Juniperus communis L., Juniper

Account Summary

Growth form and preferred habitats

This evergreen, dioecious conifer varies from a prostrate, dwarf in exposed upland and coastal situations, to an upright shrub up to 10 m tall usually growing in damp, lakeshore or riverbank sites but it can also occur over limestone, chalk and slate soils on heaths, moors and in pine and birch woodlands.

The prickly, spreading needle-like leaves borne in whorls of three represent so-called 'juvenile foliage', and the subsequent adult foliage is comprised of tightly appressed overlapping scale leaves in opposite pairs, resembling the genus Cupressus (Cypress). All, or almost all forms of juniper occurring naturally in Britain and Ireland produce only more or less prickly, linear, juvenile foliage. Fertilised female cones ripen in their second or third year, the overlapping scales swelling and becoming fleshy to form the familiar 5 mm, blackish globular edible berry-like fruit.

Fermanagh occurrence

J. communis has been recorded in a total of 16 Fermanagh tetrads, 14 of which contain post-1975 records. They probably constitute juniper's main stronghold in Northern Ireland following its decline in Co Antrim in the 1980s and 1990s (N Ireland Vascular Plant Database 2002).

Response to grazing pressure

The juniper population decline in Co Antrim is very probably a direct result of the drastically increased grazing pressure on the Antrim plateau due to European Community sheep headage payments in the 1970s which encouraged overstocking of upland pastures.

In the Republic of Ireland, a similar outcome has been found in upland grasslands in Connemara, causing peat erosion and a loss of species diversity (Bleasdale & Sheehy Skeffington 1995). Studies on J. communis conservation and regeneration in the English Chiltern downlands, the Lake District and in the Scottish highlands, all indicate that the species is sensitive to grazing pressure, particularly by sheep (Fitter & Jennings 1975; Ward & Lakhani 1977; Miles & Kinnard 1979a & b; Dearnley & Duckett 1999).

British occurrence

Juniper is a locally common and very variable shrub in various areas of Britain over a wide range of soils and habitats, but it is most widespread in the north and west of the country and is especially frequent in Scotland. It occurs in NW Wales and is also rather thinly scattered in southern England, where there have been many lowland extinctions over the last century or so (Ward 1981; Preston et al. 2002).

Reproduction

In common with most other species, juniper requires open, bare ground or very short turf for regeneration from seed to occur, and while the necessary vegetation gaps are usually present on steep slopes, on more level terrain openings that might permit colonisation are generally provided by heavy grazing pressure, or more rarely by fire. After germination takes place it is vital for the establishment and longer-term survival of juniper seedlings that they are allowed to grow on under managed conditions providing a protected, considerably lightened grazing regime.

With respect to seedling survival, the seasonal timing of sheep browsing, as well as the extent or degree of grazing pressure, has been shown to be significant in chalk downlands in Oxfordshire. Autumn and winter grazing increases juniper seedling mortality and stunting, whereas summer grazing of the grassland is apparently beneficial: the sheep having sufficient browsing material, simply leave the young juniper plantlets alone, and their grazing curtails competition from grasses, tall herbs and hawthorn scrub without killing the juniper (Fitter & Jennings 1975).

Variation

In Fermanagh, all juniper shrubs are confined to the limestone areas of the county, on scarps, scree, pavement and rocky grassland. It is thus restricted to the Monawilkin, Hanging Rock, Marlbank and Florencecourt areas of the county. Although the growth form in Fermanagh is invariably prostrate, the plants are almost certainly exposed ecotypes of subsp. communis. In truth, however, this form of the plant is very difficult to distinguish from subsp. nana (Hook.) Syme in many parts of Ireland, and probably the same is true elsewhere. It is therefore possible that the latter may also occur in some of our most exposed Fermanagh sites (Webb & Scannell 1983; Dearnley & Duckett 1999).

In many places in Fermanagh, juniper grows on scarps and rocky ground closely associated with Taxus baccata (Yew). Some individual juniper plants cover several square metres, and since prostrate growth forms of the species are very slow-growing, this suggests they might possibly be ancient clones. Studies elsewhere in Britain, however, have shown that juniper is a notoriously difficult species to accurately age without cutting live samples of the stem – a destructive process and something we could not justify under any circumstances. The difficulty in measuring the age of individual specimens arises because, firstly, the stem diameter of J. communis is not closely related to age, and secondly, the stems are usually eccentric in shape. This makes sampling for girth measurements or taking cores to ring count for age an inherently inaccurate process (Dearnley & Duckett 1999).

Seed viability

The maintenance of high seed viability has been shown in several studies in England to be important for the conservation of J. communis (Ward 1989). In the English Lake District, a study found that sites with large populations of juniper (ie more than 1,000 bushes) had significantly higher seed viability than those in small populations, and a reference site which had for 70 years been protected from sheep grazing, produced the greatest juniper seed viability index of all (Dearnley & Duckett 1999).

As far as we are aware, no study has yet been made of the ability of juniper in N Ireland to regenerate, and in the light of the obvious contraction of the species on the Garron Plateau in Co Antrim, not to mention the question of the effect on this northern-montane species of Global warming (or as we have come to experience it over the last decade - Global wetting and winding!), conservation background research work needs to be carried out to ascertain the threats to the species, and its ability to survive at all its northern Irish sites under current management practices and levels of disturbance.

European occurrence

J. communis is very widespread over most of northern and western Europe, while to the south it becomes increasingly rare and mainly a mountain plant (Jalas & Suominen 1973, Map 181).

World occurrence

A very variable, polymorphic species, Hultén & Fries (1986, Map 82) recognised six subspecific taxa (either varieties or subspecies), and they plotted the total species distribution as almost completely circumpolar.

Toxicity and uses

Juniper 'berries' are the ripe female cones of the plant, and they contain high levels of resin (10%), an unnamed essential oil, plus terpene derivatives and a bitter substance (probably an alkaloid), which has been given the name 'juniperine' (Cooper & Johnson 1998). The berries have a long history of culinary use, as a flavouring of both meat dishes and gin. The plant is poisonous if eaten in quantity, being particularly dangerous to pregnant animals since it can cause severe cramps and even abortion. Fortunately, however, no specific cases of such poisoning have been reported in Britain or Ireland (Cooper & Johnston 1998).

Names

The genus name 'Juniperus' is Latin not Greek, being a name first given by Virgil to the plant (Chicheley Plowden 1972). The Latin specific epithet 'communis' means either 'common' or 'clumped', ie 'growing in company' (Gledhill 1985).

Threats

Apart from excessive grazing pressure throughout its range in Northern Ireland, a couple of Fermanagh sites on the Marlbank could be threatened by improvements for agriculture. Elsewhere in parts of both Britain and Ireland there has been loss of scrub habitat suitable for juniper due to burning of moors and heaths, succession to woodland and afforestation (M.E. Braithwaite, in: Preston et al. 2002).

• Find out more about the format of the species accounts
• View abbreviations
• View references for all species accounts
• View a gazetteer of all the sites