Juniper (Juniperus communis)

Juniper, an important understorey shrub or small tree in the Caledonian Forest, has declined recently and is now the subject of conservation concern.

Global distribution

Common juniper has the largest geographic range of any woody plant in the world. It is circumboreal in distribution, occurring from western Alaska throughout Canada and northern parts of the USA, in coastal areas of Greenland, in Iceland, throughout Europe and in northern Asia and Japan. It is widespread in Europe, except for some low-lying areas around the Mediterranean, and it also occurs in North Africa. In North America it extends from beyond the northern limit of trees south to the Carolinas and the mountains of California, Arizona and New Mexico.

Throughout its range, juniper occurs at varying elevations, and at its southernmost extent it has been recorded at altitudes of up to 3,500 metres. Several subspecies or varieties have been described, but further research is needed to clarify the taxonomic details of these.

Distribution in Scotland

Juniper occurs throughout most of Scotland, including the Borders area, Orkney and some of the Hebridean islands, but it is only common in the Highlands. Two subspecies are found in Scotland, of which the erect, shrubby form, Juniperus communis ssp. communis , is most widespread, while the prostrate form, Juniperus communis ssp. nana , occurs mainly as a component of the montane scrub vegetation community, at the tree line in the mountains, where it can reach an elevation of 1,000 metres.

In the Highlands, juniper is more abundant in the drier, eastern part of the country, where it reaches a larger size. Good examples of large junipers can be seen in the Cairngorms area, for example at Rothiemurchus, while some of the best examples of prostrate juniper occur on Beinn Eighe in Wester Ross.

As a shade-intolerant species, juniper is found in more open types of woodland, typically birch woods or pine woods in the Highlands.

Because of its vast geographic range, juniper is not considered threatened at an international level. However, in Britain there has been a substantial decline in both the distribution of juniper and the size of juniper colonies, particularly in England. As a result, juniper is now the subject of a Biodiversity Action Plan (BAP), under the government's response to the Convention on Biological Diversity agreed at the 1992 Earth Summit.

Juniper is an evergreen shrub or small tree in the cypress, or Cupressaceae , family. It is slow-growing and in Scotland few specimens are taller than 5 metres, although heights of up to 10 metres have been recorded elsewhere in its range. It displays several different growth forms, which vary from erect and columnar to bushy, spreading or mat-forming and shrublike. Juniper is unusual in being able to grow on both acid and alkaline soils.

The foliage consists of small blue-green needles which are up to 1 cm. long and have a broad white stripe, or stomatal band, on their inside surface. The needles grow in alternating whorls of three on the twigs, and are prickly to the touch. The bark is brown on young plants, but turns grey as it gets older, and is shed in thin strips.

Bushes live on average for about 100-120 years, while the oldest recorded juniper in the UK was aged at 255 years, based on counting the annual growth rings.

Juniper is dioecious, which means that individual plants are either male or female, unlike most tree species, where both male and female flowers occur on the same tree. Male flowers appear as yellow blossoms near the ends of the twigs in spring and release pollen, which is wind-dispersed. Female flowers are in the form of very small clusters of scales, and after pollination by the wind, these grow on to become berry-like cones. Shaped like irregularly-sided spheres, the berries are green at first, but ripen after 18 months to a dark, blue-purple colour and are 0.6 cm. in diameter. Each berry contains 3 - 6 seeds, although some of these are often infertile, due to insect infestations. The seeds are triangular, hard and black, and are dispersed by birds which eat the berries.

Because of their long ripening period, berries occur on juniper throughout the year, and it is usually possible to see them at different stages of development on the same plant. The seeds are slow to germinate and normally require two winters of dormancy before they will sprout and begin growing. Juniper is a slow-growing species, and in Scotland it typically grows about 3-5 cm. per year. In good growing conditions, it can grow up to 28 cm. in a year, and flowering and seed production begins when the plants are 7-10 years old. 

Juniper can also reproduce vegetatively, with this typically occurring more in old, dying stands. Old bushes sometimes collapse, and where their branches touch the ground, they will form roots and produce new growth, so that a circular pattern of plants forms around the original bush. 

In contrast to many tree species, juniper has comparatively few mycorrhizal fungi associated with it. In these mutualistic or symbiotic relationships between trees and fungi, both partners in the association benefit from their interactions, through an exchange of nutrients between their root systems. In the case of juniper, 5 arbuscular mycorrhizal species (those where the fungal hyphae penetrate the cells of the tree's roots) and 2 ectomycorrhizal species (where the fungus surrounds the tree roots, without penetrating them) are known.

Two species of rust fungus occur on juniper, and these induce galls to form on the stems and twigs. One of these species (Gymnosporangium clavariiforme) produces yellow or lemon fruiting bodies on the galls it gives rise to, and the spores from these in turn produce galls on hawthorn (Crataegus monogyna) which then infect juniper again. The other rust fungus (Gymnosporangium cornutum) produces brown blobs on the needles of juniper, and then also has another phase on a different tree species, in this case rowan (Sorbus aucuparia), where it produces galls in the form of yellowish pustules on the upper surface of the leaves. Juniper is susceptible to some pathogenic fungi, including juniper blight (Phomopsis juniperovora), phytophthora root rot (Phytophthora cinnamomi) and brown felt blight (Herpotrichia juniperi), which grows on the needles.

Galls are also induced on juniper by two gall midges (Oligotrophus juniperus and Oligotrophus panteli). Adults of these species lay their eggs on new terminal shoots in late spring, and the larvae burrow into the buds, where the galls develop the following spring as whorls of modified needles which protect the larval chamber. The adult midges emerge from the galls after another year.

A total of 42 species of phytophagous (ie plant-eating) insects are associated with juniper, although some of these have originated from non-native ornamental junipers which have been introduced to the UK as garden plants. These feed on different parts of the plant, such as the needles and the seeds, and insect predation on the seeds is the main causal factor for the relatively low rate of fertility amongst the seeds in the berries. Insects which damage the seeds include the juniper seed chalcid (Megastigmus kuntzei), the juniper shield bug (Elasmostethus tristriatus) and the juniper berry mite (Trisetacus quadrisetus), which has been shown in some situations to affect up to 82% of the berries.

Larvae of the juniper berry miner moth (Argyresthia praecocella) also feed on the seeds, and this is one of 5 species of Argyresthia moths whose larvae live on juniper - the other species make mines in the leaves and buds. 4 species of Geometrid moths are associated with juniper, including the juniper carpet moth (Thera juniperat) and the juniper pug (Eupithecia pusillata). The larvae of a rare moth (Dichomeris juniperella) which is associated with juniper live in tight silk webs the size of golf balls, amongst the needles. The adults of these moths are inconspicuous and rarely seen, but the species is characteristic of the Caledonian Pine Forest in Speyside and Deeside.

Because of its dense cover of prickly needles, juniper provides a good nesting site for birds such as the goldcrest (Regulus regulus) and the song thrush (Turdus philomelos), and it is also important in providing winter cover for the black grouse (Tetrao tetrix). The berries are eaten, and the seeds distributed, by birds such as the fieldfare (Turdus pilais), which is a seasonal migrant from Scandinavia, and the ring ouzel (Turdus torquatus).

Juniper foliage is eaten by mammals such as red deer (Cervus elaphus) and rabbits (Oryctolagus cuniculus), and the current excessive populations of both of these species has contributed to the current decline in juniper's distribution in the UK. The burning of moorland also limits the ability of juniper to regenerate.

People have had a long association with juniper, through the use of the oil obtained from its berries for flavouring gin, and the species now requires human assistance to recover and expand its range again - this is the intention of the current Biodiversity Action Plan.

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