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Saxifraga tridactylites L., Rue-leaved Saxifrage

Account Summary

Native. Very rare, but probably previously overlooked. European southern-temperate.

15 May 1986; Wolfe-Murphy, S.A. & Austin, L.W.; Knockmore wood.

Growth form and preferred habitats

This very variable, glandular-hairy, semi-rosette, winter annual germinates in the autumn and flowers early in the following spring, between April and June. Depending upon available soil moisture the whole plant can vary widely in size between tiny (c 2.5 cm tall) and single stemmed, to small but branched and tufted (10-20 cm tall). The species epithet 'tridactylites', meaning 'three-fingered', refers to the commonest form of the leaves, although they can be 1-7 lobed (Webb & Gornall 1989). Despite the extent of the variation mentioned, most of it is phenotypic and environmental, there being little ecotypic differentiation (Webb & Gornall 1989). Being usually diminutive and fairly rare in Ireland, S. tridactylites is inconspicuous and easily over-looked unless it is abundantly present.

Elsewhere in B & I, Rue-leaved Saxifrage typically occurs in dry, open, sunny, short-turf, stony, nutrient-poor, overgrazed limestone grassland, or in droughted conditions on rock surfaces, including in solution cups in limestone pavement, in limestone or base-rich quarries and gravel pits, in crevices and on the tops of walls. S. tridactylites is classed as a stress-tolerant ruderal by Grime et al. (1988), meaning it avoids competition with larger, more vigorous, dominant plant species by frequenting unfavourable growing conditions in drier, more shallow, exposed, rocky and less fertile soils, often subject to summer drought and disturbance. Conditions include semi-stabilised, calcareous sand dunes in some coastal areas.

While Webb & Gornall (1989) regard it as not strictly calcicole, there is a definite tendency in this direction, in many instances S. tridactylites being much commoner on limestone than on any other substrate, and largely restricted to calcareous or base-rich soils in the pH range 6.0–8.0 (Grime et al. 1988).

Fermanagh occurrence

Data included in The Flora of Co Fermanagh (Forbes & Northridge 2012) had a cut-off date of 31 December 2010. The Flora contained mention of a solitary record of this tiny saxifrage from the Fermanagh western limestones, as listed above. The recorded site sounded quite atypical for the species, ie on or around the ash-wooded slope below Knockmore limestone cliffs. However, the fact that the species occurs further west on the Ben Bulbin limestones, of which the Fermanagh strata are a NE extension and that there is so much suitable habitat for it in Fermanagh, suggested that it just might be present in the VC, even perhaps as a very rare colonist. Wolfe-Murphy's voucher-less record was very welcome, although it remained doubtful and unconfirmed.

Shortly after the book database was finalised, S. tridactylites was discovered and confirmed from a site on the shore of Lough Melvin near a plantation, 1.2 km [originally given as 0.75 miles] NW of Garrison found by John Faulkner and Robert and Hannah Northridge, on 11 May 2011. It was re-found here by RHN & HN on 28 May 2012. Two additional sites have since been added by the Northridges, at Inishmore Viaduct, Upper Lough Erne, 29 April 2015 and again on 15 May 2018; and at Killyhevlin, 2.3 km SE of Enniskillen, 22 May 2015.

In view of these five new records, the current author feels Wolfe-Murphy's earlier record should probably be accepted.

Irish occurrence

Elsewhere in NI, S. tridactylites is rare or very rare and is mainly found near the coast, on sandhills, rocks and walls, as well as on bare, base-rich gravel at one site on the shore of Lough Neagh, where it is considered a very rare, accidentally introduced colonist (Flora of Lough Neagh; FNEI 3). In the RoI, S. tridactylites is very frequent in the south and in western parts of the centre, but rather rare elsewhere (Parnell & Curtis 2012). It is very frequent and often locally abundant in the limestone karst area of the Burren, Co Clare (H9) and especially so on limestone pavement, calcareous sand-dunes, and in and on lime-mortared walls near the coast (Webb & Scannell 1983).

Flowering reproduction

This little plant should be searched for early in the season, as it has more or less finished flowering by the end of May. Having said that, the white flowers, although borne either solitary or in a rather diffuse cymose branched inflorescence, are often so minute they do not greatly advertise the presence of the species. The perfect (hermaphrodite) flowers, in contrast to those of most other saxifrages, are usually somewhat protogynous (ie the female parts mature before the anthers) (Webb & Gornall 1989). Not surprisingly, however, on account of their small size, they are chiefly self-pollinated (Garrard & Streeter 1983; Webb & Gornall 1989). Seed is set from June to July, the dehiscent fruit capsule often containing more than 100 very tiny seeds. Dispersal usually occurs close to the parent plant, but as the seeds are so minute, they are easily carried by wind or vehicle slipstream and sometimes travel further than normal, including high up on walls (Grime et al. 1988).

Being a winter annual, after a post-release ripening period, the seed germinates in the autumn and the plantlet overwinters as a tiny leaf rosette. Flowering is induced after cold vernalisation, but there is no long-day requirement for floral initiation. Flowering follows in early spring (Grime et al. 1988). A short-term persistent soil seed bank is also formed, ie buried seed surviving >1 and <5 years (Thompson et al. 1997).

When in full sun in a droughted, shallow, lime-rich, sandy soil, the whole plant often goes a bright scarlet red when in fruit, making it much more conspicuous for a time in June and early July. It then quickly dries out, dies off and disappears.

British occurrence

Locally common on calcareous soils in lowland Britain, S. tridactylites becomes increasing scattered, rare and coastal northwards into Scotland, although it still reaches the northern-most point of the mainland. The New Atlas indicates that although better recorded than in the 1962 BSBI Atlas, there has been a widespread decline of the species in S & E Britain. Analysis of the New Atlas database reveals that the losses have occurred since 1950 (R.J. Gornall, in: Preston et al. 2002).

However, the British partial re-survey, Local Change 1987-2004, found that S. tridactylites was amongst a group of lowland species of calcareous grassland and marginal or linear landscape features that showed a significant increase. The calculated 'Change Factor' for the species was +15 (Braithwaite et al. 2006).

European occurrence

S. tridactylites is the most widespread Saxifraga species in Europe, ranging over the whole continent, including the Mediterranean isles, but absent from the extreme north and much of the north-east. In E Europe, it reaches only the western and southern fringes of Russia through NE Ukraine, just N of the Black Sea (Jalas et al. 1999, Map 3241). Beyond Europe, it is known only from NW Africa and SW Asia, its eastern limit lying on the borders of NE Iran (Webb & Gornall 1989, Map 74; Hultén & Fries 1986, Map 1022).

Threats

'Improved' limestone pastures (that is, in terms of grazing productivity) through fertilizer application, leading to increased plant competition.