Arabis hirsuta (L.) Scop., Hairy Rock-cress
Account Summary
Native, occasional. Circumpolar boreo-temperate.
1881; Smith, T.O.; Ardunshin.
April to February.
Growth form and preferred habitats
The erect fruits, small white flowers and flat, overwintering basal rosette of bristly, hairy, dark green leaves, make this a distinctive and easily recognised short-lived perennial or biennial species (Grime et al. 1988; Ryser 1990). The typical habitats of this species in B & I are shallow, warm, well-drained soils on exposed limestone cliff ledges, rock crevices, scree slopes and fairly steep rocky pastures. In Fermanagh, however, this tap-rooted species also colonises a few more obviously man-made or disturbed sites, including limestone gravel foundations on the sides of forest tracks and rock faces and spoil in a limestone quarry. In other areas of B & I, it also occurs on sand dunes and in lime mortar on old bridges and walls (D.A. Pearman, in: Preston et al. 2002). The species is completely absent from heavily or regularly disturbed sites, however, and also from wetlands.
A. hirsuta is an excellent example of what is meant by a 'calcicole' plant. This term means that it is closely confined to base-rich rocks (in Fermanagh – Carboniferous limestone) and their derived, infertile (nutritionally unbalanced), unproductive, often open, well-drained soils, which frequently support a moderately species-rich plant assemblage. Vigorous, potentially dominant competitors are either scarce in these soils, or limited by other factors including grazing pressure and drought. Occasionally, Hairy Rock-cress is also found on adjacent more acid, peaty ground, always provided this is percolated by base-rich drainage water (Grime et al. 1988; Rich 1991). At higher levels of fertility, competition from more vigorous taller growing species quickly ousts rosette plants like A. hirsuta, so it must be assumed that even in these percolated peaty areas, the productivity of the vegetation remains low (Ryser 1990, p. 51).
Fermanagh occurrence
Currently there are records in the Fermanagh Flora Database from a total of 38 tetrads, 7.2% of those in the VC. Apart from Smith's early record listed above, A. hirsuta is completely confined to the limestone terrain west of Lough Erne. The isolated nature of the Ardunshin record in north-eastern Fermanagh (which has a voucher in BEL), strongly suggests that Smith collected the plant elsewhere in the county at a time when site location was not regarded as significant. Along with all his other biological records, the site was simply given as his home address!
Flowering reproduction and genetic variation
Although usually Hairy Rock-cress is a polycarpic perennial, it has no means of vegetative spread or reproduction, relying entirely on plentiful seed production for the species increase, survival and dispersal. Numerous (often more than 50), small white flowers are produced on an erect, 5-60 cm tall stem from April or early May onwards, with the slender fruits ripening through into August or September (Grime et al. 1988). Just exactly how A. hirsuta carries out fertilization is still not fully resolved, since throughout its range the plant forms a polyploid complex of microspecies, four or five of which occur in Europe (Jalas & Suominen 1994; Roy 1995).
An American study by Roy (1995) found that allozymes of A. hirsuta were variable at the population level and that progeny arrays revealed fixed levels of heterozygosity, suggesting that apomixis (ie asexual seed formation) was taking place. However, Roy (1995) could not rule out the possibility of selfing polyploids, since many European populations have been shown to be tetraploid and these can be either apomictic or sexual, since the even chromosome number which they possess allows normal meiotic pairing to occur.
Titz (1972), who has studied the species aggregate in great detail, found that European A. hirsuta plants require pollen for seed set. From his studies, he has concluded that the plants are sexual, normally self-pollinating and autogamous (ie self-fertilizing). Titz also believes that the species has normal meiosis and he made successful inter- and intra-specific crosses involving it. Data in Roy's (1995) study suggested that the populations he examined were pseudogamous, since there was no segregation of alleles in progeny arrays. However, since these arrays probably were the product of self-pollination of the wild collected seed he used, Roy feels that further study is required to distinguish between pseudogamy and autogamy in A. hirsuta.
Seed production and dispersal
Seed production is quite high for a relatively small plant, each pod or siliqua containing approximately 40 seeds (Grime et al. 1988). Thus a minimum of around 2000 seeds per plant is quite usual, with larger plants achieving production several times this figure. The seed is small, light, flattened and narrowly but variably winged. While there are no special adaptations for its dispersal, we may assume it relies on wind for transport (Titz 1972; Rich 1991).
Seed germination and buried longevity
The seed does not require chilling for germination and the vast majority of seedlings emerge in the autumn (86-96%). The exact period of the autumn depends upon growing conditions, but generally it occurs before the end of September (Grime et al. 1988; Ryser 1990). The survey of soil seed banks in NW Europe found that A. hirsuta seeds were either transient (persisting less than one year), or short-term persistent (surviving buried for between one and five years) (Thompson et al. 1997).
Population studies
In a detailed experimental population study of nutrient-poor species-rich limestone grassland in N Switzerland, Ryser (1990) found that moderate cover or shelter provided either by the proximity of neighbouring plants or by a relatively loose 1-1.5 cm moss layer carpeting the soil in vegetation gaps, slightly delayed, but actually enhanced the levels of germination and establishment of A. hirsuta seedlings. These same growing conditions also favoured spring and early summer survival of the young A. hirsuta plantlets growing in a Swiss environment, since winter frost-heave of the soil surface disturbed their roots and exposure to subsequent dry, sunny April conditions resulted in desiccation and major population losses. Total mortality was high when compared with five other species in this study, only 23% and 15% of the autumn cohort of seedlings surviving through their first year of growth in 1986 and 1987 (Ryser 1990, p. 24). The pattern of establishment shown by A. hirsuta in this study was clearly very much governed by the harsh abiotic conditions of late winter and early spring in Switzerland.
A similar investigation of A. hirsuta population behaviour and ecology carried out under the milder, oceanic growing conditions typical of Ireland or western Britain might well provide a totally different analysis of the species' growth and survival strategy and such a comparison would be very welcome.
British and Irish occurrence
Two varieties are recognised within A. hirsuta by Sell & Murrell (2014), of which var. hirsuta is the most widespread. This variety is locally concentrated in its British distribution on base-rich, mainly chalk or limestone derived soils. In Ireland, both varieties probably occur; var. brownii (Jord.) Titz being frequent on sand dunes and more rarely on rocks in the West. Overall, in Ireland, A. hirsuta is much more infrequent and sparsely scattered than in Britain and it is mainly (but not exclusively) westerly and coastal in its occurrence (Rich 1991; Preston et al. 2002).
A. hirsuta has declined in Britain over the past 80 years, probably due to intensification of agriculture combined with the species low competitive ability, which prevents it colonising more disturbed and/or artificial habitats (Grime et al. 1988).
European and world occurrence
Beyond the British Isles, the A. hirsuta species complex is widely represented throughout W, C and NW Europe from N Africa and N Spain to the W shore of the Black Sea and northwards to within the Arctic Circle in Norway, although it is absent from Iceland and the Arctic Islands (Jalas & Suominen 1994, Map 2397). A. hirsuta s.l. extends right across Eurasia and the whole complicated species complex spreads around almost the entire northern hemisphere in boreal and temperate continental latitudes, although it is absent from higher latitudes in most of Canada and from Greenland (Hultén 1971, map 152; Hultén & Fries 1986, Map 940).
Names
The genus name 'Arabis' is Greek and literally means 'of or from Arabia'. Since none of the plants currently referred to by the name occur in that country, most authors of books on plant names skirt this embarrassing difficulty and refer to it as being 'of obscure derivation'. However, Chicheley Plowden (1972) suggests the connection might lie in the ability of the genus members to thrive in dry situations, which must be accepted as something of a truism! The Latin specific epithet 'hirsuta' translates as 'hairy' or 'rough-haired', the latter being appropriate in this case (Gledhill 1985).
The plant has not attracted any uses or folklore and the English common name 'Hairy Rock Cress' simply informs us that it is a hairy member of the cabbage family, that frequents dry, rocky sites.
Threats
None.