The recent general decline of the native common juniper (Juniperus communis var. depressa Pursh), which we have been observing in eastern Massachusetts, can be readily explained by the plant's light requirements. A rapid process of reforestation of post-agricultural and other man-made forest openings has resulted in decline of many sun-loving native species including junipers. Fertile, healthy common-juniper clones in eastern Massachusetts have become exceedingly rare. One patch of common juniper growing along Ponkapoag Trail in the Blue Hills Reservation has literally disappeared within five-six years in front of our eyes. That's why our attention was attracted to a large, healthy, fertile population of a prostrate juniper on slopes of a road/ramp crossing a highway and aggressively spreading down into sparse pine forest.

The juniper covers man-made slopes of a ramp in Wareham, MA, where Charge Pond Road crosses a state highway, Rt. 25. Wareham is a Greater Boston's South Shore community and a gateway to Cape Cod: Rt. 25 is leading to Bourne Bridge built across the Cape Cod Canal. Present on both eastern and western slope of the ramp crossing Rt. 25 and on both northern and southern side of the highway, the population is covering an estimated total area of about 0.5 acres (2,000 sq. m), producing abundant cones in its most lighted parts.

A closer examination of samples has shown that the plant, even though much resembling the native common juniper, appears to be Japanese shore juniper, J. conferta Parl.

Introduced to the US less than a hundred years ago, in 1915 (Rehder 1940), shore juniper is now widely cultivated in North America, available at many nurseries. This vigorous prostrate salt-resistant shrub is recommended by the US Department of Agriculture and Massachusetts Office of Coastal Zone Management for slope, beach, and dune stabilization. As far as we know, no cases of naturalization have been reported. This species is not on the Massachusetts Checklist (Sorrie and Somers 1999), not even in the Flora of North America (Watson 1993 in FNA, Vol. 2).

J. conferta superficially resembles the native J. communis var. depressa. Both belong to sect. Oxycedrus, whose members have only acicular (prickly, needle-like) leaves and no scale leaves. The leaves are jointed at base, with a single white band of stomata on the upper (adaxial) side. (The latter character is observed more consistently on live plants or fresh samples. In old herbarium samples white bands may be less pronounced or altogether disappear.)

Another native juniper, J. virginiana L., while belonging to a different section (Sabina), in its juvenile form, when lacking mature scale leaves, may also be confused with common juniper. However, in all representatives of the section Sabina including J. virginiana, the juvenile (prickly) leaves are decurrent, that is, running down onto the branchlet, while in the section Oxycedrus leaves are jointed at base, not decurrent.

Therefore, in Massachusetts the real challenge is to differentiate only between the alien J. conferta and closely related native J. communis var. depressa. A comparison of morphological descriptions yielded the following differences.

1. Leaf and cone size. While in J. communis var. depressa leaves are 0.8-1.8 cm long and cones are only 0.6-1.0 cm in diameter (Fernald 1950), in J. conferta, both leaves and cones are larger: leaves are 1.5-2.5 cm long (Voroshilov 1982), cones to 1.2 cm in diameter (Rehder 1940, Voroshilov 1982).
2. Leaf cross-section. While in J. communis var. depressa leaves are only slightly concave above and bluntly keeled underneath, in J. conferta they are deeply grooved above (becoming folded on drying), their keels more pronounced below.
3. Habit. In the Japanese species, leaves are very crowded, the whole plant thus looking more vigorous, yet at the same time more prostrate and compact, forming denser mats (indeed conferta means 'dense'). While the native prostrate form of common juniper may have branches ascending as high as 1.5 m (Fernald 1950), Japanese shore juniper is strictly procumbent (Rehder 1940), in this respect resembling J. horizontalis Moench. more than any other eastern North American juniper.

The enormous range of J. communis has apparently resulted in development of geographical varieties within the species, var. depressa being only one of a few. Among known varieties of common juniper, there is a northern (Canadian) var. megistocarpa Fernald et H. St. John characterized by abundant, large cones and very depressed, trailing clones. However, in all its prostrate varieties including var. megistocarpa, J. communis appears to retain relatively short leaves—shorter than leaves in its upright forms, much shorter than in J. conferta, and not as dramatically concave.

Sad experience with many introduced plants is to result in much more cautious approach to new introductions. The least vigilance has been applied to alien gymnosperms (conifers), which are less known as invasive plants, as compared to angiosperms. A latent period has been typical for all newly introduced plants that later became invasive, though its duration was different for different species. It definitely takes a conifer a longer time to develop a vigor in a new setting enough for it to become invasive.

One interesting example of an aggressive alien conifer is Norway spruce (Picea abies Karst.), whose year of introduction to the US is unknown because it happened so early. This is one of those few conifers that has had enough time on the North American continent to develop a few generations and attain a menacing vigor; yet Norway spruce is not currently recognized as a threat in many states including Massachusetts. Here it is still playing an important role in plantings, even within reservations. The reason might be that, once it is planted, Norway spruce has to go through a prolonged latent period, during which it appears harmless. From now on, Norway spruce expansion in Massachusetts parks may speed up significantly due to presence of new generations of fertile trees.

The population of Japanese shore juniper in Wareham may also eventually expand faster, especially if there are viable seed in existence. It most probably is originating from a few shrubs planted along the newly constructed road/ramp less than 25 years ago, in 1987. Over the years the juniper has descended to the pitch pine forest at a lower elevation, away from the road. It appears to be spreading vegetatively—despite the presence of abundant cones—or at least we cannot report any findings of seedlings. Separate small patches appeared to be connected with other parts of the population under the forest litter. A similar spreading planting of J. conferta (planted together with J. horizontalis) is found at the next bridge across the same highway, about 2 miles west.

Alarmed upon finding a thriving population of an exotic juniper in eastern Massachusetts, we have been revisiting other juniper locations known to us. None of those re-examined so far have proved to be non-native. However, we have to look out for more populations of Japanese shore juniper, especially in the coastal situations in Plymouth and Barnstable counties. In the case of the successful Wareham population, we may be dealing with an early manifestation of a future problem.

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Irina Kadis & Alexey Zinovjev
31 March 2011 - 23 May 2011