Epilobium parviflorum × E. palustre
(E. × rivulare Wahlenb.), a hybrid willowherb
Account Summary
Very rare, but probably overlooked and under-recorded.
1947; MCM & D; shore of Annashanco Lough, Annaghmartin Td.
This hybrid, for which the above is the solitary Fermanagh record, is almost certainly under-recorded as both parents are widespread and common species.
As expected, the hybrid is intermediate in many of its characters, the stigma being either of a confused degree of lobing, or thick and entire. Middle and upper stem hairs are most like E. parviflorum, but are shorter and somewhat crisped. The pubescent leaves are sessile, linear-lanceolate and their margins are ± revolute like those of E. palustre, with the marginal teeth tucked underneath. Filiform stolons may be produced, like in E. palustre, but they are more leafy in the hybrid than in the species (Stace et al. 2015).
The hybrid is recorded from wet marginal and disturbed habitats including semi-open areas in lowland marshes, fens and around ponds and only rarely does it appear on waste ground habitats (Stace et al. 2015). In his Flora of Cardiganshire, Chater (2010) described E. × rivulare as frequent there in tall herb fens and bogs, usually growing amongst Juncus acutiflorus (Sharp-flowered Rush) or Molinia caerulea (Purple Moor-grass) communities, where it sometimes was more abundant than E. palustre and often occurred in the immediate absence of E. parviflorum. The hybrid frequently resembled tall, much-branched E. palustre (Chater 2010).
E. × rivulare is mostly sterile, although some wild material from Britain shows more seed-set than found in experimental hybrids, and it may represent the F2 generation or backcrosses. Whenever the hybrid is found in quantity, some of the plant spread and abundance may result from stoloniferous growth, rather than seed production (Stace et al. 2015).
The only three other Irish VCs with records of this hybrid in the Cen Cat Fl Ir 2 are Mid-Cork (H4), Antrim (H39) and Londonderry (H40). The latter two NI records are also Praeger finds dating from 1937 and 1935 respectively, and the FNEI 3, which details them, adds another solitary record for Co Down (H38), made by the late Miss Doreen Lambert near Portaferry in 1971.
Stace (1997) states that E. × rivulare occurs scattered in Britain and NI, but it is not mapped in the New Atlas. The hectad map of this hybrid in the Hybrid Flora of the British Isles plots only two records from Ireland, one from Fermanagh and the other from Mid-Cork (H4). The other three records from NI listed here are not represented. The map also indicates that E. × rivulare is rare and very thinly scattered in Britain (a total of 32 hectads), with a concentration of records in S Wales (VCs 44-46). The distribution stretches northwards, reaching coastal NW Scotland at Westerness (VC 97) (Stace et al. 2015). The hybrid is described as widespread in temperate Europe (Stace et al. 2015).
Growth form and preferred habitats
E. parviflorum is one of four erect Willowherb perennial species with four-lobed stigmas present in the flora of B & I. The combination of dense, soft, short, spreading hairs on the stems, plus leaves softly felted on both surfaces, produces the usual 'hoary' appearance of this distinctive perennial (Clapham et al. 1987). Unfortunately, the hairs are not always present in quantity and a gradation of forms exist which approach hairlessness (Kitchener 1992a). The latter near-glabrous condition is very rarely found, however, and generally the appearance very well fits the English common name. This is a smaller plant (up to 60 cm) and usually it is more narrowly upright than E. hirsutum (Great Willowherb). Its four stigma lobes in the smaller purplish rose-pink flowers are variably compact and never curved back upon themselves as they do in the larger species (Kitchener 1992a). The leaves are very variable but are usually 3-7 × 1-1.5 cm, the lower ones opposite and those above the mid-stem alternate. The leaf shape is oblong-lanceolate and the base rounded, but neither clasping the stem, nor decurrent at the sessile base (Clapham et al. 1987).
E. parviflorum is occasional to common in damp, marshy or swampy tall-herb fen conditions over a variety of lowland waterside habitats. Unusually and rather surprisingly, E. parviflorum also frequents very much drier, disturbed ground, as a pioneer colonising ruderal species (see below). Its established strategy is therefore categorised by Grime et al. (1988, 2007) as C-S-R, meaning it is intermediate between a Competitor, Stress-tolerant and Ruderal species, and displaying features of each ecological approach to survival.
Fermanagh occurrence
In Fermanagh, Hoary Willowherb most often occurs on damp, calcium-enriched soils, including organic fen peats around Upper Lough Erne and the SE part of Lower Lough Erne. In these wetland situations, it is especially frequent where there is a recent history of soil disturbance, such as light trampling by cattle, ie animals, however heavy in weight, paying visits of brief duration, thus providing sporadic, occasional grazing pressure and manuring that keeps the ground ecologically open and primed for colonisation.
Hoary Willowherb is seldom or rarely recorded in unproductive soils more acidic than around pH 5.5 (Grime et al. 1988, 2007). Interestingly, E. parviflorum, like E. hirsutum (Great Willowherb), also occurs widely, but less frequently and abundantly as a pioneer colonist of dry or well-drained disturbed ground (Kitchener 1992a). Often, but not always, this involves stony or gravelly soils, including limestone talus slopes (eg below Carrickbeg cliffs), old quarries (it is present in at least eight of these in Fermanagh), alongside gravel or sandy track-sides and in waste ground, including the margins of car parks and other similar, moderately to slightly disturbed urban sites. The ruderal population remains associated, albeit to a minor degree, with the old disused railway lines and derelict station areas which were abandoned in Fermanagh in 1942 and 1957.
Taking these two very different types of habitat moisture populations together, E. parviflorum is found in 192 Fermanagh tetrads, 36.4% of those in the VC. The fact that it belongs to a genus equipped with plumed, air-borne seed undoubtedly assists E. parviflorum in behaving both as a widespread ruderal colonist of disturbed ground, while holding on to its much larger, probably original, wetland territory, by means of longer-lived, repeatedly reproducing, species-sustaining populations.
Flowering reproduction
Overwinter perennation is by vegetative buds and short, leafy stolons and, of course, by seed. The leafy buds on stolons are produced in autumn and winter, but they are generally formed near the base of the plant and essentially they maintain existing individual plants and are neither an efficient means of vegetative spread nor of species reproduction.
Flowers are produced in July and August and are borne in a ± corymbose terminal raceme. They are 6-13 mm in diameter (parviflorum means small-flowered), of a pale purplish-rose colour (rarely white) with deeply notched petals; the style is white, tinged mauve, club-shaped when young but the stigma opens out into four fairly upright lobes. The style and stigma together about equal the length of the stamens (Sell & Murrell 2009). The ovary, 1.2-4.0 cm long, is inferior and resembles a thickened flower stalk. The flowers are homogamous (the two sexes maturing simultaneously) and are occasionally visited by hive bees and other Hemiptera, although they often self-pollinate (Clapham et al. 1987; Fitter 1987). The fruit capsule is elongated, 30-70 mm, splitting into four narrow recurved valves, releasing numerous tiny seeds each with a tuft of silky hairs attached at the top that enables ready wind dispersal (Hutchinson 1972; Clapham et al. 1987).
Persistence of dormant seed in the soil greatly facilitates the rapid, opportunistic occupation of vegetation gaps created by occasional disturbance. However, the survey of NW European soil seed banks found that five of the six records that exist for E. parviflorum, described the seed as merely transitory, while the other record suggested short-term persistence only (Thompson et al. 1997). In this context, the persistence of the small populations on long-abandoned railway land is very interesting, indicating the success the species is somehow experiencing in this portion of its habitat range, given our decidedly damp, humid, often overcast weather conditions where drought conditions are rare.
Obviously other environmental factors assist survival by minimising competition from potentially dominant species that might otherwise oust E. parviflorum. It is not known for certain what these ecological conditions might be although low soil fertility and an intermediate level of vegetation disturbance are probably involved (Grime et al. 1988). Further study of this topic is definitely required.
Hybridization
Thus while it regularly consorts with its larger cousin, E. hirsutum (which unlike the majority of willowherbs, very rarely hybridizes with its relatives), when it is operating as a ruderal, E. parviflorum frequently overlaps and shares ground with another perennial member of the genus, E. montanum (Broad-leaved Willowherb), although it manages to avoid much contact with E. obscurum (Short-fruited Willowherb), again due to their differing soil pH preferences. Hybridization with E. montanum can and does occur at least occasionally, but to date this is recorded only rarely in B & I.
Fermanagh has one of the four Irish records of this hybrid displayed in the New Atlas map. RHN and the current author (RSF) tend to agree with G.D. Kitchener (in: Preston et al. 2002), that plant recorders must all be overlooking this hybrid to some unknown extent.
British and Irish occurrence
E. parviflorum is common and widespread throughout most of lowland England, Wales and Ireland, but becomes scarce and local in Scotland, apart from the urban Central Belt and coastal areas. However, the species does reach as far north as Orkney, but is absent from Shetland (New Atlas).
European and world occurrence
E. parviflorum belongs to the European temperate biogeographic element and is widespread throughout Europe northwards to S. Sweden. It is also present in N Africa, W Asia to India and in the Atlantic Islands (Sell & Murrell 2009). It is probably only casual in C Fennoscandia and other outlying localities in both Europe and Asia (Hultén & Fries 1986, Map 1358). It is introduced to New Zealand, but was not collected there until 1967 and is thus probably a recent arrival. It may now be widespread in moist, fertile farming country around Hawke's Bay and Poverty Bay on North Island (Webb et al. 1988).
Threats
None.