Juniper (Juniperis communis) is a highly polymorphic species with a widespread circumpolar distribution. Juniper, like willow, is dioecious. It is a wind-pollinated shrub or small tree and produces cones which develop into berry-like fruits which are readily dispersed by birds. Several subspecies are commonly recognized, which together with intermediate forms provide a source of extensive genetic diversity. As a genus, the junipers are slow-growing conifers of marginal areas as they can survive in full sunlight on dry mineral soils and are capable of withstanding exposure and drought. Juniper is therefore capable ofliving on sand dunes and chalk grasslands, as well as in birch woods and heaths, and as scrub communities at high altitudes and latitudes. Two subspecies are regularly recognized. The typical lowland variety Juniperus communis ssp. com-munis (2n = 22) occurs as a low-growing tree or columnar (fastigiate) bushes or scrub up to and also above the treeline (Fig. 9.19). This subspecies is distinguished by having spreading leaves almost at right angles to the stem, which makes the species prickly to touch. The second form (J. communis ssp. nana syn. alpina, J. sibirica) is a procumbent variety with ascending or loosely appressed leaves which grows on rocks and moors and mountains (Fig. 9.20). It occurs up to 1730 m in central Norway as well as on exposed coastal sites in western Scotland and lowland bogs in western Ireland (Stace, 1997). The is subspecies (J. communis ssp. nana) attracts attention with its northern, circumpolar distribution extending to 70° N at Disko in West Greenland and to the north of Norway where it reaches an altitude of 700 m (Hulten, 1971). Intermediates also occur between these two forms and
other subspecies are recognized in different regions of Europe, Asia, and North America.
In terms of species success in marginal areas in the British Isles the dwarf subspecies favours areas with cool summers and relatively mild winters and a high rainfall (Rodwell, 1991). In the Scottish north-west Highlands Calluna-Juniperus heath is found typically between 300 and 600 m at the junction of the sub- and low-alpine zones except in areas where there has been frequent burning. So sensitive is juniper to fire that there are even places where it has apparently vanished after a single burning (McVean & Ratcliffe, 1962). Juniper also needs protection by the winter snow pack. However, young seedlings do not tolerate prolonged snow cover due to their susceptibility to the brown snow felt fungus Herpotrichia (Holtmeier & Broll, 2005).
Examination by DNA fingerprinting in 12 different populations of juniper from widely dispersed locations has shown that the species as a whole can be considered as two main groups: one from the western hemisphere, and one from the eastern hemisphere (including Greenland and Iceland). There is also a minor Kamchatka population that is dissimilar to any other population so far examined (Adams et al., 2003). This study suggested that the current arctic populations are recent in origin as the present sites for this species in the north were probably covered with ice or otherwise inhospitable for juniper during the late Pleistocene (c. 12 000 BP). The genetic affinities of the differing populations appeared to indicate that the path of recolonization was northward in North America while Greenland appears to have been colonized from Icelandic plants, which in turn came from northern Europe. The Kamchatka population seemed likely to have come from Japan.
In Britain, juniper has two main centres of distribution: a highland zone in the north and west, in which populations of dwarf juniper (J. communis ssp. nana) are still extensive and sexually reproducing, and a southern zone on chalk downlands. In the latter populations of common juniper (J. communis ssp. communis) are small and fragmented and currently suffer from a decline in fertility. Vigorous stands of columnar juniper can be seen in coastal areas in the Baltic as well as in north German heathlands (Fig. 9.19).
It might have been expected that the large sexually viable populations in the north would possess high levels of within-population genetic variation, while the declining southern populations would be genetically depauperate. However, an analysis of amplified fragment length polymorphisms (AFLPs) has shown that all the populations studied had high levels of genetic variation (van der Merwe et al., 2000). Juniper, if not disturbed by fire, can be long lived. On the upper slopes of Ben Eighe (north-west Scotland), many plants are over 100 years old and the oldest individual found in a recent survey was at least 202 years old (Dr Sarah Woodin, pers. com.) while in northern Finnish Lapland there can be found junipers that are almost 1000 years old (Kallio et al., 1971). Thus, even though sexual reproduction may not be frequentlongevity compensates for lack of new recruitment.
Geographical variation in seed production, predation and abortion has been analyzed for 31 populations of juniper in seven distinct regions throughout the species' distribution range in Europe, including both the northern and southern boundaries (Garcia et al., 2000). Seed abortion shows a significant quadratic relationship with latitude, with higher values of abortion at either end of the gradient, especially at the southern limit. The production of filled seeds declined gradually towards both northern and southern distribution limits. In the Mediterranean mountains (southern limit), low seed production coincided with a marked limitation placed upon natural regeneration by summer drought, leading to a demographic bottleneck in populations. Although seed abortion levels were relatively high in the subarctic tundra (northern limit) populations, they were free from predispersal seed predators, which suggests that population viability here may be under less pressure (Garcia et al., 2000).
There also exist equally peripheral lowland areas where common juniper is a characteristic feature of the vegetation provided burning is not frequent. On acid sand dunes in north-east Scotland, Denmark and the Baltic coasts of North Germany, Latvia and Estonia, juniper can form a scrub community along with the nitrogen-fixing sea buckthorn (Hippophae rhamnoides; Fig. 4.36).
In southern Europe, and around the Mediterranean, juniper scrub is a feature of high mountain vegetation, but here again it is vulnerable to human disturbance. Aerial images of the high summits of the Spanish Central Range reveal significant changes in vegetation over the period 1957 to 1991 in which high-mountain grassland communities, formerly dominated by Festuca aragonensis, have been recently replaced by shrub patches of Juniperus communis ssp. alpina along with Cytisus oromediterraneus from lower altitudes. Climatic data for this mountainous region indicate a shift towards warmer conditions since the 1940s, with the shift being particularly marked from 1960. Changes include significantly higher minimum and maximum temperatures, fewer days with snow cover and a redistribution of monthly rainfall. Total yearly precipitation showed no significant variation. There were no marked changes in land use during the period under examination, although there were minor changes in grazing practice in the ninetenth century. It may be that the advance of this drought- and heat-tolerant woody species into higher altitudes is related to climate change (Sanz-Elorza et al., 2003).
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