Juniperus horizontalis Moench
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Basis for Listing
In spite of being geographically widespread in Minnesota, Juniperus horizontalis is quite uncommon. Furthermore, it is invariably localized in small, discontinuous habitats, typically sand dunes and bedrock outcrops. These habitats support a highly specialized community of plants that are maintained in a delicate balance by complex natural forces. They are also in a general state of decline resulting from a disruption of natural processes (specifically fire suppression), invasion of non-native species, incompatible recreational use, and conversion to commercial, residential, or agricultural use. Juniperus horizontalis was listed as a special concern species in Minnesota in 1996.
Juniperus horizontalis is a low-growing, coniferous shrub with long, slender branches that hug the ground. These branches can grow several meters long and produce small branchlets that grow erect to a maximum height of about 25 cm (9.8 in.). The leaves are opposite on the branchlet and persist for 4-5 years. The leaves are of 2 types: juvenile leaves, as found on seedlings and vigorous shoots of mature plants, are needlelike, divergent from the branchlet, and 3-6.5 mm (0.12-0.26 in.) long; adult leaves are scalelike, tightly appressed to the branchlet, and 1-3 mm (0.04-0.12 in.) long. The seed cone is fleshy and berrylike, 5-8 mm (0.20-0.31 in.) in diameter, and bluish to blue-black in color. It contains 3-5 seeds and matures the 2nd year, but remains closed. In many ways, J. horizontalis is a prostrate version of J. virginiana (eastern redcedar), which is an upright tree. In fact, the only reliable way to distinguish the 2 species is growth form. The only other native juniper in Minnesota is J. communis var. depressa (common juniper), which is a shrub, but it is usually at least 1 m (3 ft.) tall with upswept branch tips (Smith 2008).
The primary habitat of J. horizontalis in Minnesota is sand dunes. It seems that nearly every example of an intact, well-developed dune field has a population of J. horizontalis. However, it must be stated that such dune habitat is extremely rare in Minnesota. Talus slopes, cliffs, and exposed bedrock ridges in the southeastern and northeastern counties also provide habitat, although a very small percentage of such habitat actually harbors J. horizontalis.
Biology / Life History
Juniperus horizontalis is dioecious, meaning that each shrub bears either female cones (strobili) or male cones, but not both. The cones are wind-pollinated and the seeds are dispersed in the droppings of animals. The long, prostrate branches lay directly on the ground, and if the conditions are right, they can send down new roots anywhere along their length. In this way, they can enhance their water-collecting capacity and establish large clones that are better able to resist damage or injury. In fact, a single clone can form a dense carpet 20 m (66 ft.) or more across. In favorable habitats the individual branches can grow relatively fast, yet may live only 10-15 years. An entire interconnected clone can live much longer however. Juniperus horizontalis requires direct sunlight at all stages of development and is easily shaded out by trees, other shrubs, and even tall grasses. It is also easily killed by ground fires and does not resprout. On the presettlement landscape, it probably survived only in habitats where fuels were too light to carry a fire, or where there was some naturally occurring firebreak. Paradoxically, the suppression of wildfires that has accompanied settlement has not favored this species. Without fire, the previously open dunes and outcrops have become overgrown by Populus tremuloides (trembling aspen), J. virginiana, and various shrubs, or they have been deliberately planted with non-native conifers. This has effectively crowded J. horizontalis out of its native habitats, and unlike the other native junipers, it does not readily colonize human-created habitats (Smith 2008).
Conservation / Management
Juniperus horizontalis is very sensitive to ground fires. Although the foliage is slow to ignite, if it does catch fire the resins and volatile oils burn explosively. The bark is thin so it does not protect the vital cambium from fire, and the roots will not resprout or sucker after a fire. Essentially, a fire will kill the entire plant and recolonization is not assured. For this reason, populations of J. horizontalis must be protected from fire. The natural habitat of J. horizontalis is sparsely vegetated and not likely to experience wildfire, or at least carry a hot enough fire or one that burns evernly across the ground. The real concern is poorly considered prescribed burns that may be used to clear unwanted brush or encourage fire-adapted grasses or prairie vegetation. This will kill J. horizontalis at anytime of the year. If brush or trees are encroaching into J. horizontalis habitat, they should be controlled by mechanical removal.
Conservation Efforts in Minnesota
Juniperus horizontalis is known to occur at several sites that are owned by public land management agencies and private conservation groups. It is not known, however, if management of any of these sites considers the unique requirements of J. horizontalis.
Adams, R. P. 1993. Juniperus. Pages 412-420 in Flora of North America Editorial Committee, editors. Flora of North America north of Mexico. Volume 2. Oxford University Press, New York, New York.
Smith, W. R. 2008. Trees and shrubs of Minnesota. University of Minnesota Press, Minneapolis. 703 pp.