Alder buckthorn

Appearance

Alder buckthorn is a non-spiny deciduous shrub, growing to 3–6 m, occasionally to 7 m tall. It is usually multistemmed, but rarely forms a small tree with a trunk diameter of up to 20 cm. The bark is dark blackish-brown, with bright lemon-yellow inner bark exposed if cut. The shoots are dark brown, the winter buds without bud scales, protected only by the densely hairy outer leaves.

The leaves are arranged alternately on 8–15-millimetre petioles. They are ovate, 3–7 cm long by 2.5–4 cm wide. They have 6–10 pairs of prominently grooved and slightly downy veins and an entire margin.

The flowers are small, 3–5 mm in diameter, star-shaped with five greenish-white acute triangular petals, hermaphroditic, and insect-pollinated, flowering in May to June in clusters of two to ten in the leaf axils.

The fruit is a small black berry 6–10 mm in diameter, ripening from green through red in late summer to dark purple or black in early autumn, containing two or three pale brown 5-millimetre seeds. The seeds are primarily dispersed by frugivorous birds, which readily eat the fruit.

Naming

Alder buckthorn was first formally described by Carl Linnaeus in 1753 as "Rhamnus frangula". It was subsequently separated by Philip Miller in 1768 into the genus "Frangula" on the basis of its hermaphrodite flowers with a five-parted corolla; this restored the treatment of pre-Linnaean authors, notably Tournefort. Although much disputed historically, the separation of "Frangula" from "Rhamnus" is now widely accepted, being supported by recent genetic data though a few authorities still retain the genus within "Rhamnus".

The genus name "Frangula", from Latin frango "to break", refers to the brittle wood. Both the common name alder buckthorn and species name "alnus" refer to its association with alders on damp sites. Unlike other "buckthorns", alder buckthorn does not have thorns. Other recorded names include glossy buckthorn and breaking buckthorn; historically, it was sometimes called "dogwood" through confusion of the leaves with those of dogwood "Cornus sanguinea"."Frangula alnus" was probably introduced to North America about 200 years ago, and in Canada about 100 years ago. It was planted for hedgerows, forestry plantings, and wildlife habitat, but has become an invasive species, invading forests in the northeastern United States and wetlands and moist forest in the Midwestern United States. It is predicted to continue to expand its North American range with time. Its invasiveness is assisted by its high adaptability and pollution tolerance.

It invades forests and grows in the understory in spots with a lot of light. These areas, usually where a tree has fallen, normally allow locally native tree seedlings to grow and eventually fill in the gap in the canopy. But when "Frangula alnus" invades and grows in these locations, its dense canopy prevents light from reaching the ground and therefore prevents other seedlings from growing. It tends to grow more densely and with larger individuals in lower topographical areas with moist, fertile soils, and is very problematic for land managers. Uplands forests are not invaded as easily as lower lying ones. Hemlock-oak stands, which tend to be older stands of trees, are much less suitable for "Frangula alnus" because the density of the tree canopy creates a more shady environment that is not as suitable for "Frangula alnus". Eastern white pine stands are easily invaded because they allow more light to reach the forest floor, and tree stands that are cut are very quickly invaded while undisturbed stands are rarely invaded.

Habitat

Alder buckthorn grows in wet soils in open woods, scrub, hedgerows and bogs, thriving well in sunlight and moderate shade, but less vigorously in dense shade; it prefers acidic soils though will also grow on neutral soils.

"Frangula alnus" is one of just two food plants used by the common brimstone butterfly. The flowers are valuable for bees, and the fruit an important food source for birds, particularly thrushes.