant [is] a more luxuriant product of cultivated land.’
Rodwell (1991-2000) lists it only as a plant of arable fields in the SE of England, in OV1 Viola arvensis-Aphanes microcarpa and OV5 Digitaria ischaemum-Erodium cicutarium communities.
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Future work
It is not entirely apparent what is happening to Scleranthus annuus. Its range has not contracted, but it has seemingly declined from its arable situations. Whether the more natural populations have changed or not is an unanswered question.
There is need for more study of the putative subspecies, including cultivation experiments to demonstrate that they breed true and possibly genetics work to see what taxonomic rank they merit.
More work is needed on its vegetation communities, especially in non-arable habitats.
References
Dickie, G. (1860) The Botanist’s Guide to the Counties of Aberdeen, Banff and Kincardine. Brown, Aberdeen.
Hanbury, F.J. & Marshall, F.J. 1899. Flora of Kent. Frederick J. Hanbury, London.
Acknowledgements
Photographs by Geoff Toone.
Citation
Lockton, A.J. & Pearman, D.A. (date accessed). Species account: Scleranthus annuus. Botanical Society of the British Isles, www.bsbi.org.uk.
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Send us your comments and Feedback
David Welch (22/8/2008): regarding the maximum altitude given by Dickie, Corgarff at the head of Strathdon could be at 1200 ft. And Mortlach parish would go equally high south of Dufftown, but probably with no suitable habitat.
My present highest localities are c. 120 m, and the furthest inland was at Insch Station (old sidings but now gone as a car park). I reckon Scleranthus is becoming rare.
Angus Hannah (12/2/2010): Analysis of associated species at Scleranthus annuus sites on Bute indicates that the local habitat is unlike that described from elsewhere, e.g. in Rodwell (2000) or Braithwaite et al (2006). Though recorded in only 13 tetrads nationally in the 2003/4 Local Change survey, it was found in at least four different broad habitats. These included arable fields on light acidic soils, acid grassland, dwarf shrub heath and inland rock. None of these encompasses the habitat in which it appears to be rather successful in Bute, where it occurs invariably on or close to a track regularly trampled by cattle. Trampling creates and maintains the openness which the plant seems to need, while the gravelly nature of the substrate ensures adequate drainage even in circumstances where the track serves as a watercourse during and after heavy rain. That the impression of a moist habitat is misleading is evident from the companion plants, which confirm the correctness of the Ellenberg F value of 4 attributed to the species (Hill et al., 2004). It is clear, however, that it is able to tolerate temporarily very wet conditions.
However, consideration of the associated species at its Bute sites does not support the Ellenberg values of 4 given by Hill et al. for R (reaction) and N (nitrogen level). These are probably appropriate for its heathland/dwarf scrub habitat and on certain types of inland rock, but it seems unlikely that its arable context could be so acidic and relatively oligotrophic, while the localities which it favours on Bute are always well manured and only mildly acidic. Among the constant companions of Scleranthus here are Polygonum arenastrum, Plantago major, Poa annua and Lolium perenne, all of which require relatively high nutrient levels and often occur as arable weeds. Values of 6 or 7 for both R and N would seem to be more correct for this type of site. These species, along with Matricaria discoidea, are characteristic of trampled trackways, in the OV21 community, and it is here that the Bute sites for Scleranthus seem to belong. The interesting question which follows is why Scleranthus generally figures so infrequently in this very common community.
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