Natural History

Geographical Origin and History

Common buckthorn is native to Europe, western Asia, and high altitudes of northern Africa (Godwin 1943b). It has long been planted as an ornamental and for hedgerows. In Wisconsin it was used as a hedgerow plant as early as 1849 (Hoffman and Kearns 1997). In the early 1900s common buckthorn was promoted as a shelterbelt species on the northern Great Plains, until the realization that it was the alternate host of oat stem rust (Archibold et al. 1997, see below). In Michigan, common buckthorn was first collected outside of cultivation in 1914 (Voss 1985). Two years later it was reported as locally common in southern Michigan (Voss 1985), indicating that it was already naturalized there by that time. Today common buckthorn is widely established across most of eastern and central temperate North America (USDA-NRCS 2004, EC 2003).

Characteristics

Common buckthorn is typically a shrub or small tree, to 6 m (20 ft) tall. The leaves are broadly oval, glabrous (hairless), 3-6 cm long and usually about half as wide, with 2-4 pairs of upcurved veins on each side (Voss 1985, Gleason and Cronquist 1991). They may be rounded or pointed at the tip. The leaf margins are evenly toothed with small teeth. Leaf arrangement varies from opposite to subopposite to occasionally alternate on the twigs. The twigs have grey brown bark and scaly buds, and are often tipped with a short thorn.

Common buckthorn is functionally dioecious, with male and female flowers on separate plants, though male flowers may include a vestigial (nonfunctional) pistil (Gleason and Cronquist 1991). Small, inconspicuous, yellow-green flowers are produced from mid May through June. They are arranged in clusters of 2-6, and appear at the base of some of the leaf stalks.

After flowering, the female plants produce clusters of berry-like, deep purple to blackish drupes, each typically containing 4 seeds. Fruits of this (and probably other) species of Rhamnus have laxative properties (Voss 1985).

Like some other introduced species (e.g., Eurasian Lonicera spp.) the leaves of common buckthorn stay green until late in fall, weeks after most native species have lost their leaves (Archibold et al. 1997). They turn yellow (not red) before falling (Godwin 1943b).

Similar Species

Another Eurasian invasive, glossy buckthorn (Rhamnus frangula L.), has also widely escaped across temperate North America (Gleason and Cronquist 1991, USDA-NRCS 2004). Glossy buckthorn has glossy, shiny leaves that are mostly or all alternate on the twigs (Gleason and Cronquist 1991). Its flowers have 5 sepals and 5 petals. Its fruits are reddish at first, turning dark purple to nearly black as they ripen. The twigs have buds that are hairy but lack scales. Like common buckthorn it also invades mesic upland forest. Glossy buckthorn prefers wetter soil than common buckthorn, though, and is very invasive in wet woods and riparian areas.

Dahurian buckthorn [R. citrifolia (Weston) W. Hess & Stearn, also known as R. davurica Pallas] is introduced from northeast Asia. It is similar to the native lance-leaved buckthorn (see below) but has mostly opposite or subopposite leaves. It has much shorter petioles (1-3 mm, or 0.04-0.12 in) than common buckthorn, and 1-3 flowers per cluster (Gleason and Cronquist 1991). So far Dahurian buckthorn has not been found in Wisconsin or Minnesota, though it has been found in all the states bordering these two, including Illinois and Michigan (USDA-NRCS 2004).

Asian buckthorn (R. utilis Decne.) is native to China. It is sparingly introduced into Michigan and Illinois (Voss 1985, USDA-NRCS 2004). It is similar to common buckthorn, except that the leaves are shiny and longer (8-12 cm, or 3.1-4.7 in), more than twice as long as broad, and usually have 5-6 pairs of veins instead of 3-4. The fruits typically have just two seeds instead of 4 (Voss 1985).

Two buckthorn species are native to the upper Great Lakes region (Gleason and Cronquist 1991). Alder-leaved buckthorn (R. alnifolia L'Her.), Is a low, multistemmed shrub, reaching only about 1.5 m (4.9 ft) tall at most (Gleason and Cronquist 1991). The flowers have 5 sepals but lack petals, and the drupes have 3 seeds each (Gleason and Cronquist 1991). It has alternate leaves that are similar to common buckthorn's, but that tend to be somewhat larger and more elongate (Voss 1985). It tends to form low patches up to several meters across. It inhabits floodplains, wetlands, and bogs throughout the northern US and adjacent Canada, south to Tennessee and California (USDA-NRCS 2004).

The second native buckthorn is lance-leaved buckthorn (R. lanceolata Pursh). It is a shrub growing to 2 m (6.6 ft) tall, with alternate, narrow leaves that have petioles 4-10 mm (0.16-0.39 in) long, and blades that taper to a point at the tip (Gleason and Cronquist 1991, Hoffman and Kearns 1997). It is found in eastern and central North America, but is known in the upper Great Lakes states only from southern Wisconsin, where it is considered rare (USDA-NRCS 2004, WIS 2004).

A third buckthorn species, Carolina buckthorn (R. caroliniana Walter), is native to the southeastern US, as far north as southern Illinois, Indiana, and Ohio (USDA-NRCS 2004).

Of all these buckthorn species, only common buckthorn and Asian buckthorn have a short terminal thorn at the tips of some branches (Voss 1985). In eastern Lower Michigan these two species have interbred, forming a fertile hybrid swarm (Gil-ad and Reznicek 1997).

Reproduction and Dispersal

Common buckthorn reproduces almost entirely be seed (Godwin 1943b). The berries have a bitter taste, and are eaten and dispersed mainly by birds (Catling and Porebski 1994, Archibold et al. 1997). Seeds are generally dispersed from September through April (Gill and Marks 1991). Much of the fruit simply falls to the ground beneath the shrubs, producing a dense layer of seedlings (Wieseler 1999). Seeds must be removed from the fruits (by animals or through decomposition) for them to germinate (Archibold et al. 1997). Gill and Marks (1991) found that small mammal predation of buckthorn seeds in old fields was lower than on two common eastern North American shrubs, northern swamp dogwood (Cornus racemosa Lam.) and arrow-wood (Viburnum dentatum L.).

Once removed from the fruits, germination rates of common buckthorn seed may be relatively high - Archibold et al. (1997) found rates of nearly 90%. Germination is higher on bare soil, than on soil with a litter layer or a layer of herbaceous plants (Gourley and Howell 1984).

Habitat Preference and Tolerance

Like many invasives, common buckthorn has a wide habitat tolerance. Though it is typically an upland species, strongly associated with calcareous soils in Europe (Godwin 1943b, Archibold et al. 1997), it can colonize a variety of soil types and tolerate a broad range of moisture levels. It is at least moderately shade-tolerant (the seedlings are very shade-tolerant) and capable of invading mature forest (Godwin 1943b). It does best at light levels of 12.5% or more of full sunlight (Gourley and Howell 1984). Early leaf-out and late leaf-fall gives this species an advantage over most native shrubs in the forest understory (Harrington et al. 1989).