Atriplex prostrata Boucher ex DC., Spear-leaved Orache

Account Summary

Growth form, recognition and preferred habitats

Despite its botanical name, A. prostrata is an erect or a near-prostrate (procumbent), much-branched summer annual with large, basal, triangular or hastate (ie lobed, spear-shaped) leaves, upper ones covered with a mealy (farinose) tomentum of hairs beneath (only). The species is distinguished from the very much more common A. patula (Common Orache), by having paired green or membranous ovate or triangular bracteoles around the female flower, which are united only near their base as opposed to about half way. The prostrate form of the plant is also much more flattened to the ground than A. patula, and the leaves are not in the least tapered at the base, but rather, they are very definitely truncate (ie cut off abruptly at the base) (Taschereau 1985a; Webb et al. 1996).

Like A. patula, this leafy annual occupies open, disturbed, damp to moist, unshaded, fertile or nutrient-rich substrates (silt, sand or shingle) of near-neutral soil reaction, either at the coast, or inland in cultivated ground, trampled, muddy margins of ditches, lakes and ponds, or in waste places or on rubbish tips (S.J. Leach, in: Preston et al. 2002). The inland, ruderal, anthropogenic biotype is often accompanied by A. patula. In common with other ruderal species, the species favours disturbed, fertile, productive ground with plenty of patches of bare soil. A. prostrata does not tolerate strong competition from its neighbours and it therefore tends to occur as a pioneer colonist of open, moist, unshaded sites, where growth of perennials is delayed or prevented by instability or disturbance. In freshly disturbed soil, it is often only transient, colonising quickly and soon disappearing. Having said this, plants of A. prostrata are also quickly ousted by grazing or trampling animals (Grime et al. 2007).

A. prostrata has a particularly prostrate salt-tolerant form that is more or less confined to the upper reaches of coastal salt marshes. Very probably this halophyte is the original form of the species, although in recent decades it has evolved and colonised a wider range of inland habitats in Britain and N Ireland, following the routes of trunk roads salted to reduce accidents in icy conditions. The majority of roads in the RoI are not salted or de-iced in this way. This type of species migration has not yet been observed in Fermanagh, although one very noticeable salt-tolerant species, Cochleria danica (Danish Scurvygrass ), has been observed advancing inland from Belfast along the M1 motorway towards the county.

Variation

Like other members of the family Chenopodiaceae, most characters of A. prostrata are very variable and plastic in response to its local environment. The A. prostrata group comprises a number of partially inter-fertile and morphologically similar taxa found on the coasts of W Europe and elsewhere. In Britain and Ireland, it is represented by four species, of which A. prostrata is the most widespread, and the only one to occur inland from the coast. The other three species in the group are A. glabriuscula Edmondson (Babington’s Orache), A. longipes Drejer (Long-stalked Orache) and A. praecox Hülph. (Early Orache). In the A. prostrata group, leaf size and outline shape show extreme variation from the base to the apex of the plants. The most constant and characteristic leaves to examine are those on the central axis (main stem) in the middle portion of the plant, half way between the base and the terminal inflorescence. These are referred to as the 'lower leaves' or the 'lower principal leaves', and they often drop off before the bracteoles and seed are fully mature (Taschereau 1977).

Reproduction

A. prostrata reproduces entirely by seed. The seeds are of two types; larger brown seed 1.5-3.0 mm wide and smaller, and in ruderal habitats, more frequent black seed 1.0-2.5 mm. The brown seed are non- or less-dormant than their small, black, shiny counterparts, while the latter are dormant and long-persistent in the soil seed bank. Seed, of whatever colour germinates in the spring, and the plants flower from July to September, or until the first frost. The flowers are unisexual, but both sexes are borne separately on the same plant (ie the plant is monoecious). Normally this means the upper flowers on the inflorescence are male and female flowers are arranged towards the base of the spike-like flowering stem. The number of flowers produced will depend on growing conditions in the particular environment, but typically there are over 100 flowers per inflorescence. The flowers are wind-pollinated, but they do also attract insect visitors (syrphid flies) which feed on the pollen and may assist in its transfer to the female stigma. Like other species in the genus, the flowers are described as facultatively autogamous, meaning they can self-fertilize if other means of pollination fail to occur. However, the male and female flowers are clustered so closely together in tight glomerules, and they mature and open almost simultaneously, that this greatly reduces the possibility of cross-pollination (Taschereau 1985a).

Seed dispersal and survival

The seed is about 1.5 mm wide, and when dispersed is enclosed within two 5 mm persistent bracteoles (ie small, modified leaves). Since these attachments increase the surface area of the fruit propagule, they must assist its wind-dispersal. They probably also make the fruit more edible, and thus encourage birds and mammals to ingest and transport them internally. Ridley (1930, p. 361) includes A. prostrata (as A. hastata L.) amongst a selected list of plant species carried internally by domestic animals in Sweden and subsequently deposited with excreta in a viable condition. The seed is long-persistent in the soil seed bank, a proportion surviving for at least five years (Thompson et al. 1997).

Fermanagh occurrence

Recorded as A. hastata L., it was first found by Praeger in Fermanagh during 1934, but was not recorded again until the summer of 1986 by members of the EHS Habitat Survey Team investigating the conservation potential of Upper Lough Erne shores. Known chiefly as a coastal plant and considered rare inland, I personally have never seen this annual during Fermanagh field work. It may possibly have been overlooked to some extent by myself and others, though we are not aware of doing so.

There are just four Fermanagh records, confined to two tetrads in the Upper Lough Erne basin. Apart from the first record given above, the other record details are: W shore of Corradillar Td, 6 August 1986, L.W. Austin & S.A. Wolfe-Murphy; Sand Lough, Killyclowny Td, 14 August 1986, L.W. Austin & S.A. Wolfe-Murphy; Clonmin Lough, Corsale Td, 19 August 1986, S.A. Wolfe-Murphy.

Hybrids

There are three Atriplex hybrids involving A. prostrata, two of which are occasional or rare at the coast. The most frequent hybrid is A. prostrata × A. longipes Drejer = Atriplex × gustafssoniana Tascher., which regularly occurs where the two species overlap in distribution in both coastal and inland sites. Hybrid intermediates derived from this combination are also found in sites remote from both parents, particularly on exposed coastal beaches in the far north of Scotland and more occasionally in Shetland. A. longipes differs from A. prostrata most clearly in that its bracteoles are enlarged and foliaceous (ie green, leaf-like), up to 25 mm and with stalks up to 25 (or, exceptionally, 30) mm, whereas in the latter they do not exceed 6(-8) mm and they are stalkless (ie sessile). There is a range of variation within the hybrid forms, however, making determination rather difficult. Having said this, A. × gustafssoniana is a particularly frequent hybrid, and it must still be under-recorded (Stace et al. 2015).

Threats

None.

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