Plant of the Week – 28th December 2020 – Juniper (Juniperus communis L.)

Juniperus communis (Juniper) is an evergreen dioecious gymnosperm shrub, one of only three native gymnosperms in the British Isles.  There are two main population centres in Britain, one southern and the other northern (Fig 1). Those in the south are more often the subspecies communis whilst the northern representatives are often the dwarf form,  ssp. nana. A third, southern European, ssp. hemisphaerica, has two small populations on maritime cliffs in the UK.

Fig. 1. Distribution maps. Left, undifferentiated records of Juniperus communis; centre, ssp. communis; right, ssp. nana. Note that ssp. nana has a single centre in NW Scotland whereas ssp communis has three centres, one in a different region of northern Scotland, one in northern England and one in southern England. Downloaded 24.12.20. By permission of the Botanical Society of Britain and Ireland.

Morphology:   It can be a small tree (up to 11 metres in ssp. communis), a spreading bush or, in the case of spp. nana, a somewhat procumbent shrub. The distinction between the two subspecies is not merely on height. Ssp. nana has smaller leaves (up to 15 mm), whereas the leaves of  ssp. communis are up to 20 mm. Both leaves are pointed but those of nana are described as mucronate (a mucro is a sharp point, and ‘mucronate’ is a sharp point made by an abrupt narrowing of the leaf tip). Another difference is that the leaves of ssp. communis form a right-angle with the stem but those of ssp. nana meet the stem with a lower angle. Yet another difference is that the berries of ssp. communis have globose fruits but those of nana are longer than broad.

Most Cupressaceae have juvenile leaves at first, then adult leaves. Juniperus communis however never develops the adult leaves. These ‘juvenile leaves’ are in whorls of three, with a single broad white stomatal band on the upper side. Like most conifers, leaves remain on the plant for several years, usually three years. Twigs are circular in cross-section, slender, smooth and often shiny. Shoots are spreading in three dimensions, not flattened into one plane as in other genera of the Cupressaceae (think of Lawson’s cypress Chamaecyparis lawsoniana). 

Fig. 2. Juniperus communis, forming an understorey beneath Betula, Tullochgrue, Cairngorm Mountains, Scotland. Photo: Chris Jeffree.

Reproduction: Like all conifers, juniper is wind-pollinated.  Male cones are yellow, produced in March-April.  Female cones are pollinated in July, but fertilization occurs 12-13 months later.  The ‘berries’ ripen over a period of one or two years, turning blue-black by September/October of their third year.  They are eaten by birds and field rodents.  Germination, which may be delayed by up to 5 years, normally takes place in April-May. There is no long‐term seed bank.

Fig. 3. The variety of cone-types seen within the genus Juniperus. Left: J. communis, showing ‘berries’ which are green at first then becoming black; centre: J. osteosperma, Monument Valley, Utah; right: J. cedrus, above Aguamansa, Tenerife. Photos and information: Chris Jeffree.

Habitat and distribution: Juniperus communis has an enormous world-wide distribution, possibly greater than that of any other tree or shrub. It spans the globe in the northern hemisphere and in Europe is occurs from the Arctic Circle to southern Europe, with a few records in north Africa (Fig. 4).

Fig. 4. Global distribution of Juniperus communis. Downloaded from GBIF, 24.12.20.

In the British Isles the southern plant communities are: (i) grasslands of Festuca ovina–Hieracium pilosella–Thymus praecox/pulegioides); (ii) nutrient poor grasslands with Cladonia  as seen at Porton Down, Wiltshire; (iii) freely-draining calcareous  hawthorn scrub; (iv) on shallower, drier soils on steeper and more exposed sites where it is the main nurse plant necessary for the establishment of yew seedlings (Watt 1934), leading to Taxus baccata woodland in the south‐east and elsewhere such as the sea cliffs of north Wales [see my notes for the Taxus baccata PotW].

In Scotland it is often associated with Scots pine and birch, but it is also a component of many vegetation types. It is a fairly abundant and frequent member of (i) the Salix repens–Empetrum nigrum subcommunity of the Dryas octopetala–Carex flacca heath in the lowlands of north‐west Scotland; (ii) wind‐swept short carpets of shrubs typical of the Armeria maritima subcommunity of the Calluna vulgaris–Scilla verna heath on cliffs of the west coast of Britain; (iii) in tree-line vegetation it may form a narrow belt between pine and dwarf-shrub heath (Birks 1977); (iv) in the north-west Highlands on Calluna vulgaris–Racomitrium lanuginosum heath; (v) on very exposed ridges of higher altitudes in north‐west Scotland, and to a lesser extent at lower altitudes in the Viola riviniana–Thymus praecox subcommunity of the Vaccinium myrtillus–Racomitrium lanuginosum heath. Some of these plant communities are shown in the fine watercolours by Barbara Nicholson, in The Oxford Book of Trees (Nicholson and Clapham 1975).

Throughout its British range it is more likely to be found as an occasional component of a plant community rather than as a more or less pure stand, and Juniper woodlands sensu stricto are rare and occupy just a small area (eg Gilbert 1980, Forbes and Proctor 1986, Lavery 2016).

Conservation. Its range in the British Isles has shrunk in recent decades. General awareness of its status as a native conifer have led to ‘Juniper recovery plans’ by various agencies, and publication of advice on cultivation (Broome 2003). It is designated a priority species in the UK Biodiversity Action Plan (Plantlife 2004) and is one of the key woodland species identified for action under the Scottish Forestry Strategy 2006. It is still considered ‘vulnerable’, being sensitive to heavy grazing, fire, disease and shading by tree cover. A useful and most detailed analysis of the health of juniper in Perth and Kinross was commissioned by Scottish Natural Heritage1 (Lavery 2016).

Mycorrhizas:   It is mostly endomycorrhizal although ectomycorrhizas have occasionally been reported (Harley & Harley 1987). The following arbuscular fungi were found  by Błaszkowski et al. (1998): Acaulospora lacunosa, A. paulinae., Glomus deserticola, G. constrictum, G. dominikii, G. fasciculatum, G. ?geosporum, G. ?heterosporum, Scutellospora dipurpurescens and S. pellucida.

Predators, parasites and pathogens:    In the British Isles, ripe juniper ‘berries’ and seeds are eaten by the woodmouse and possibly by bank voles and field voles (Gilbert 1980; Hester 1995; García et al. 2000a).  They are also dispersed by the mistle thrush, blackbird, song thrush and, in Scottish pinewoods, possibly by the crested tit. These relationships are similar to those between oaks, squirrels and jays, and cannot be described as being purely predatory.  Sheep may shelter under the larger, prostrate juniper shrubs, push the main stems over, and rub against the bark so as to expose the heartwood.  Although juniper heartwood contains fungicidal substances, I am convinced that this exposes live tissues to pathogens and thus hastens the mortality of mature junipers (personal observation in the Pentland Hills, south of Edinburgh).

Fig. 5. Infestation by a rust, Gymnosporangium. The species of juniper here is likely Juniperus phoenicea. Photo and information: Chris Jeffree.

Like all higher plants, juniper has parasites and pathogens.  There are many insects of several orders, and mites, too numerous to mention.  Some of these infest the female cones and seeds.  There are also parasitic fungi, too numerous to list (they are listed in an Appendix to Thomas et al. 2007), one of which deserves special mention.  This is Phytophthora austrocedri (a.k.a. P. austrocedrae), a member of the class Oomyceta, and therefore a fungus only in an honorary sense: its relationship to true fungi (Ascomycetes and Basidiomycetes, class Dikarya) is exceedingly remote.  This pathogen was described on Austrocedrus chilensis in 2007 (Greslebin et al. 2007; Greslebin and Hansen 2010), but is thought to have been present in Argentina for at least 50 years before that, and possibly/probably arrived from elsewhere (Henricot et al. 2017).  All species of Cupressaceae are susceptible.  It usually attacks the roots and root collars, moving up into the bases of stems (trunks), and the lower parts of branches: the pathogen can extend 50cm or more up diseased stems.  It was first reported in the Scotland in 2011, then found more widespread in 2012-2013 (Green et al. 2014).  Since not even Phytophthora can cross the Atlantic unaided, it must be assumed that in had human assistance in its distribution: Homo sapiens, his machines and appurtenances are now the principal vectors of plant pathogens around the globe (however, the strain in the BI is not the same as the one in Argentina, so the story may be more complicated).  Significant outbreaks have occurred in the Cairngorms National Park; at a Site of Special Scientific Interest at Glen Artney, Perthshire; in the Upper Teesdale National Nature Reserve; and at various sites in the Cumbrian Lake District.  There have been smaller outbreaks elsewhere, including southern Scotland.  It is now causing widespread mortality of J. communis in the British Isles, and is of grave concern to everyone interested in the conservation of juniper.

Fig. 6. The Juniper Tree, a gruesome fairy story. Illustration by Louis Rhead -from a 1917 edition of Grimms Fairy Tales., PD-US, Attribution 2.5 Generic (CC BY 2.5).

Physiology:     Most research on the species has focussed on the pharmacological properties, chemistry and phyto-pathology. Juniper has not received the attention of eco-physiologists on anything like the scale of such gymnosperms as Pinus sylvestris and Picea abies, because it is not of such great commercial importance. We do however know that Juniperus communis is slow-growing, intolerant of deep shade (Grubb et al. 1996) and drought tolerant (e.g. Lysova 1980). There has been work on its epicuticular waxes: Dodd & Poveda (2003) found that the alkane chains of these waxes of Juniperus communis ssp. nana were longest at low and high elevations in the Pyrenees. This correlated with temperature and an aridity index, and is possibly a result of adaptation to minimize cuticular transpiration during hot summer temperature at low elevation and physiological drought caused by freezing temperatures at high elevation. 

It is cold‐tolerant and will survive to a minimum of 150 growing degree days (calculated after budburst and using a 5°C threshold); this is comparable to Betula pubescens.

Uses of Juniper wood: In Scandinavia, juniper wood has been used for making small items of  decorative kitchenware, including cutting boards and butter knives. In former times its great strength made it suitable to form the dowels (trenails or treenails) in wooden shipbuilding.  In Estonia juniper wood is valued for its pleasant and long lasting aroma (like many Cupressaceae), its very decorative annual rings, as well as its density and strength. In France, it is frequently used to make handles for pocket-knives.

On Easter Monday boys in Kashubian (Northern Poland) chase girls, whipping their legs gently with juniper twigs. This is to bring good fortune in love to the chased girls.

Fig. 7. Butter knife made of Juniper wood, made in Finland. So much better for spreading butter than a steel one, and just as long-lasting. Photo: John Grace.

Culinary uses: We know of no-one who likes to eat the raw berries, but dried berries  are widely used to flavour meats, sauces, and stuffings. Since juniper berries have a strong taste, they should be used sparingly. They are generally used to enhance meat with a strong flavour, such as game or tongue.

The berries are used to flavour certain beers and of course gin (the word “gin” derives from an Old French word meaning “juniper”). In Finland, juniper is used as a key ingredient in making sahti, a traditional Finnish ale.  The Slovak alcoholic beverage Borovička and Dutch Jenever are flavoured with juniper berry or its extract. Juniper is also used in the traditional farmhouse ales of Norway.

Juniper-flavoured chocolate has recently become popular.

Beware, over-consumption of berries may damage the kidneys (Hensel 2008). In The Juniper Tree, a fairy tale by the brothers Grimm, a woman becomes gravely ill after eating juniper berries. Later though, after unexpectedly giving birth to a baby, she dies of happiness. In 1990 the story became an award-winning Icelandic film  directed and written by Nietzchka Keene. Squeamish people should not read the story or watch the film.

Fig. 8. Dried berries for culinary use, available in UK supermarkets. Photo: John Grace.

Traditional medicine: Pliny the Elder, in his epic work Natural History, written about two thousands of years ago, tells us that Juniper berries may be chewed for toothache (“the berry breaks the teeth and makes them fall out”). He is sceptical about oral use of the juice and its topical use as a contraceptive. Culpepper (1653) tells how country people may eat ten or a dozen ripe berries every morning; among the many benefits of this practice is that “they strengthen the brain exceedingly, help the memory and fortify the sight..” Moreover, Juniper berries have long been used as medicine by many cultures including the Navajo people (McCabe et al. 2005). Western American tribes combined the berries of Juniperus communis with Berberis root bark in a herbal tea. Native Americans also used juniper berries as a female contraceptive (Tilford, 1997).  Juniper is listed in at least one modern pharmacopeia (Hensel 2008); however, we are warned that “prolonged use may lead to kidney damage”.

by Roger West

1Scottish Natural Heritage is now called NatureScot.


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