Salix fragilis × S. alba (S. × rubens Schrank), Hybrid Crack-willow
Account Summary
Introduction, archaeophyte, deliberately planted, very probably under-recorded and possibly occasional.
30 August 1967; Parker, R.E.; roadside near Lisnaskea.
There is only the solitary record listed above for this hybrid in the Fermanagh Flora Database. A voucher exists in BEL, but the identification needs to be expertly confirmed. It is likely that some of the plants that are listed as S. fragilis, but are growing in unlikely planting sites, may really be this hybrid, rather than the usual form of S. fragilis in our area, ie var. russelliana. The entirely female var. russelliana crosses with pollen from S. alba to form this intermediate fertile hybrid. Back-crossing can occur with both parents, creating a hybrid swarm of intermediates. Fortunately this development has never been suggested in our area, although more work is definitely required on the variation within Fermanagh tree willows.
Taxonomic difficulties
In the Revised Typescript Flora, Meikle et al. (1975) state that typical S. fragilis L. (sec. Sm.), that is the Smith 'type', var. fragilis, had not been seen by Meikle in Fermanagh. In his Willows and Poplars Handbook, where he enlarges upon the situation regarding this complex group of large, tree-willows in a B & I context, Meikle affirms that in Ireland S. fragilis var. fragilis is generally replaced by var. russelliana, exclusively female and the commonest and most widespread form of the four variants (var. fragilis (dioecious), var. furcata (male), var. russelliana (female) and var. decipiens (male)) of this species in these islands (Meikle 1984, p. 28; Meikle 1992). The three single-sex variants, in Britain at least, are individually so uniform that each appears to be represented by a single clone. Thus from this argument, almost certainly the records in the Fermanagh Flora Database for S. fragilis refer to either var. russelliana (most probable), or just possibly to the other form that Meikle reckons occurs in Ireland, var. decipiens (Hoffm.) W.D.J. Koch. The latter is part of the taxon recognised by continental botanists as the 'type' for the species S. fragilis L.
Meikle (1992) cogently discussed this problem, listing the points where his opinion at that time differed from that of the continental taxonomists. In a recent Scandinavian critical Flora (Jonsell et al. 2000), where the editors take a narrow Linnaean view of S. fragilis, it appears that three of Meikle's four variants of S. fragilis (all except var. decipiens), fit into what they refer to as 'S. × rubens Schrank', albeit along with several other named forms and hybrids. S. fragilis var. decipiens W.D.J. Koch is now referred to by Stace (2010, 2019) as S. euxina I.V. Belyaeva, and the three remaining 'fragilis' variants, that previously were referred to as S. × rubens Schrank, have been transferred to S. × fragilis L.
This continental reworking might prove sensible here, since it greatly simplifies life and makes the three single-sex Meikle S. fragilis variants redundant (which we believe are of horticultural origin anyway). This approach is encouraged by the fact that nowhere in B & I does S. fragilis s.s. nor any of the Meikle species variants occupy a naturally occurring plant community. Rather, they are always, or almost always, found in obviously planted situations. Some proportion of the less obviously planted tree clusters might possibly be fertile hybrids with S. alba (White Willow), which can back-cross with S. fragilis and create additional identification problems.
The critical Flora of Great Britain and Ireland, 1 (Sell & Murrell 2018) takes the European approach to S. fragilis (or S. × fragilis, S. × euxina), but lists numerous more splits and subdivisions. Three of Meikle's four subspecies are listed as nothovars, and two new ones are added, nothovar. basfordiana (Scaling ex Salter) P.D. Sell, which is a hybrid between S. alba var. vitellina and S. euxina. Within this latter nothovar, two formas are named, forma basfordiana and forma sanguinea (Meikle) P.D. Sell. The fifth nothovar is rubens (Schrank) P.D. Sell, a hybrid between S. alba var. alba and euxina. Meikle's var. decipiens (Hoffm.) Koch becomes a separate species, S. euxina I.V. Belyaeva.
Growth form and preferred habitats
S. fragilis var. fragilis is a sturdy, round-headed tree with wide-spreading branches that can grow up to 15 m tall. Young twigs are brittle at their point of attachment to the older branch. See below for the comparative growth form of the var. russelliana in a separate account.
All variants of S. fragilis tend to occur in linear collections or plantations by water, or in damp hedgerows near ditches, typically, and rather obviously, in low-lying situations. Occasionally trees appear singly or in small groups in wet hollows in fields, where they may have self-sown, or rather since the most likely form of the tree we have in Fermanagh (Meikle's var. russelliana) is entirely female and does not set seed, where wind or animal broken and transported branches may have self-rooted. Where clusters of the tree occur we presume the original planted individual has fallen over and layered itself, or else it has formed new plants through branches breaking off at joints and re-rooting, something they readily do. On the other hand, it is possible that some proportion of these less obviously planted tree clusters may be hybrids with S. alba (White Willow).
The fragile nature of S. fragilis, ie the ease with which young side branches snap off at the joint with the stem, is a much over-rated species distinguishing character (Howitt & Howitt 1990). This feature should not be relied upon to distinguish the tree, since it also applies to the other very common large tree willow, S. alba. The difference between these two taxa in terms of brittle branches is simply a matter of degree.
Fermanagh occurrence
As the tetrad map shows, S. fragilis (ie probably largely, or almost entirely, Meikle's var. russelliana), has been recorded in 72 tetrads in Fermanagh, 13.6% of the total number in the VC. It is widely scattered throughout the county along riverbanks, lakeshores and roadways, but with a definite concentration towards the SE of the area around the Upper Lough Erne basin.
Irish occurrence
Because of the confusion of S. fragilis var. fragilis with var. russelliana and var. decipiens and other forms and hybrids (especially that with S. alba), it is almost impossible at present to map with any exactitude the distribution of S. fragilis var. fragilis in B & I (Meikle 1984). However, for what it is worth, the New Atlas hectad map for S. fragilis in Ireland shows that it is widespread and scattered throughout, with the most consistent presence being in NI. In the light of what Meikle (1984) explains, very probably this map really shows the occurrence of var. russelliana, rather than that of var. fragilis.
British occurrence
The same New Atlas map (with the same disclaimer applied), displays how very widespread the tree is in lowland Britain, the distribution however petering out in N & W Scotland.
Threats
None.