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Ranunculus bulbosus L., Bulbous Buttercup

Account Summary

Native, frequent. European southern-temperate, also naturalised in N America and New Zealand.

1882; Stewart, S.A.; Knockmore Hill.

April to September.

Growth form and habitat preferences

Early in the growth season, when plants of this perennial are not yet in flower, the lower leaves of each individual's basal rosette lying flat to the ground are a distinctive identification feature of Bulbous Buttercup and, in addition, the corm-like, swollen stem-base can often be felt underneath the leaves. The lowest leaves being close-pressed to the ground (a feature described by Harper (1957, p. 332) as, 'strongly epinastic' − an interesting term), appears to confer a distinct competitive advantage over other plants in the sward, since it allows the R. bulbous individual to form a 5-7 cm saucer-shaped hollow in the turf, from which it manages to exclude other species. When Bulbous Buttercup is in flower, the down-turned (ie reflexed) sepals are very obvious and they provide another ready means of recognition.

While Bulbous Buttercup is a characteristic perennial of Fermanagh's limestone pastures, it is not a strict calcicole, ie it may prefer and occur most abundantly on well-drained, lime-rich soils, but it is not entirely confined to them. Rather, the species also occurs on dry roadside banks and verges as well as on well-drained unimproved meadows and pastures, including some over more neutral to moderately acidic soils (ie around pH 5.0 and above). This can also involve the shallow peaty soils that form over leached Carboniferous limestone in Fermanagh and other areas in western Ireland.

Essentially a lowland plant of open, full-sun situations, R. bulbosus can withstand moderate amounts of disturbance such as trampling and since it contains protoanemonin and is unpalatable, stock naturally avoid grazing it (Cooper & Johnston 1998). Severe trampling and the associated degree of soil disturbance and compaction will, however, obliterate the plant and its complete absence from frequently used pathways in grassland is often very obvious. Bulbous Buttercup also avoids strongly acid, wet, shaded or indeed overly fertile, productive conditions. In the latter circumstance, taller and more vigorous plants out compete it (Harper 1957; Grime et al. 1988).

Fermanagh occurrence

R. bulbosus is much less common in Fermanagh than either R. acris (Meadow Buttercup) or R. repens (Creeping Buttercup) and has only been recorded in 66 tetrads, 12.5% of those in the VC. Nine of these tetrads contain only pre-1976 records, indicating a loss of suitable habitat which is easily identified with agricultural improvements made to lowland limestone grasslands in recent decades. As the distribution map shows, Bulbous Buttercup is widely scattered across Fermanagh, but it is definitely most frequent in the Monawilkin, Knockmore and Marlbank limestone areas.

Flowering

R. bulbosus flowers earlier than its closest buttercup relatives, R. acris and R. repens, the peak of its flowering normally occurring in mid- to late-May. Fruit has normally ripened by the end of June and the aerial parts may brown and die off soon afterwards, so that the summer-dormant plant survives as the corm just below the soil surface − a drought-avoiding mechanism not very necessary in the wet climate of western Ireland! The species can thus be very inconspicuous for a few weeks in mid-summer, particularly if we have a dry spell of weather. However, in our damp, mild climate, with rainfall typically occurring regularly throughout the summer, Bulbous Buttercup plants produce fresh basal leaves soon after flowering (ie by the end of July or in early August), and they maintain their growth until winter cold eventually stops them in October or November. The leaf rosette is maintained throughout the winter and it recommences vegetative growth early in the year once temperatures begin to rise. Although the corm-like stem base and all the other parts of the plant are renewed annually (Harper 1957, p. 333), this method of perennation means that the plant tissues never become old and senescent and thus established individuals of R. bulbosus can be exceedingly long-lived and, indeed, in stable habitats they may persist indefinitely. There is nothing against saying that, in certain circumstances, some individuals could be thousands of years old!

As with other common buttercup species, R. bulbosus shows considerable variation in form in relation to prevailing environmental conditions (ie phenotypic plasticity), especially with respect to the size of corms and leaves and the number of flowering stems per corm (Harper 1957; Coles 1973). The typical plant has a corm 1.5 cm in diameter which produces just one flowering stem, but larger corms can carry up to eight or more flower stems and Harper (1957) reports a record breaking plant bearing 42!

The flowers open daily for about 4-7 days and are visited for both pollen and nectar by honey bees and many other short-tongued insects. Cross-pollination is the norm since a high degree of self-incompatibility exists, but while some evidence of selfing and a low degree of agamospermy, ie seed formation without fertilization, had been reported in the past (Harper 1957). However, Coles (1973) concluded after he had carried out greenhouse tests that R. bulbosus is totally self-incompatible, cross-pollinated and sexual, ie the flowers are fully outcrossing (xenogamous), and the breeding system is ‘panmictic’ (ie it involves random matings)(Richards 1997, page 6). Each flower produces between 20 and 30 achenes or seed (ie the achenes are single-seeded dry fruits).

Variation in seed production

The scale of seed production, or the reproductive capacity of the species, is another characteristic that varies enormously with the environment and particularly with the competitive situation of the plant. It is hard to measure or summarise reproductive capacity, but Salisbury (1942) provided experimental evidence by comparing the growth and productivity of plants in a meadow under three regimes: no competitors, and slight and severe levels of plant competition. As Harper (1957) pointed out, the competitive measurements Salisbury made were carried out without any experimental control, yet they still give a clear impression of the overall scale of the effect of competition on the reproductive capacity of the species. In the case of R. bulbosus, the sample under severe competition produced 69 fertile carpels per plant, while that with no competitors produced 687, a tenfold difference in sexual productivity.

Seed dispersal

The ripe seed drop off the receptacle and there is no specialised means of dispersal. Internal transfer in the gut of animals through being eaten by birds or stock animals, plus external carriage in mud by animals, including man and his vehicles, are probably significant. Wind and rain-wash may also play some role in dispersal of R. bulbosus achenes (Harper 1957). Other bird species, such as Pigeons, are known to be responsible for a considerable level of seed predation, and Voles and Field mice also make depredations.

Phenology

Bulbous Buttercup germinates mainly in the late summer and early autumn on bare ground in gaps created in turf by disturbance such as trampling and overgrazing (Sarukhan & Harper 1973; Sarukhan 1974, 1976). In Britain, molehills may be significant in this respect, providing fresh, bare soil, but fortunately there are no moles in Ireland. Although tremendous rates of seedling loss are involved, effective establishment from seed is the only reproductive mechanism that Bulbous Buttercup displays, corm division being extremely rare.

Seed survival in the soil seed bank

Buried seed longevity frequently appears to be brief: eight out of 15 studies in a major survey in NW Europe which gave survival estimates in years, indicated that the seed bank of R. bulbosus in soil is transient (surviving less than 1 year), and four indicated that it is short-term only (surviving more than 1 year but less than 5) (Thompson et al. 1997). The remaining three studies gave estimates of R. bulbosus seed longevity that ranged from 5 years to over 30! Clearly there is considerable variation in estimates of seed longevity, but the predominant mode remains transient to short-term only.

Fossil record

The fossil record of R. bulbosus is more slight than that of other species in the genus, but the pattern is similar through numerous glacial and interglacial stages and thus provides conclusive proof that the species is native in Britain and Ireland (Godwin 1975).

British and Irish occurrence

R. bulbosus is widespread in Britain and Ireland, but less so than either R. acris or R. repens. In Ireland, it is widely distributed but is less frequently recorded in western and southern areas of the island. The pattern in Britain shows the species is less frequent in the N & W of Scotland. While this may in part be due to the upland, acidic nature of the terrain in the N & W of both islands, the fact that the species is only conspicuous and readily identified in the early summer, probably means that it is under-recorded in areas where botanical recorders are themselves scarce, or indeed, rare (New Atlas).

The calculated Change Index value measuring change between the two BSBI plant Atlases (published in 1962 and 2002) is -0.48, which is taken to indicate very little loss over the 40 year period (R.A. Fitzgerald, in: Preston et al. 2002). The results of the 2003-04 BSBI Local Change re-survey in Britain of the 1987-88 BSBI Monitoring Scheme survey squares suggest that R. bulbosus is actually on the increase in Britain. Unfortunately, this re-survey was not extended to the island of Ireland. In comparison with other widespread plant species of predominantly calcareous or neutral grasslands, Bulbous Buttercup can cope relatively well with factors which lead to ranker swards, including eutrophication (essentially, nitrogen enrichment) (Braithwaite et al. 2006).

European occurrence

In continental Europe, depending upon which source is consulted, either two or three subspecies of R. bulbosus are now recognised: subsp. bulbosus which occurs in the British Isles, and subsp. adscendens which is confined to the Mediterranean region, presumably including N Africa (Coles 1973). Subsp. adscendens may be subdivided into subsp. castellanus in NW Spain and subsp. aleae in S Europe, extending eastwards to Hungary (Tutin & Akeroyd, in: Tutin et al. 1993). Plants of R. bulbosus in N Africa, N Turkey, Georgia and Azerbaijan also lie within subsp. aleae (Hultén & Fries 1986, Map 845).

Subspecies bulbosus is widespread over W and C Europe, but it is absent from much of the N and E of the mainland. Having said this, it is frequent in Denmark and southern Sweden, rarer on the south coast of Norway, and extends east to Belarus and the Balkans (Hultén & Fries 1986, Map 845; Jalas & Suominen 1989, Map 1743; Jonsell et al. 2001, p. 289). Some outlying occurrences in C Finland and C Russia are considered occasional only (Hultén & Fries 1986).

World occurrence

Bulbous buttercup is a naturalised alien in N America, apparently invading from both E and W coasts, although much more frequent in eastern states. It is also a commonly naturalised weed in New Zealand (Harper 1957; Hultén & Fries 1986, Map 845; Jonsell et al. 2001).

Names

'Ranunculus' is derived from the Latin 'rana' meaning 'a frog', an allusion to the fact that so many members of the plant genus live in or near water, the habitat of frogs (Johnson & Smith 1946). The Latin specific epithet 'bulbosus' means 'having a bulb', but of course technically it is not a leafy storage organ, that is, a bulb, but rather stem tissue (Gilbert-Carter 1964). Thirty-eight English common names are listed by Britten & Holland (1886), many of which are widely applied and some of which are merely mis-spellings, for example, 'Bolt', for 'Bout', which is derived from the French 'Bouton d'or', referring to yellow flower-buds of this and other species (Prior 1879). One of the more interesting names is 'Lodewort', said to be an Anglo-Saxon name for the species, but also applied by some to R. aquatilis (Common Water-crowfoot) (Britten & Holland 1886). Another name of interest is 'St Anthony's Rape' or 'St Anthony's Turnip', from its corm being a favourite food of pigs, and he being the patron saint of pigs (Prior 1879, p. 204).

Uses

Although in Britain and Ireland it is not nearly as common a species as several other buttercups, R. bulbous is very conspicuous early in the growing season and, perhaps for this reason, it has long been used in herbal medicine for its blistering properties. Grieve (1931) summarises the many uses, some of which like the cure for a headache which involves applying the acrid juice to the nostrils, makes the present author's mind boggle at the very thought. THIS DEFINITELY IS NOT ADVICE TO BE FOLLOWED.

Threats

Improvement of grasslands involving ploughing, reseeding and application of fertiliser.