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Rorippa amphibia (L.) Besser, Greater Yellow-cress

Account Summary

Native, common. Eurasian temperate, but widely naturalised.

1806; Scott, Prof. R.; Upper Lough Erne.

May to October.

Growth form and preferred habitats

Individuals of this rather large, stout but straggling, clump-forming, stoloniferous perennial show a degree of variation in leaf shape that approaches heterophylly, but without there being sharp limits between the leaf types. Early in the growing season, the plant has submerged pinnatifid or deeply pinnately-cut basal leaves, which tend not to persist, but its aerial leaves are simple, quite broad and either shallowly lobed or distinctly serrately toothed. In Irish material of this species, the leaf margins have many small, deeply incised but irregular teeth (see Jonsell 1968, p. 105, Figs 15, H & I). The curiously dissected leaf-shape of Irish specimens of R. amphibia might represent a character introgressed from R. sylvestris (Creeping Yellow-cress) through hybridisation, plus several (perhaps numerous) generations of subsequent back-crossing.

The leaves of Great Yellow-cress are also of a distinctive bright green or yellowish colour and, when flowering, the 3–6 mm yellow petals are about twice the length of the sepals (Clapham et al. 1962; Stace 1997).

R. amphibia prefers fertile growing conditions and, as its botanical name suggests, it tolerates fluctuating water levels, often growing on wet ground that only dries out at the surface for short periods in summer, if at all. It frequents tall, emergent swamp and fen margin vegetation on sheltered lakeshores and riverbanks. Along slow moving rivers and streams, it grows where the emergent waterside vegetation provides sufficient shelter to allow at least temporary anchorage and maintenance of shallow-rooted species like R. amphibia (Haslam 1978; Holmes 1983).

The species also copes well with semi-shaded conditions and thus is frequently associated with tall-herb fringing vegetation in eutrophic and generally calcareous waters. It also occurs on muddy clay or silted soils of similar chemistry, which again are always subject to at least winter-flooding conditions (Preston & Croft 1997).

Fermanagh occurrence

In Fermanagh, R. amphibia has been commonly recorded from 104 tetrads, 19.7% of those in the VC. It is locally abundant amongst tall, emergent swamp and fen vegetation on sheltered lakeshore and riverbank sites in the county. The fluctuating levels and decidedly eutrophic, calcium-rich waters and muddy, clayey, sandy or silted shingle shores of the more sheltered, shallow inlets of Upper Lough Erne, provide eminently suitable growing conditions for R. amphibia, as they also do for several other Rorippa species.

However, R. amphibia also occurs in a number of other situations scattered throughout the VC, including the lake shore and margins of wider ditches around Lower Lough Macnean. It also occurs in the somewhat taller, wetter, 'floodgrass' vegetation around the turloughs (ie vanishing limestone lakes) at Roosky and near Fardrum, plus beside calcareous marl lakes and ponds along the River Finn (known as the Finn Floods). Other populations occurs along the lower stretches of the Silles and Swanlinbar rivers where the water flow is slow and the substrate is sufficiently deep and stable to support emergent vegetation amongst generally taller, more firmly anchored aquatic plants such as Sparganium erectum (Branched Bur-reed), Schoenoplectus lacustris (Common Club-rush) or Glyceria maxima (Reed Sweet-grass).

Flowering reproduction

R. amphibia flowers in its second season of growth, from June to September, and both vegetative growth and fruiting reaching their maximum development in late summer. Like R. sylvestris, the flowers of R. amphibia show a high degree of self-incompatibility. Seed set is, therefore, variable; it is often good, but sometimes rather poor despite the conspicuous flowers, the clumped growth of the plant and the consequent plentiful pollination by small bees and flies of various sorts (Jonsell 1968; Rich 1991).

Vegetative reproduction

In addition to seed production, or to compensate if the sexual process should fail, the plant also regularly reproduces vegetatively by means of axillary rosettes, which are formed later in the season on lateral stolons at the base of the aerial stems. These vegetative plantlets either enable the formation of clonal clumps, or else they assist seed dispersal in transporting this semi-aquatic species further afield, through their breaking off from the parent plant and subsequent floatation (Preston & Croft 1997; D.A. Pearman, in: Preston et al. 2002).

Genetic variation

It has been known for almost 40 years that R. amphibia consists of two cytotypes, a rare diploid, known in B & I only from the Thames valley, and a very much more widespread tetraploid elsewhere in these islands. The tetraploid form is also found throughout most of temperate Europe (Jonsell 1968; Jalas & Suominen 1994, Map 2318). A very powerful sterility barrier exists between these two genetic forms, such that no triploids have ever been found (Preston & Croft 1997). Although there are minor morphological differences between the two cytotypes, they are regarded as too similar to merit taxonomic differentiation (Jonsell 1968; Rich 1991).

Hybrids

While the species remains on the whole morphologically uniform (Jonsell 1968, p. 116), the variation that exists may well be the result of hybridization. R. amphibia is known to form hybrids with three other water-cress species, R. sylvestris (Creeping Yellow-cress), R. palustris (Marsh Yellow-cress) and R. austriaca (Austrian Yellow-cress), but only the first of these is at all widespread in B & I, and its hybrid with R. amphibia (R. × anceps (Wahlemb.) Rchb.) has just a few Fermanagh records, although here, as elsewhere in B & I, it is very much overlooked.

Jonsell (1968) raised the possibility that the curious dissected leaf-shape of Irish specimens of R. amphibia, might represent a character intergraded from R. sylvestris through hybridization and subsequent back-crossing.

Irish occurrence

R. amphibia is better recorded in the New Atlas than in the earlier BSBI Atlas (Walters & Perring 1962). This is especially so in Ireland, where it is chiefly found scattered around Lough Neagh and Lough Erne, with a similar presence in the Irish Midlands along the Shannon basin, but with only a sparse sprinkling of stations elsewhere.

British occurrence

In Britain, R. amphibia is common in C and SE England, but absent or a rare introduction north of a line between Liverpool and Hull. It also appears to have retreated in recent years from some previous sites in the English West Country and in Wales (Preston et al. 2002).

European and world occurrence

The wider world distribution shows the species native from temperate W Europe and N Africa, to E Asia. In W Europe, it becomes rare and scattered southwards in Spain and Portugal and towards the Mediterranean and is absent from many Mediterranean islands (Jalas & Suominen 1994, Map 2318). R. amphibia is also widely introduced and naturalised in areas including eastern N America and Australasia, for example, North Island (New Zealand), where it was first recorded in 1870 (Webb et al. 1988). In most cases, these introductions are undoubtedly dispersed by human agencies, as for example in the St Lawrence River in Canada, where the species was carried long distances with ships (Marie-Victorin 1930). Jonsell (1968, p. 111), concluded that R. amphibia in C Sweden shows a distribution whose borders are probably not determined by environmental factors, but rather by historical causes, including past patterns of boat traffic.

Threats

None.