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Rorippa sylvestris (L.) Besser, Creeping Yellow-cress

Account Summary

Native, common but rather local. European temperate, but also widely naturalised.

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

April to November.

Growth form and preferred habitats

R. sylvestris is a yellow-flowered perennial species, 15-60 cm tall, that generally develops thick white tap-roots and storage roots from a basal 'crown' immediately below the rosette leaves. Sometimes it also produces additional very slender, horizontal, creeping roots which are branched off from the vertical tap-root and help the plant to spread and form clonal patches. Typically, plants of R. sylvestris also have rather weak stems, which appear to 'lodge' or become decumbent under the weight of the inflorescence, particularly when growing in loose, disturbed or moist, muddy soils. These more or less prostrate stems generally bear a sparse smattering of short adventitious roots, even on the flowering portion of the stem but, nevertheless, they could hardly be accurately described as 'rhizomatous', as they sometimes are (eg Webb et al. 1988; Rich 1991).

This often sprawling perennial occupies very much the same range of damp ground, waterside open habitats as R. palustris (Marsh Yellow-cress), but it is sometimes found growing amongst fairly dense, obviously shading, shoreline vegetation. However, it does also appear as a pioneer weedy colonist of drier, more open ground in villages, harbours and along waysides. All Rorippa species tend to disappear when the open vegetation they originally colonised becomes closed, since the competition they subsequently experience becomes too strong for them to survive (Jonsell 1968). The need for more open conditions explains why Rorippa species appear to favour ± disturbed situations, and thus they are often very closely associated with human activity, even sometimes to the extent of their becoming completely dependent weeds. This colonist behaviour limits both the type of cultivated or disturbed habitats they occupy, and their means of seed dispersal. Creeping Yellow-cress regularly occurs along with R. palustris, but generally it occupies somewhat drier, somewhat less open bare ground than the latter typically frequents (Jonsell 1968).

Identification

As with R. palustris, the flowers of R. sylvestris are small and yellow, yet with petals about twice as long as the sepals. The fruits are longer than those of other Rorippa species that have regularly divided (ie pinnate) leaves, and taken together these features enable R. sylvestris to be readily distinguished from other yellow crucifers. Considerable morphological variation within the species has frequently been observed and led to the description of infraspecific taxa, none of which, however, are based on NW European material

Jonsell (1968). Much of the variation described, particularly in C Europe, is obviously due to hybridization, above all with R. amphibia and R. austriaca (Jonsell 1968).

Variation

Jonsell (1968) found there is considerable variation in leaf shape, plant habit and fruit shape, associated with polyploidy within the species (tetraploid, hexaploid and hybrid pentaploid forms occur). The pentaploid forms are particularly vigorous, aggressive weeds, but they are very rare (Rich 1991). From his study of pollen types and variation in vegetative characters, Jonsell (1968) also showed that both tetraploids and hexaploids are composed of numerous biotypes. Oddly, perhaps, while the tetraploid is the most common form in Europe (68% tetraploid versus 20% hexaploid), the hexaploid is most common in N America (65% hexaploid versus 29% tetraploid). Pentaploid hybrids are much more common in the United States (21%) than in either Canada (10%) or Europe (3%), yet even so, the usual situation of aborting siliques in nature indicates that at most sites there is just a single chromosome race propagating vegetatively (Mulligan & Munro 1984).

Fermanagh occurrence

This is another Rorippa species that is very much concentrated in the Lough Erne basin and around the Upper Lough in particular. It is most frequently found in wet or damp ground by lakeshores, often near points of shoreline access such as quays, jetties and bridges or as a weed of drier ground in villages and along waysides. R. sylvestris is common in these types of habitat and has been recorded in 78 tetrads, 14.8% of those in the VC.

Flowering reproduction

Unlike R. palustris, R. sylvestris is known to be self-incompatible, but it is variably so (Jonsell 1968). Three chromosome races in R. sylvestris have been reported in Europe, but beyond Scandinavia where hexaploids are the norm and pentaploids very rare, tetraploid forms dominate. Most R. sylvestris clones have reduced pollen fertility, but this is more the case in hexaploid than in tetraploid clones, and only among the latter are there clones of good quality pollen. Jonsell regarded this tendency to be expected in plants derived by a process approaching autopolyploidy, which would be valid for the hexaploids.

Curran (1984) reported that most studies of R. sylvestris reproductive ability indicate it achieves only a small proportion of its seed production potential. It appears to be the case that populations or colonies very often consist of one genotype only, and thus seed set tends to be severely limited in such circumstances (Webb et al. 1988; Rich 1991). The small Irish population Curran studied illustrates this effect very well, with less than 10% of fruits setting any seed, and many of the fertile fruits having only small seed numbers (ie between one and eight seeds present in them) (Curran 1984).

The sexual reproductive ability of the populations around both parts of Lough Erne has not been examined in this respect, but the large number of records and the concentration of them particularly around the Upper Lough, suggests that reproduction, by vegetative means or seed or both, is perfectly healthy, and is more than sufficient to support the species.

British and Irish occurrence

The New Atlas hectad map clearly shows that R. sylvestris is less frequent than R. palustris throughout these islands, yet it is widespread in lowland Britain, becoming rarer further north in Scotland and the Western Isles. In Ireland, it is very much more scarce and scattered than R. palustris. The distribution of R. sylvestris in semi-natural habitats is probably stable, but it has become more widespread as a weed in B & I since the 1950s.

The index of species change in the 40 years between the two BSBI Atlases is + 0.73, a figure which reflects the fact that R. sylvestris is now recorded in many more hectad squares than was the case in the earlier BSBI Atlas (Walters & Perring 1962).

European and world occurrence

Beyond B & I, R. sylvestris is regarded as native and widespread in temperate latitudes of W & C Europe, but it becomes scarce or absent in the Mediterranean basin, and it is only of introduced status in most of Scandinavia, having originally arrived as ship ballast in many harbours (Jonsell 1968; Jalas & Suominen 1994, Map 2319). Rich (1991) lists R. sylvestris as native also in W Asia and N Africa and it is widely introduced elsewhere in the world, including in NE USA where it was first recorded in 1818, SE Canada in 1894 (Mulligan & Munro 1984), and New Zealand in 1952 (Webb et al. 1988). R. sylvestris rarely produces seed in N America, and it appears therefore to spread there along streams, rivers, ditches, railroads and highways almost entirely by vegetative reproduction, assisted by transport of garden and nursery stock (Mulligan & Munro 1984).

Names

The Latin specific epithet 'sylvestris' in botanical Latin always simply means 'wild', rather than suggesting any woodland connotation (silva = woodland) (Gilbert-Carter 1964). The plant has no medicinal or other uses and therefore has not attracted any folk names.

Threats

None.