Helosciadium repens, commonly known as creeping marshwort, is a species of plant belonging to the Apiaceae family. It occurs in Western and Central Europe, being rare throughout its range. It grows in wetland areas where it does not have to compete with taller plants due to grazing by animals, periodic flooding during the winter-spring seasons, or mowing. It is considered a species of near-threatened status at the continental level, critically endangered and legally protected in Poland. It is subject to protection within the European Natura 2000 network.
Geographical distribution The creeping marshwort is a subatlantic species. It grows in Western Europe from the Iberian Peninsula to the British Isles and Denmark in the north, as well as Poland, Slovakia, Hungary, Croatia, and Montenegro in the east. Isolated occurrences outside its core range are found in Turkey, Morocco, and the Canary Islands. As a naturalized species, it has been reported from the Czech Republic, North America, and even from the Swartland region in the Western Cape province of South Africa.
Throughout its range, the species is rarely encountered, influenced by both its specific ecological requirements and its poor ability to spread over long distances. In the southern part of its range, it grows in mountainous areas (e.g., in Spain and Morocco, at altitudes above above sea level).
Morphology
Habit The naked, shiny perennial with creeping stolons typically reaches in terrestrial habitats, sometimes up to or , and when growing in water, even up to or in length. In the nodes, the stolons root and develop leaves, which are sometimes absent in the node from which the flower stalk emerges. There are no erect and leafy stems. The roots are filamentous. The stem is light green, smooth, and naked.
Leaves They grow from the nodes of the stolons, reaching up to , sometimes up to in length (in plants submerged in water, even up to 40 cm), but most often only up to 5 cm in length and 1.5 cm in width. The leaf sheaths are inconspicuous. The leaf blades are attached to long petioles and are individually, odd-pinnately compound. The leaflets, numbering 3–6 pairs, are sessile, ovate to rounded, with unevenly and coarsely toothed margins. They reach up to 1 cm in length. One of the notches between the teeth, especially on the third pair of leaflets from the bottom, is distinctly deeper, which is a significant diagnostic feature distinguishing this species from terrestrial forms of fool's watercress. The leaves are usually yellowish-green, lighter underneath, and naked.
Flowers Gathered in umbellules arising from the node of the creeping stem, the umbels, along with their peduncles, reach a height of , maximum up to , sometimes . The umbel consists of 2–7 umbellules, each supported by numerous, white-bordered, elliptic, and three-veined bracteoles. The pedicels of the umbellules are usually longer than the pedicels of the flowers, although they may be strongly shortened on heavily grazed pastures. At the base of the umbel peduncle, there are usually 3 to 7 quickly shedding, lanceolate bracts. The flowers are small, with white (sometimes cream or pinkish) corolla petals reaching about 0.5 mm in length, mostly with a pointed and inwardly bent tip. Sepals are absent. There are 5 stamens with white filaments and pale yellow anthers. The pistils are two, bent in two directions. The stigma is small and capitate. The pistils are seated on a stylopodium reaching half their length.
Fruits The spherical schizocarps have a length of 1–1.3 mm and a width of 1.1–1.4 mm (the schizocarp is slightly wider than it is long), with persistent styles bent at the apex. The size of the fruits is influenced by the plant's growth conditions – they are larger under optimal conditions. Individual schizocarps are bluntly 5-angled in cross-section with weakly marked ribs. The fruit mass is 0.27 ± 0.04 mg.
Similar species The creeping marshwort can be confused with common young specimens of the lesser water-parsnip (Berula erecta) and the great water-parsnip (Sium latifolium), which grow in similar habitats. However, the flowering stems of these species are erect and leafy, with the inflorescences almost sessile (in contrast, the stems of creeping marshwort are always prostrate, and the inflorescences are elevated on stalks several centimeters long). Very similar is the fool's watercress (Helosciadium nodiflorum), which, however, has a maximum of two bracts and shorter stalks of umbels compared to the stalks of umbellules. This species, in its terrestrial form, can coexist with creeping marshwort in meadows and can be almost indistinguishable to such an extent that genetic research is necessary to determine the taxonomic affiliation of the plants. Another similar species, , lacks bracts altogether, and its leaflets have wedge-shaped bases, not rounded as in creeping marshworts.
Biology
Development The creeping marshwort can be classified as a hemicryptophyte, helophyte, or hydrophyte depending on the habitat conditions. Among plants with a molecular cloning type, creeping marshwort is included in the Fragaria vesca group characterized by the predominance of spreading through stolons with relatively short individual ramet lifespans. Flowering occurs from June to October, with self-pollination playing a significant role. Fruits ripen from August to October.
In Central Europe, it is assumed that the species primarily reproduces vegetatively through stolons. On the other hand, studies on seed germination ecology suggest that, contrary to previous beliefs, at least in northern Germany, there are no significant problems or limitations for the species to spread via seeds. A complicating factor for generative reproduction may be excessive turf density and the lack of initial microsites free from vegetation.
The seeds have the ability to float in water for at least 50 days without losing their germination capacity, which is significant for their dispersal (hydrochory). They can be carried by water over long distances (in lake basins or along streams), but they can also be moved on a smaller scale within grassland communities during periodic floods, for example, after heavy rains. Outside of water, the fruits are transported by wind and animals, both attached to fur and hooves (epizoochory) and consumed (endozoochory).
Seeds partially germinate in the same season they mature, even those floating in water. The seeds form a persistent soil seed bank, retaining the ability to germinate when buried in the soil (even up to 70 years). The density of seeds on species' habitats is recorded at about a thousand seeds per square meter. Seed germination occurs under a wide range of abiotic conditions. In terms of temperature, germination is limited only by high temperatures, which rarely occur in habitats wet and humid enough for the species. It thrives in variable temperatures, although it is not necessary. Good light conditions are necessary for germination. Seeds germinate well on moist substrate, even when flooded for two weeks, which usually limits competition from other species (in places where water stagnates, seeds of most species enter a dormant phase).
Creeping marshwort populations exhibit large fluctuations in the number of individuals depending on prevailing water conditions or other interactions, such as grazing. In locations on pastures, plants may seemingly disappear during periods of intense cattle grazing, only to quickly recolonize gaps in the sward during breaks in grazing. Naked patches of soil exposed by the trampling of wet soil by cows' hooves are significant for maintaining creeping marshwort on pastures. Due to the length of the stolons, these plants utilize gaps in the sward up to 10–15 cm away from them. For creeping marshwort to persist on pastures, grazing frequency is important – too infrequent grazing results in strong development of vegetation competing with this species and, consequently, its decline.
Genetics The chromosome number is 2n=22. A low genetic diversity within populations was observed, particularly in the lowland (northern) part of Germany, while in the southern part of the country (Bavaria), the differentiation between populations was slightly higher.
Ecology
Habitat Creeping marshwort is a light-demanding pioneer species on wet, nitrogen-rich soil. It occurs along water edges, especially lakeshores, in communities of low vegetation flooded outside the growing season, and in gaps in reed beds. Another primary habitat is small depressions in the terrain, flooded in the spring and later drying out, in the Netherlands also interdune depressions on the seaside. In places flooded in winter and spring, the bare soil surface is exposed after the water level drops, enabling the development of light-demanding creeping marshwort. Grazing by animals also favors its growth by chewing up competitive vegetation. Secondary habitats are pastures, usually extensive (over 50 ha), where cattle are extensively grazed, sometimes also horses, as well as places trampled on the edges of waters, e.g., on beaches and boat launching sites, and within recreational areas, including those landscaped as lawns. However, excessive trampling and grazing by animals can eliminate this species.
These plants inhabit fertile, moderately fertile, and poor soils, rich in nitrogen compounds, neutral to slightly acidic or slightly alkaline. The mechanical and chemical composition of soils varies at different sites. A constant feature of habitats is the near-neutral soil pH and high humidity (although often with a significant drop in water level by the end of summer). The species' habitats are periodically flooded or inundated with water for a longer period. In particular, places flooded in winter are preferred. While these plants tolerate winter flooding well, prolonged submergence in the summer can lead to plant death (plants die when flooded for more than 56 days or repeatedly inundated for shorter periods). These plants can tolerate shorter and less frequent floods, losing their roots and floating on the water surface. However, creeping marshwort growing in water does not flower. Submergence of the species' habitats in the summer stimulates seed germination. However, these plants wither when watered with saltwater, although sometimes they grow on slightly saline coastal alluvia.
Due to the pioneer character of the species, a constant feature of its occupied sites is the high frequency and intensity of interactions leading to the limited development of competitive vegetation. The habitat characteristics conditioning the development of creeping marshwort include low vegetation height (optimally not exceeding 20 cm) or at least the presence of gaps (e.g., in reed beds), lack of shading caused by trees and shrubs, and absence of competitive expansive species (strongly growing ones).
The plants tolerate frosts down to -20°C.
Phytosociology and interspecies interactions
Creeping marshwort grows in Central Europe in communities of floodplain meadows from the Lolio-Potentillion anserinae association (Agropyro-Rumicion crispi), usually species-poor, as well as in richer communities of Triglochino-Agrostietum stoloniferae and Ranunculo-Alopecuretum geniculati. The most common accompanying species of creeping marshwort (growing with them on over 50% of sites in northern Germany) include: creeping bentgrass (Agrostis stolonifera), creeping buttercup (Ranunculus repens), silverweed (Potentilla anserina), water mint (Mentha aquatica), jointleaf rush (Juncus articulatus), hairy sedge (Carex hirta), white clover (Trifolium repens), common marsh bedstraw (Galium palustre), and Kentucky bluegrass (Poa pratensis).