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Betula pendula Roth, Silver Birch

Account Summary

Native, occasional. Eurosiberian boreo-temperate.

1884; Barrington, R.M.; Ely Lodge Forest.

April to September.

Growth form and taxonomic status

Three species of Betula are native in Britain and Ireland. One of them, B. nana L. (Dwarf Birch), does not occur in Ireland and in Britain is almost restricted to C and N Scotland. It is a distinctive, very dwarf, deciduous subshrub of high alpine habitats, is easily recognised and normally presents no taxonomic problems for botanists. The same cannot be said of the two deciduous tree birches, however, B. pendula Roth (Silver Birch) and B. pubescens Ehrh. (Downy Birch). The taxonomic status of these two trees has been a matter of long dispute. Linnaeus (1753) treated the tree birches as a single species, B. alba, and some still consider the two forms in Britain and Ireland as subspecies of this entity (Tuley 1973). There is a chromosome difference however: B. pendula is a diploid species or subspecies with 2n=28, whereas B. pubescens is a tetraploid aggregate (2n=56). Despite the fact that there are only two species and they differ genetically, the tree birches constitute a difficult taxonomic group as a result of common and widespread intermediate forms that are not readily separable from the parent species. Most intermediates are fully fertile and therefore they probably backcross with the parents. The taxonomy can really only be understood when examined on a world scale using cytogenetic experimental study (Walters 1968).

To deal with the problem of intermediates, Atkinson & Codling (1986) devised a reliable leaf discriminant coding equation based on three easily measured characters measured on five leaves collected from side branches from the lower crown of the tree and averaged. The methodology is illustrated, but in the opinion of the current writer, it not adequately explained in Stace (2010). Reference to the original paper in the journal Watsonia is therefore recommended.

'Pure' B. pendula is described as a tree up to 30 m, with slender, pendant, glabrous twigs covered with wart-like resin-glands, smooth silvery-white bark and subglabrous, biserrate (doubly toothed) leaves. In addition to the described species identification difficulties, there are as many as ten named cultivars of B. pendula in horticulture (Schilling 1984; Griffiths 1994).

Hybird birches

Hybrids between the two species occur to complicate matters. B. pubescens in particular is extremely variable, and the hybrid appears to overlap in leaf morphology more with it than with B. pendula (Kennedy & Brown 1983). The overlap of numerous characters occurs to such an extent that the existence of hybridisation became a matter of dispute (Brown et al. 1982; M.D. Atkinson, in: Rich & Jermy 1998; Stace et al. 2015). The real extent of hybridisation between the two tree birches remains unresolved (Stace et al. 2015). In view of the great difficulty of recognising birch hybrids in the field, they have not yet been looked for in Fermanagh at all.

Distinguishing the two Birch tree forms

The Flora of Connemara and the Burren unequivocally states that B. pendula is grossly over-recorded in Ireland due to the fact that many botanists, particularly visiting, non-native recorders, fail to appreciate that in Ireland the twigs of the much more common B. pubescens are often without a hairy pubescence. This observation renders the possession of glabrous twigs almost useless as a distinguishing character, and this is particularly the case in the north and west of Ireland. Experience shows Irish material of the two tree birch species can best be distinguished by the larger, paler, more raised warts (c 1 mm in diameter), present especially on the younger twigs of B. pendula, and secondly by the appearance of its leaf tip, which is much more drawn out into a slender point (ie acuminate) in comparison with the leaf apex of B. pubescens, which is described as sub-acute to acute (Atkinson 1992; Parnell & Curtis 2012).

B. pendula is regarded as the more 'shapely' of the two species, having fine long-pendulous shoots that hang from elegantly arched branches borne on a straight trunk. In comparison, B. pubescens is described as having an untidy, shapeless and twiggy crown (Mitchell 1996). Unfortunately, tree shape again is not a reliable means of distinguishing the two genetic forms, since pendulous forms of B. pubescens do also occur (Nelson & Walsh 1993).

Preferred habitats

Populations of B. pendula of native Irish status are found mainly on the margins of lowland raised bogs, or by limestone or in woods and scrub around stony lakeshores. It prefers well illuminated sites on well drained acidic soils, including less heavy clay situations. B. pendula can tolerate somewhat wetter and more shaded conditions as already mentioned, but in such sites B. pubescens is often the more prevalent of the two. In addition, elsewhere throughout Ireland, this colonising species with very light, readily dispersed winged nutlet fruits, originates as occasional to common naturalised self-sown seedlings, often derived from nearby planted garden, parkland or demesne trees (Webb 1994; An Irish Flora 1996). Having a small seed, B. pendula is unable to colonise ecologically occupied sites such as grassy swards that might remain open to larger seeded tree species (Worrell & Malcolm 1998). In an unknown number of cases, the gene stock may be derived from imported, non-native seed. Seedlings are most frequently found on recently cleared ground, especially where vegetation has been burnt, but it can also colonise other forms of open ground, including roadsides and the gravel drives of houses.

Growth, ecology and longevity

Silver Birch is light-demanding and, while it does not very readily grow in the shade of other trees, occasionally it does. Seedling birches are especially rare under the canopy of young trees of their own species (Kinnaird 1968). Birch therefore tends to be found most often in open areas, including in clearings and on woodland margins. Birches can show very rapid growth during their first 20 years, often reaching 20 m in height over this time span. After the initial spurt in development, growth is slow, terminating in a decline that is often rapid due to fungal decay (Kinnaird 1968).

As pioneer colonists, Silver Birch trees are not likely to survive to an advanced age. No dated tree in cultivation is known before 1905, but so few are of known age that this means less than it might otherwise. It is generally assumed that in Britain and Ireland the trees die back, break up, or fall over when less than a hundred years old. In Scottish highland glens, the oldest trees were reckoned to be around 220 years old (Mitchell 1996).

In birchwoods, it is often observed that apart from the occasional birch seedling, saplings are rarely observed under the woodland canopy. This is all the more remarkable as birch can establish itself in the shade of other tree species. Birchwoods consequently regenerate on suitable disturbed or open ground lying adjacent to pre-existing birch stands, rather than within them. Exceptions to this can occur where gaps appear in deciduous woodland canopy, especially when this is accompanied by soil disturbance or is the result of a fire. Birch regeneration depends on the vigour of the seedlings, and as with any species, establishment is greatly affected by the local grazing intensity. Seedling vigour is a function of soil fertility, but on poorer sites it may also depend on the successful establishment of a mycorrhizal association with soil fungi (Kinnaird 1968).

Seed production and interactions with birds

Birch fruits are generally produced in abundance, although not in every year by an individual tree. In other words, there is a sort of birch mast, as in beech and several other species, a good fruiting year being followed by several of low productivity (Kinnaird 1968).

Birch trees are highly attractive to birds. The small, light, winged, single- seeded dry fruits are much sought after by small finches and tits including siskins, goldfinches and blue tits. The foliage in spring and autumn supports large populations of aphids, the first crop of which feed nestling tits and warblers. The autumn fruit crop feeds the adult birds, the warblers building up fat reserves to fuel their migration. Rotting tree bases and stumps of birches are bored for nest holes by willow-tits (Mitchell 1996).

Fermanagh occurrence

B. pendula, which is also referred to as Pendulous or Warty Birch (the latter name much deplored, being a quite horrible name to give to any plant), is almost a rarity in Fermanagh. It has only been recorded from a total of 26 tetrads, 18 of them containing post-1975 records. While the distribution map shows B. pendula is thinly and widely scattered across the vice-county, to some extent it reflects areas of likely plantation woodland associated with landed estates.

British and Irish occurrence

The New Atlas hectad map shows B. pendula as being common and very widespread throughout most of Great Britain, but absent from high ground, and becoming less frequent in NW Scotland. On account of the ability of the two birch tree species (B. pendula and B. pubescens) to readily colonise felled woodland, Kinnard (1968) speculated that they might well be the commonest trees in Britain. In Ireland, the hectad map indicates that B. pendula is much less frequent than in Britain, and it become rare or absent in the west of the country (Preston et al. 2002). Recently, doubt has been cast on the Scottish distribution of B. pendula, as Worrell & Malcolm (1998) have found anomalies which suggest that it is greatly over-recorded in much of that country, few recorders being able to reliably distinguish it from B. pubescens. While no one knows for certain at present, a similar situation might very possibly apply in Ireland, and perhaps even throughout the whole of the British Isles (Wurzell 1992; Webb 1994).

Names

The genus name 'Betula' is the Classical Roman name for the tree, for example in Pliny (Gilbert-Carter 1964). The familiar Latin specific epithet 'pendula', means 'hanging down'. The English common name 'Birch' is from the Anglo-Saxon 'birce' or 'beorc', and it has many similar names in other northern Indoeuropean languages, for example, 'berk' in Dutch, 'Birke' in German, 'birk' in Danish, 'beith' in Gaelic and Old Irish, 'begh' in Irish. The Indoeuropean root of the name means 'white' or 'shining', probably referring to the bark (Grigson 1974). Many place-names contain the elements 'birk' or 'birch' in England, or 'beagh' and 'behagh' in Irish, meaning 'birch land' (Milner 1992).

Uses

The timber of both tree birches is hard, strong and works and turns well, so it has been used for a wide variety of chiefly indoor household purposes (it is not durable outdoors). Uses include furniture, as brooms (besoms), tool handles and thread spools (until replaced by plastic). Birch branches also provide excellent firewood and the jumps for steeplechasing (Milner 1992). The birch (hereafter meaning the two species undifferentiated), had the rank of a 'Commoner of the wood', the second grade of four in the eighth century Irish Laws of Neighbourhood. Most likely it achieved this rank on account of its timber value (Nelson & Walsh 1993).

The bark produces Birch Tar oil, which is, or was in the past, used in the tanning of leather. The distinctive smell of Russia leather is due to its use as a dressing. Russia leather is very durable and it was the preferred material for book binding since the tar oil dressing also prevented or discouraged attacks on books by fungi and insects.

Birch sap has had a long history of use in brewing, which continues today in parts of E Europe (Grieve 1931; Milner 1992). As a consequence of its long history and many uses, a very well-developed folklore has grown up around birch, particularly in northern regions of Europe where the trees are most prevalent, and there are numerous folklore rituals and superstitions attached to it (see Grigson 1987 and Milner 1992). Birch sap and birch tar oil have also been used medicinally (see Grieve 1931).

Threats

None.