Vaccinium myrtillus L., Bilberry
Account Summary
Native, common and locally dominant. Eurosiberian boreal-montane.
1818; Walford, T.; Co Fermanagh.
Throughout the year.
Growth form and preferred habitats
This very variable calcifuge, semi-deciduous subshrub forms large clonal patches by horizontal vegetative growth of its branching underground rhizome. Frequently, but locally, this enables Bilberry to become the dominant or subdominant species of the under-storey field layer in damp, upland, acidic, mixed deciduous woodlands and also, to a certain extent, in more open areas within coniferous plantations (Ritchie 1956). Since the plant can tolerate more shade than both Calluna vulgaris (Heather) and Erica spp., to a limited extent it can occupy darker areas in conifer plantations than can those shrubs (Ritchie 1956).
The breadth of the species woodland tolerances is demonstrated by the fact that Rodwell et al. (1991a) lists its presence in nine different woodland communities in the British NVC classification (not all of which, of course, occur or are relevant in an Irish context); it is considered a 'constant species' in five of these communities. While the subshrub grows largest and most bushy (c 60-90 cm tall) in fairly open woodland, it noticeably tolerates more shade than the other ericaceous species with which it almost always closely associates and competes.
Small plants of it are frequently present in an often very subsidiary role in heaths, heathy grassland and bogs dominated by other species, so that Rodwell et al. (1991b) lists it occurring in six types of NVC mires (bogs) and as many as 17 forms of heath vegetation, and as a constant species sufficiently significant to feature in the community name, in six of the latter. It is therefore not surprising that Grime et al. (1988) described it as a "stress-tolerant competitor".
Bilberry is common on peat bogs where it tends to dominate the very tops of the characteristic vegetation hummocks. It also becomes abundant around the margins of isolated rock outcrops in bogland and likewise its presence increases on the somewhat better drained sloped margins of the peat dome of lowland raised bogs. On bogs that are cut for turf (peat fuel) on an irregular, occasional basis, Bilberry often becomes prominent on the drier edges of peat banks or those of cut surfaces after the spade or 'turf slipe' is used for peat sod extraction (Evans 1942, p. 136-7). The steadily increasing use nowadays of modern mechanised turf extractors which tunnel below the surface of raised and blanket bogs (appropriately enough nicknamed 'sausage machines') can, at least temporarily and very locally, increase drainage in the peat and thus favour the growth of Bilberry over C. vulgaris and Erica tetralix (Cross-leaved Heath), at least in the short term. Unfortunately, this method of peat extraction eventually causes the collapse and frequently also the rupture of the thin layer of surface vegetation, so that erosion (either cyclical or longer term) often results.
On and below cliff scarps in upland areas, especially as in the Lough Navar Forest Park where many of the slopes are covered with a relatively well stabilised, overgrown block scree, V. myrtillus may also be locally abundant, competing well in these generally completely ungrazed circumstances with Calluna vulgaris, Erica cinerea (Bell Heather), Pteridium aquilinum (Bracken) and Luzula sylvatica (Great Wood-rush). Bilberry is often closely associated with V. vitis-idaea (Cowberry) growing under its canopy in this type of habitat.
Variation
V. myrtillus is phenotypically extremely variable and 'plastic' in its response to growing conditions in its wide variety of habitats. It generally performs best however in well-drained, somewhat sheltered situations. At the same time it is able to persist and is often quite abundant (albeit in a much dwarfed form, c 5.0 cm tall) in sheltered spots near exposed mountain summits. Dwarfism and reduced stem rigidity are genetically controlled, and both are correlated with increasing total leaf nitrogen at higher altitudes. This in turn controls maximum rates of photosynthesis and levels of stomatal conductance, assisting absorption of carbon dioxide as the concentration of the gas thins with altitude, thus compensating the plant for a shortened growing season. The thinner, less rigid stems enable increased levels of leaf nitrogen to occur and the stems also survive wind buffeting better. However, they do so at the expense of sexual reproductive capacity, each stem being only able to bear the weight of one or two fruits at the most (Woodward 1986).
Fermanagh occurrence
In Fermanagh, V. myrtillus is frequent or common and locally abundant, recorded in 244 tetrads, 46.2% of those in the VC. Here, as elsewhere in B & I, Bilberry grows in a very wide variety of strongly acidic, peaty conditions, ranging from sheep pasture grasslands, even on those formed over very shallow raw humus, or on heathy podsols formed directly over hard Carboniferous limestones. Examples of the latter occur around Knockmore, above Florencecourt and in the Cladagh River Glen (= The Marble Arch), as well as on ombrogenous raised and blanket peat bogs (ie Atlantic mires). Bilberry also thrives on drier, steeper, heathy areas of moorland, especially in sheltered pockets of ground around rocks, or in hollows and on, or close to, mountain summits, cliff scarps and on stabilised, talus scree slopes below cliffs.
Flowering, fruit and seed production
When in full sun, V. myrtillus flowers freely with two peaks of anthesis between April and early July. Flowers are either insect-pollinated by bees and wasps, or self-pollinated by gravity (Richie 1955a). Inbreeding is described as 'habitual' (Richie 1956) and a recent study has shown a drastic 'early-inbreeding' depression in fertility takes place in Bilberry due to embryo abortion in the early stages of seed development following successful self-pollination. It has been plausibly suggested that the observed partial self-sterility results from the accumulation of lethal recessive gene alleles, ie rather than any form of self-incompatibility system existing in the plant (Guillaume & Jacquemart 1999).
Masting behaviour
The familiar small but plump, blue-bloomed, purple-black berries (called 'Fraughans' locally, a Gaelic Irish name of the plant) are produced from July to September in numbers which generally appear, or which are assumed, to vary with the prevailing seasonal weather conditions. In Fennoscandia, however, Bilberry has been reported to produce large berry and seed crops at three to four year intervals, ie it is observed to follow a masting cycle synchronised within the population (Myrberget 1982; Selås 1997). Mast years in S Norway have been followed by temporal population increases of a large number of animal species of all trophic levels and an analysis of a 50 year record of seed production found that both the previous degree of Bilberry reproduction and weather helped to explain the pattern. At the same time, regularity was commonly interrupted by unfavourable weather conditions and Bilberry production was depressed by, amongst other factors, low temperatures in spring and when flowering was taking place, and low or high levels of precipitation during berry ripening in summer (Selås 2000).
Seed population behaviour
Each berry may contain up to 20 small yellow seeds, but the calculated average is just over 16 seeds per berry. In the shade of woodland canopy, or in the more exposed upland habitats over c 300 m, far fewer flowers are produced by comparison with well lit, sheltered conditions on open hillsides. In these more stressful environments, berries tend to be scarce or rare, some of them only partially formed or filled and, in view of the above revelation regarding self-pollination, we may assume that most seed contained in the poorly formed berries are sterile (Guillaume & Jacquemart 1999).
Studies in Canada, albeit further north in latitude than we lie, involving six species of Vaccinium in heathland, showed seed production is variable in time and space and varies from year to year. Even where Bilberry and V. vitis-idaea are dominant in the vegetation, their seed is curiously rare or absent in the soil seed bank. This is often the case despite the fact that experimental measurements of V. myrtillus seed longevity showed that their survival varies between two years and more than six (Vander Kloet & Hill 1994). Fungal rot appears to play a part in determining, or rather limiting, seed viability; Cippollini & Stiles (1992) have suggested that the seeds of summer-ripening berries in the Ericaceae generally have ineffective defence against fungal invasion and rot. This often overlooked topic has previously been reviewed by Janzen (1977) and by Herrera (1982), both of whom concluded that the rot factor affecting both fruit and seed could be as significant a limitation on reproductive performance as seed predation, or it might be of far greater consequence.
A parallel study on seed production and its fate on moorland in Scotland found that even when V. myrtillus produced many berries containing highly viable seed, collection of the berries by birds and mammals was minor. Most berries fell into the mossy ground under the bushes that produced them and the soil seed bank was small (c 0-274 seeds/m²). Observations of seedlings were extremely rare and then only in bare peat in disturbed ground conditions. Furthermore, the viability of buried seed declined from 90% to 20% in three years, due to a combination of decay in the soil and premature germination, ie germination soon after dispersal, in or on top of the litter zone in conditions inimical for seedling growth, establishment and survival (Welch et al. 2000).
Contrary to this, Ridley (1930) quotes numerous reports of birds of the thrush, crow, pigeon and grouse families feeding on Bilberries in areas of Britain and Europe, while on the other hand, in their book Birds and Berries, Snow & Snow (1988) make no mention of this happening, and these authors almost totally overlook Bilberry. On the basis of these reports we might conclude, as have some other studies of British moorland soils, that the seed bank of V. myrtillus is virtually non-existent. It certainly contrasts very strongly with the huge seed store of Calluna vulgaris that exists under similar habitat conditions eg 52,900 viable seed/m² (Hester et al. 1991).
Seed dispersal and seedling rarity
Despite the above, it appears that Bilberry does achieve occasional long-distance jump-dispersal to new 'vacant' sites by being transported by frugivores (ie fruit-eating birds and mammals). Guitian et al. (1994) reported migratory flocks of Redwings (Turdus iliacus coburni) consuming wild fruits of Empetrum nigrum (Crowberry), Vaccinium uliginosum (Bog Bilberry), V. myrtillus, Arctostaphylos uva-ursi (Bearberry) and Rubus saxitilis (Stone Bramble) in SW Iceland. This form of transport occurs at an enormous expense in terms of lost seed (Flower-Ellis 1971). Low levels of seed persistence in soil and the observed rarity of seedlings in the field in sites where the species grows are very probably due to poor defence of the propagules from pathogens and predators in the soil and, rather unexpectedly, a poor competitive ability after germination (Welch et al. 2000).
The most likely fruit and seed consumers in the Fermanagh area are sheep, small rodents ie mice and voles (probably very frequent) and, much more occasionally, birds of the thrush family. The habit of rodent species storing food in caches may apply to bilberries in moorland and, since this could assist dispersal, it deserves further study (Price & Jenkins 1986). However, some of the potential animal vectors are probably as much seed predators as transporters (Janzen 1969, 1971); the predators include insects, as well as birds and mammals. None of the mammalian dung examined in two studies in NE Scotland produced any seedlings of V. myrtillus and it therefore appears likely that passage of the seed through the gut of animals as different as cattle and voles most of the time effectively destroys embryo viability (Welch 1985; Welch et al. 2000). On the other hand, game bird studies of Red Grouse and Capercaille have reported viable seedlings growing on their dung in parts of England and Scotland (Welsh et al. 2000).
Another significant observation which helps explain the rarity of V. myrtillus seedlings on moors and in woodland is the extremely slow growth that Bilberry seedlings are capable of achieving in damp, cool, acidic, nitrogen-limited and other nutrient-deficient soil, which means that a rhizome does not develop until at least the third year of a plant's life (Flower-Ellis 1971). The result of this limitation is that the young seedling cannot extend laterally to escape competition from other more rapidly growing ericoid or graminoid seedlings, or from the established moorland or woodland ground level sward surrounding it.
Vegetative reproduction and clonal spread
It is clear from the above that in both shaded and more exposed conditions, the photosynthetic resources and growth of Bilberry is often directed not towards sexual reproduction, but rather to vigorous vegetative spread achieved by underground horizontal extension of its sympodial branching, woody rhizome. Thus many Bilberry plants form large clonal patches of indeterminate, but presumably often considerable age, in stable woodland, scrub or exposed heath situations (Richie 1956). Numerous observations indicate that recruitment from seedlings is infrequent in many species of clonal plants, perhaps especially in those which, like Bilberry, advance using a 'phalanx growth form' (ie tightly aggregated shoots or ramets, with clonal extension growth confined to the perimeter); V. myrtillus appears to be a good exemplar of this type of primarily vegetative reproductive behaviour, which directs the long-term growth and dispersal of the species in a locality (Eriksson 1989).
Response to sheep grazing
The phenology of Bilberry is unusual for a shrubby plant in B & I in being deciduous yet having angular stems which stay green and relatively free of woody tissue during the winter months, so that considerable winter and spring utilisation by browsing herbivores may be expected. Bilberry growth also starts earlier in the year than in most other moorland plants (Welch et al. 1994) and so it is liable to be heavily grazed by sheep or cattle when nutritious, digestible fodder is typically at its most scarce in February and March. In mixed stands with Calluna vulgaris and in ± pure Bilberry heaths in the Derbyshire Peak District, sheep given all year access consistently chose to graze the Bilberry swards much more heavily in autumn (August-October) than in the rest of the year, while for C. vulgaris the increased grazing pressure fell between October and January.
Autumn peaks in carbohydrate (Stewart & Bannister 1973) and fructose and glucose (Pakonen et al. 1991) contents have been found in Bilberry and this, coinciding with a sharp decline in the nutrient content and digestibility of moorland grasses in autumn, appears to shift sheep feeding preferences towards ericoids at this season (Powell & Malcolm 1974). In mixed stands, it is often noticeable that Heather and Bilberry exist in distinct patches, each of which has one species dominant and the other absent or very subsidiary. Under three different experimental grazing regimes, these mixed heaths showed a sharp decline in Bilberry cover and an equally marked increase in C. vulgaris cover and height (Welch 1998). Flowering and fruiting will be more depressed (at least in the short term) by grazing in Bilberry than in Heather, since V. myrtillus flowers develop on the previous year's shoots, whereas C. vulgaris flowers appear on the current year's shoots, which generally are not sheep browsed until wintertime.
Insect herbivores
Many insects feed on Bilberry, predominantly the larvae of geometrids, tortrids and sawflies. These larvae utilise leaves, bark, buds and reproductive parts of the plant as their sole food. High larval densities in late June and early July coincide with the hatching period of many woodland breeding insectivorous birds which rely, at least in part, on the larvae to feed their nestling broods. Exclusion of birds from experimental Bilberry plots in Swedish boreal Pine, Spruce, Birch forests found that the total density of larvae was 63% lower outside the enclosures where the birds had access and that the scale and type of damage to Bilberry annual shoots by the insect larvae was significantly affecting the competitive potential and reproductive performance of the shrub (Atlegrim 1989).
Other damage and comparative recovery potential
In common with the woody rhizome, branching of aerial ramets in V. myrtillus is sympodial and, since apical dominance is weak and dormant buds are abundant, branching is both frequent and flexible, allowing compensatory growth to occur in the different habitat-related light levels which bushes experience after environmental stress caused by eg herbivory, late frost, burning, trampling or fungal attack (Tolvanen 1995). The woody stems of Bilberry and other ericaceous subshrubs, however, are very limited in their ability to tolerate trampling wear and tear and their recovery is slow to negligible (Cole & Spildie 1998; Malmivaara et al. 2002). Since the rhizome typically grows at depths of 15-20 cm, which is somewhat deeper than C. vulgaris roots, it is generally believed that regular burning of heathland favours Bilberry over Heather, but further studies to monitor the composition of mixed Bilberry-Heather stands over the whole cycle of burning are required to test this hypothesis scientifically (Welch 1998). In comparison with its evergreen relative and associate, V. vitis-idaea, the deciduous growth form of V. myrtillus increases its regrowth potential after damage by the various factors under consideration here, since its photosynthetic and growth rates are both greater than those of the former shrub (Tolvanen 1995).
British and Irish occurrence
The New Atlas hectad map shows that in Britain, V. myrtillus has a very marked N & W distribution, being almost entirely absent from south central and eastern England, which broadly corresponds with the major areas of chalk and limestone geology. In Ireland, by comparison, V. myrtillus is shown at the hectad scale as being much more widespread throughout, although it is undoubtedly better represented in the N & S of the island. Elsewhere, it is much more restricted to higher ground. In Fermanagh, Bilberry is much more frequent and abundant on the higher ground on the Western Plateau than elsewhere, but it is well scattered throughout the county and lowland sites include lakeshore woods, scrub and peatlands. In the wet oceanic climate of W Ireland, regular, heavy rainfall and long-term leaching is known to induce shallow, acid peaty ranker soil formation directly on top of limestone rock and, in Fermanagh, some of these profiles support at least stunted Bilberry bushes.
European and world distribution
Widespread in the boreal Eurosiberian region from Iceland eastwards through Fennoscandia to C Siberia and beyond to N & C China. The range also reaches southwards more or less continuously to the Alps and the other middle latitude mountain alpine and subalpine forest zones in Europe, from N Portugal eastwards to the Caucasus. It has also been reported from Alangorssuaq Island in S Greenland (Böcher et al. 1968), but there is a suggestion that it may be introduced here (Hultén & Fries 1986, Map 1462).
Uses
In the past, berries were often collected and eaten raw − preferably with cream, or used to make pies, puddings, jam and jelly, or to add to wine or spirits to manufacture liqueurs, practices which at least locally appear to have declined or died out. Grigson (1987) reports that the berries were used for dyeing in Scotland and Ireland and that in the past the branches were fashioned into brooms and whisks to use in spraying the potato crop with Bordeaux mixture against blight. In the Hebrides, Bilberry leaves were used as a tea substitute and, being a strong astringent and diuretic, it was used medicinally for dissolving kidney stones and the treating of other ailments of the urinary tract. Allen & Hatfield (2004, p. 123) catalogue numerous other medicinal uses which closely mirror those of 'heather' (ie the more common species of Erica plus C. vulgaris). As a result V. myrtillus has accumulated many local names.
Names
The origin of the genus name 'Vaccinium' is something of a mystery. It appears to be a Latinised name of great antiquity said to date back to the same prehistoric Mediterranean language (ie Thraco-pelasgian) as the Greek 'Hyakinthos', ie commonly known as 'Hyacinthus'. Hyakinthos is the pre-Hellenic name of a boy god beloved of Apollo in Homer's poems and Greek myth, accidentally killed by a discus which was diverted to hit him by the jealous wind god Zephyrus. From his blood the poets say a flower sprang marked by Apollo's cries of grief, 'AI AI' (Radice 1973). Alternatively 'hyakinthos' may be a word in the ancient language describing the blue colour of water! It is thought that perhaps, or even probably, 'Vaccinium' is a corruption of that name, but it depends upon the authority one consults (Gilbert-Carter 1964; Gledhill 1985; Stearn 1992). From the current author's low level of awareness of the linguistics, that explanation of the name and its supposed transition to 'Vaccinium' does appear to be a very long shot indeed! The Latin 'vaccinus' is an adjective meaning 'dun in colour' (Stearn 1973), but this is not regarded as relevant, and neither is 'vacca', meaning 'a cow' (Gilbert-Carter 1964). The Latin specific epithet 'myrtillus' is a diminutive literally meaning 'a small myrtle', ie likening the blue berry to a small myrtle-like fruit (Gilbert-Carter 1964; Stearn 1992).
The English common name 'Bilberry' dates from the 16th century, and as with the Latinised genus name, its origin is uncertain. The first element 'bil' is probably of Scandinavian origin, as is 'blae', 'blea' (or 'blaa' in Old Scandinavian) meaning 'blue-black' of the 15th century English common name 'Blaeberry' (Grigson 1974). The widespread alternative name 'Whortleberry' is very probably a corruption of 'Myrtleberry', and 'Hurtleberry', 'Hurtberry', 'Hurts', 'Whorts', Huckleberry', 'Hartberry' are all likewise corruptions of one another linked to 'Myrtleberry', and used as local names around these islands (Prior 1879, p.123). 'Wimberry' on the other hand may derive from 'Wine-berry', a reference to a more sociable use of the plant.
Threats
Although currently still widespread and common throughout B & I in suitable habitats, the limited ability of the species to reproduce sexually, especially the low level of representation in the soil seed bank and the virtual lack of successful seedling establishment, resulting in a nearly total reliance on vegetative growth for maintenance and spread of existing populations, makes V. myrtillus vulnerable to local extinction and fragmentation of its distribution following any excessive environmental stress or stresses it encounters. The fungal pathogen Phytophthora ramosum, that recently arrived in B & I from N America and is rapidly spreading north and west, has a very wide host range which includes Rhododendron and other ericaceous shrubs including V. myrtillus. The disease has not yet appeared on this genus anywhere in Ireland, but we must not be complacent.