Rhamnus cathartica, the European buckthorn, common buckthorn, purging buckthorn, or just buckthorn, is a species of small tree in the flowering plant family Rhamnaceae. It is native to Europe, northwest Africa and western Asia, from the central British Isles south to Morocco, and east to Kyrgyzstan. It was introduced to North America as an ornamental shrub in the early 19th century or perhaps before, and is now naturalized in the northern half of the continent, and is classified as an invasive plant in several US states and in Ontario and Québec, Canada.
Description Rhamnus cathartica is a deciduous, dioecious shrub or small tree growing up to 10 metres (33 ft) tall, with grey-brown bark and often thorny branches. The leaves are elliptic to oval, 25–90 mm (1–3+1⁄2 in) long and 12–35 mm (1⁄2–1+3⁄8 in) broad; they are green, turning yellow in autumn, have toothed margins, and are arranged somewhat variably in opposite to subopposite pairs or alternately. The flowers are yellowish-green with four petals; they are dioecious and insect pollinated and bloom in April–June. The fruit is a globose black drupe, 6–10 mm (1⁄4–3⁄8 in) across, and contains two to four seeds.
In addition to the kindred Frangula alnus, Rhamnus cathartica may be visually confused with a plant of another family, Cornus sanguinea. The species was originally named by Carl Linnaeus as Rhamnus catharticus, but this spelling was corrected to cathartica as the genus name Rhamnus is treated as being of feminine gender.
Toxicity The seeds and leaves are mildly poisonous for humans and most other animals, but are readily eaten by birds, who disperse the seeds in their droppings. The toxins cause stomach cramps and laxative effects that may function in seed dispersal. The chemical compounds responsible for this laxative effect are anthraquinone and emodin. The species name cathartica and the common name purging buckthorn refer to this effect. In 1994, R. cathartica was implicated in the outbreak of an idiopathic neurological disease in horses, although no causative agent was officially identified. In trials where rodents were fed the leaves and stems of R. cathartica, glycogen metabolism became abnormal and glycogen deposits formed in the cytoplasm of liver cells. Abnormalities in glycogen metabolism lead to diabetes in humans.
Ecology Rhamnus cathartica is shade-tolerant, moderately fast-growing and short-lived. It is a food plant of the brimstone butterfly, Gonepteryx rhamni. The sulphur-yellow males are indicative of the presence of this species or of its alternative host, Frangula alnus. Rhamnus cathartica is the alternate host for the rust disease of cereals caused by Puccinia coronata. Crown rust fungus results in leaf damage and reductions in photosynthesis during its transit in this species. Rhamnus cathartica is also the primary overwintering host in North America for a significant agricultural pest of soybeans, the soybean aphid.
Allelopathy Secondary compounds, particularly emodin, have been found in the fruit, leaves, and bark of the plant, and may protect it from insects, herbivores and pathogens. The emodin present in R. cathartica fruit may prevent early consumption, as it is found most in unripe fruits, which allows seeds to reach maturity before being dispersed. Birds and mice significantly avoid eating unripe fruits, and if forced to ingest emodin or unripe fruit, the animals regurgitate the meal or produce loose, watery stools. Rhamnus cathartica is a constitutive isoprene emitter Allelopathic effects of exudates from R. cathartica leaf litter, roots, bark, leaves and fruit may reduce germination of other plant species in the soil. Soils in buckthorn-dominated areas are higher in nitrogen and carbon than normal soils, which speeds up decomposition rates of leaf litter. This can result in bare patches of soil being formed and R. cathartica performs well in such disturbed habitats, so this may be adaptive for the setting of its seed.
Invasive species The species is naturalised and invasive in parts of North America. Rhamnus cathartica has a competitive advantage over native trees and shrubs in North America because of root competition, the shade it produces, and it leafs out before native species. Of the annual carbon gain in R. cathartica, 27–35% comes from photosynthesis occurring before the leaves of other plants emerge. Soil in woodlands dominated by R. cathartica was higher in nitrogen, pH and water content than soil in woodlands relatively free of R. cathartica, probably because R. cathartica has high levels of nitrogen in its leaves and these leaves decompose rapidly. Rhamnus cathartica is also associated with invasive European earthworms (Lumbricus spp.) in the northern Midwest of North America. Removing R. cathartica led to a decrease of around 50% in the biomass of invasive earthworms. Soils enriched by extra nitrogen from decayed buckthorn leaves and additional earthworm activity impair North American forest ecosystems: "Invasive earthworms, which need rich litter, break [buckthorn leaves] down rapidly, destroying beneficial fungi and exposing bare soils in the process. These soils provide ideal conditions for buckthorn germination and seedling growth but many native trees and shrubs need the beneficial fungi and will not reproduce without it." Common buckthorn can be found in most northern states of the United States and it is particularly prevalent in the Great Lakes states of Minnesota, Wisconsin, and Michigan. A current map of its distribution can be found at Early Detection and Distribution Mapping System (EDDmapS). This species is largely bird-dispersed, but only a subset of bird species that eat the fruit are competent dispersers. It is winter hardy in USDA zones: 3 to 8. It is a prohibited species in a few US states; Minnesota lists it as a Restricted noxious weed making it illegal to import, sell, or transport the plant, Connecticut, Iowa, Massachusetts, New Hampshire, also restrict or ban the plant, and Vermont lists it as a Class B noxious weed.
Control
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