Betula pubescens Ehrh., Downy Birch
Account Summary
Native, very common and widespread. Eurosiberian boreo-temperate.
1900; Praeger, R.Ll.; Co Fermanagh.
Throughout the year.
Growth form, identification difficulties and preferred habitats
Although the common and the scientific name both refer to 'downy birch', plants of this species in Fermanagh, as in most of Ireland, have twigs which are almost hairless (Webb 1994). B. pubescens is a diploid with chromosome number 2n=28. Birch trees are monoecious and can begin flowering when they are only five to ten years old. Individual trees are mostly self-incompatible, and as could be expected in any widespread self-sterile species with a broad geographical and ecological range, there is considerable genetic variation and overlap between species, subspecies and forms.
B. pubescens can be subdivided into the tree form, subsp. pubescens and the shrubby upland form, subsp. tortuosa (Ledeb.) Nyman; the latter is especially frequent in N Britain, but is very probably present in N & W Ireland too (New Flora of the BI; M.D. Atkinson, in: Rich & Jermy 1998). No birch subspecies have yet been distinguished in Fermanagh, nor has the hybrid with B. pendula (which is a tetraploid taxon with 2n=56), ever been recorded. However, the whole problematic question of the true identity of our birches compared with those in Britain requires further study.
B. pubescens occupies damp open woods and scrub, frequently but not always in upland areas at or near the potential tree-line. It also frequents peat bogs (including cut-over examples of these), cliffs, stabilised boulder screes and disturbed and open abandoned ground generally. In most parts of Ireland, birch is most conspicuous when the trees are young and are colonising thickets, or when well-spaced individual trees are found invading disturbed open situations on peaty moorland, heath or bog. These colonising situations usually occur after there has been a fire or turf-cutting operations have taken place, both of which require previous drainage. The other absolute essential for successful establishment from seed is the creation of a bare, almost litter-free soil surface (Gimingham 1984).
Variation
Within both species of tree birch that occur in Britain and Ireland (B. pubescens and B. pendula), there is very great phenotypic plasticity (ie environmentally induced variation). For instance, when tetraploid trees (therefore supposedly B. pendula) were transplanted into both dry heath and rather wetter bog conditions, Davy & Gill (1984) found that the leaves of those planted into the heath soil became more like B. pendula, while those planted into boggy ground resembled B. pubescens! Similarly, a detailed biometric study of transplanted B. pubescens seedlings gathered from 25 populations which examined and compared no less than 36 variable characters, found that seedlings with small leaves and drooping branchlets were associated with westerly, wet locations, whereas most seedlings with erect branches and larger leaves originated from drier, more easterly locations (Pelham et al. 1988).
Glacial history and native status
Both tree birches are native throughout the British Isles and fossil pollen studies prove they were the earliest tall growing woody plants to colonise these islands as the glacial ice finally retreated around 10,000 BP. This accords with the position of birch zones on modern tree lines at the margins of both the arctic tundra and the high mountain tundra further south. It appears that before 10,000 BP, tree birch was well established in much of C and N England, S Scotland and parts of Wales, and tree birches just managed to reach two bridgeheads in E Ireland. This pattern very strongly suggests that the spread of birch was emanating from the east, probably migrating from the then dry North Sea basin. By 9,500 BP the tree birches had expanded to cover northern Ireland and parts of the Scottish Highlands. Interestingly, expansion was delayed (perhaps for up to 500 years), in areas of the Scottish Highlands which were re-glaciated during the Loch Lomond Stadial (ie a short spell local return of cold, glacial conditions within a warm interglacial period). Presumably the delayed birch arrival was caused by the time required for the soil to mature sufficiently after glaciation or periglacial conditions ceased. Periglacial conditions involve near-glacial, freeze-thaw perturbation that effectively turns over and rejuvenates the soil. Conditions on the glacial moraine ridges exposed by the thawing ice could also involve soil instability and 'frost drought', which would seriously affect Betula species since their seedlings are very sensitive to any form of drought (Birks 1989).
Fermanagh occurrence
Betula pubescens is the locally common and very widespread birch tree and shrub in Fermanagh, present in 396 tetrads, 75% of those in the VC. It is the eighth most frequent woody species in the county after Salix cinerea subsp. oleifolia (Common Sallow or Rusty Sallow) and the tenth most widely distributed woody plant after Acer pseudoplatanus (Sycamore). In comparison, a Britain-wide forestry survey found birch (hereafter referring to both tree birch species) was found to be the second most common broad-leaved tree in England and Wales and the most common tree in Scotland (Steele & Peterken 1982).
British and Irish occurrence
Our best knowledge of the present day British Isles distribution of the two tree birches suggests that they overlap considerably. This is more the case in Britain than in Ireland, where B. pendula is very much less frequent and widespread than B. pubescens. In general, B. pubescens is more prevalent in the north and west of both islands, while B. pendula is more common in the south and east (Gimingham 1984; Preston et al. 2002). Recently, however, doubt has been cast on the supposed Scottish distribution of B. pendula. Studies by Worrell & Malcolm (1998) found anomalies which suggest B. pendula is over-recorded in much of Scotland, 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 maybe even throughout the British Isles (Wurzell 1992; Webb 1994).
The degree of confusion and need for further research is such that some botanists have even called for a return to the Linnaean single species, Betula alba, demoting B. pubescens and B. pendula to subspecies until their true status can be researched and clarified (Tuley 1973; Nelson & Walsh 1993). Nevertheless, even given the possibility that B. pendula may be over-recorded, the overall trend observed with regard to the distributions of the two tree birches in the British Isles is believed to be reflected in Continental Europe, ie B. pubescens extends further in the colder north and east, reaching Iceland, while B. pendula stretches further south into warmer, drier regions than B. pubescens, reaching the mountainous regions of Spain, Italy and Greece (Jalas & Suominen 1976, Maps 269 & 270).
Birch ecology
Tree birches are particularly cold hardy pioneer species, but the northern limit for both species is still set by exposure to cold and dry NE winds in places like Iceland and Greenland. Even in northern parts of Scotland, seedlings of B. pendula in particular are killed or suffer severe die-back in periods of very cold, dry, winter weather, while by comparison B. pubescens seedlings suffer less damage. This suggests that in at least some parts of Scotland, both birch species are fairly close to their ecological survival limits with respect to extremely cold winds (Atkinson 1992). The latter author succinctly described the main behavioural characteristics of birch tree ecology (both species undifferentiated), in terms of: (a) ability to quickly colonise bare areas, (b) intolerance of shade, (c) lack of affinity for any particular soil type (they can tolerate a very wide range of substrate pH), and (d) ability to grow on nutrient-poor soils.
While this is a generally useful summary, it requires some qualification in that B. pendula appears more restricted to drier, lighter soils in more sheltered lowland situations than B. pubescens, which can perform perfectly on rather wetter (although seldom permanently waterlogged), more peaty, wind exposed situations on upland heaths, moors and bog margins (Gimingham 1984). A much rarer and more unusual occurrence is where birch woods or copses are found growing on wet flushes or soaks on bogs, as described by Cross (1987) from two Irish raised and blanket bogs in Cos Offaly and Mayo (H18 & H27). It is difficult to imagine the precise factors which allow the rather stunted, multi-stemmed birches (chiefly, but not exclusively, B. pubescens), to colonise these particular bogs. Human interference and disturbance of the bogland in the past may well be involved, eg grazing and burning. Straightforward human neglect of the ground in question may also be partly responsible for birch invasion. Whatever anyone thinks of them, these two birch stands on bog soaks are most unusual, and I am not aware of any Fermanagh parallel to them.
Birch in woodland
Reflecting the broad soil range of both tree birches, the massive yearly output of small, wind-dispersed, single-seeded fruit and their consequent rapid colonising ability, plus their characteristic high light demand, rapid initial growth rate and short lifespan (typically 80 years), birches are regularly present in a wide variety of woodland types throughout Britain and Ireland. In many woodland situations, however, birches are not necessarily very numerous nor very conspicuous. Rather, they simply represent a constant, minor component, filling gaps in the canopy that occur sporadically within the woodland stand due to the death of larger trees. Alternatively, they may form a marginal exposed fringe to the woodland, rather than occurring as dominant canopy-forming species within it, or forming an extensive pure birch stand (Gimingham 1984).
In Fermanagh, as in most other parts of Ireland, birch is most conspicuous when young colonising thickets or well spaced individual trees invade disturbed peaty moorland or heath, say after fire or turf cutting operations involving both drainage and, in the case of birch, what is absolutely essential for successful establishment from seed, the creation of bare, almost litter-free soil. Birch also regularly invades the drier sloping margins of raised bogs, or abandoned cut-over bogs (which have always been subjected to drainage operations of some sort). However, as in other situations, this generally requires temporary severe grazing pressure or some other form of disturbance to create the vegetation gaps and the bare ground needed for successful seedling colonisation (Gimingham 1984).
Seedling growth and colonising ability
Birches do not form a persistent soil seed bank, and with the seed being so very small, the new seedlings quickly exhaust the very limited seed food supply, and to survive must rapidly develop the mycorrhizal fungal partnership on which they are so dependant (Milner 1992; Ingram & Robertson 1999). Until the mycorrhizal fungal root sheath develops and grows out into the soil, the seedling is very vulnerable to both nutrient starvation and drying conditions. When invading newly available ground in this manner, birch is sometimes in the vanguard of a vegetation succession responding to major change in the environment, and if protected from grazing and other destructive pressures and allowed to develop, would eventually build a more diverse woodland dominated by taller, long-lived species more capable of regeneration within woodland canopy shade than the light-demanding Betula species (Atkinson 1992).
This is not to advocate that a completely stable climax vegetation of mixed deciduous woodland dominated by oak, ash (or beech), would necessarily always develop, since the terrain and its stability might not prove suitable or conducive for such vegetation in the longer term (Tansley 1949). Vegetation history from the Neolithic onwards suggests that man and his activities have played such a dominant determining role, that vegetation scarcely ever reaches anything approaching stability or a classical Climax state, but rather it is characterised by reaction to more or less continuous change (Mitchell 1995). Certainly older birch may instead be replaced by young birch if human induced disturbance is sufficient to maintain or recreate open light conditions, thus favouring opportunistic recolonisation by birch seed. As Rackham (1980) so very well puts it, "Throughout the history of birch its supreme colonising ability has been balanced against the greater competitive ability and longer life of other trees." He has also commented, "The reproductive ability of birch is so great as to outweigh even the disability of its short life-span."
Reputation as soil improvers and nurse trees
In forestry, the pioneer seral role of birch in vegetation change and development is recognised in that birches have the reputation of being both soil-improvers and useful nurse trees for longer lived, more valuable timber crops. In both these aspects, however, this reputation may be challenged. Miller (1984) found that while birch did improve a heath mor podsol, quite rapidly changing it into a less acidic brown earth with a mull humus and with a greatly increased worm population, it performed this service no better than would other tree species of similar growth rate. Likewise, in exposed sites the whippy tops of birch trees are capable of killing the apical buds on leading shoots of adjacent trees, in these situations at least, effectively negating any nursing abilities the birch might otherwise possess. The whole idea of using 'nurse trees' in forestry is really now a thing of the past due to reductions in available manpower, and thus in forest management possibilities.
In reality, the birchwoods most commonly seen in Britain and Ireland appear to have arisen secondarily as a result of felling of woods of all types on a wide variety of soils. The conditions following felling are obviously highly suitable for colonisation by birch, provided that seed parents are available reasonably close nearby. The relative abundance of lightweight birch fruit and the efficiency of its wind dispersal compared with the seed of other native trees, enables it to rapidly invade and exploit the open situation as a pioneer species. In summary, the conditions created by felling operations which enable birch to invade and compete involve the following; the surface vegetation is disturbed, the mineral soil is loosened and mixed with the litter and humus to some extent, the soil moisture regime is diversified, soil surface temperatures are elevated by increased sunlight, and there is an increased rate of decay and mineral release from soil organic matter (Kinnaird 1968).
Birch as habitat and food provider
Birch has a very important role as a habitat and food provider supporting other organisms and together the two tree Betula species have an impressive tally. They support at least 334 herbivorous invertebrate species, more than any other native tree apart from oak and willow (Kennedy & Southwood 1984). The presence of so much insect life naturally makes the trees highly attractive to birds. The leaves in spring and in autumn carry a large population of aphids, the first crop of which supports nesting tits and warblers. The autumn aphid crop builds up the fat reserves of adult warblers for their migration.
The often abundant seeds are sought by finches and by a wide variety of other small seed-eating birds including blue tits (Mitchell 1996). Browsing of saplings and seedlings is an important cause of both mortality and modification of birch tree growth. Trees can respond to mechanical damage by producing longer shoots, which favour the escape of the growing points to heights above the reach of the browsing animal. Birch leaves also respond to insect herbivory by increasing their phenol and tannin content, which renders them much less palatable (Atkinson 1992).
Fungal, lichen and bryophyte associates
Apart from the mycorrhizal fungi vital for the establishment and subsequent nutrition of the tree (Mason et al. 1984), the fungal associates of birch include one of the most familiar gall-forming species, the witch's broom. These structures, often quite numerous on individual trees, are a tight proliferation of shoots caused by attack on a shoot apical bud by the ascomycete Taphrina betulina on B. pubescens, and T. turgida on B. pendula (Milner 1992; Ingram & Robertson 1999, p. 103).
Being relatively short-lived and having very acidic bark, the tree birches support just 93 forms of lichen, a total which ranked it eighth among native trees in the British Isles in this respect after the tree willows and hazel (Rose 1974). Subsequently, however, Coppins (1984) listed no less than 235 species of lichens and 58 bryophytes (mosses and liverworts), growing as epiphytes on Betula in the British Isles. The greatest lichen development on birches was found in Scotland for a variety of reasons, not least of which was the lesser degree of sulphur dioxide pollution (Coppins 1984).
Names
See my Betula pendula account.
Uses
See my Betula pendula account.
Threats
Evidence from Britain suggests that both our native birches may be susceptible to leaf and possibly stem diseases caused by Phytophthora ramosum. Birches are not at the present moment in any conservation danger themselves, but as the fruit is so very mobile and invasive, they can become 'woody weeds' in fens, bogs and heathland. Control is expensive and very difficult to achieve.