Robinia pseudoacacia
Robinia pseudoacacia, commonly known as black locust, is a medium-sized hardwood deciduous tree, belonging to the tribe Robinieae of the legume family. Another common name is false acacia, a literal translation of the specific name.
Although fossilized traces of the genus were found in Europe, the species itself is native to a few small areas of the United States, but has been widely planted and naturalized elsewhere, including temperate North America, Eurasia, and Africa. It is considered an invasive species in some regions.
Description
The roots of black locust contain nodules that allow it to fix nitrogen, as is common within the pea family. Trees reach a typical height of with a diameter of. It is a very upright tree with a straight trunk and narrow crown that grows scraggly with age. The bark is a reddish black and gray and tinged with red or orange in the grooves; it is deeply furrowed into grooves and ridges which run up and down the trunk and often cross and form diamond shapes. The branches are typically zig-zaggy and may have ridges and grooves or may be round. When young, the branches are at first coated with white silvery down; this soon disappears, and they become pale green and afterward reddish or greenish brown.The dark blue-green leaves are lighter on the underside, and are compound, meaning that each leaf contains many smaller leaflike structures called leaflets, which are roughly paired on either side of the stem that runs through the leaf. There is typically one leaflet at the tip of the leaf, and the leaves are alternately arranged on the stem. Each leaf is long and contains 9–19 leaflets, each being long and wide. The leaflets are rounded or slightly indented at the tip and typically rounded at the base. The leaves come out of the bud folded in half, yellow green, covered with silvery down which soon disappears. Each leaflet initially has a minute stipel, which quickly falls, and is connected to the by a short stem or petiolule. The leaves are attached to the branch with slender hairy petioles which are grooved and swollen at the base. The stipules are linear, downy, membranous at first and occasionally develop into prickles. The leaves appear relatively late in spring, and turn a clear pale yellow in autumn. The leaflets fold together in wet weather and at night, as some change of position at night is a habit of the entire leguminous family.
Young trees are often spiny, especially on root suckers and branches near the ground; mature trees often lack spines. R. pseudoacacia is quite variable in the number of spines present, as some trees are densely prickly and other trees have no prickles at all. The spines typically remain on the tree until the young thin bark to which they are attached is replaced by the thicker mature bark. They develop from stipules, and since stipules are paired at the base of leaves, the spines will be paired at the bases of leaves. They are up to long and somewhat triangular with a flared base and sharp point. Their color is a dark purple and they adhere only to the bark.
The winter buds are minute, naked, three or four together, protected in a depression by a scale-like covering lined on the inner surface with a thick coat of tomentum and opening in early spring. When the buds are forming they are covered by the swollen base of the petiole. The large flowers open in May or June for 7–10 days, after the leaves have developed. They are arranged in loose drooping clumps which are typically long. The flowers themselves are cream-white with a pale yellow blotch in the center and imperfectly papilionaceous in shape. They are about wide, very fragrant, and produce large amounts of nectar. Each flower is perfect, having both stamens and a pistil. There are 10 stamens enclosed within the petals; these are fused together in a diadelphous configuration, where the filaments of 9 are all joined to form a tube and one stamen is separate and above the joined stamens. The single ovary is superior and contains several ovules. Below each flower is a calyx which looks like leafy tube between the flower and the stem. It is made from fused sepals and is dark green and may be blotched with red. The pedicels are slender,, dark red or reddish green.
The fruit is a typical legume fruit, being a flat and smooth pea-like pod long and broad. The fruit usually contains 4–8 seeds. The seeds are dark orange brown with irregular markings. They ripen late in autumn and hang on the branches until early spring. There are typically 25,500 seeds per pound. Cotyledons are oval in shape and fleshy.
Reproduction and dispersal
Black locust reproduces both sexually via flowers, and asexually via root suckers. The flowers are pollinated by insects, primarily by Hymenopteran insects. The physical construction of the flower separates the male and female parts so that self-pollination will not typically occur. The seedlings grow rapidly but they have a thick seed coat which means that not all seeds will germinate. The seed coat can be weakened via hot water, sulfuric acid, or be mechanically scarified, which will allow a greater quantity of the seeds to grow. The seeds are produced in good crops every year or every other year.Root suckers are an important method of local reproduction of this tree. The roots may grow suckers after damage or after no damage at all. The suckers are stems which grow from the roots, directly into the air and may grow into full trees. The main trunk also has the capability to grow sprouts and will do so after being cut down. This makes removal of black locust difficult as the suckers need to be continually removed from both the trunk and roots or the tree will regrow. This is considered an asexual form of reproduction.
The suckers allow black locust to grow into colonies which often exclude other species. These colonies may form dense thickets which shade out competition. Black locust has been found to have either 2n=20 or 2n=22 chromosomes.
Flavonoid content
Black locust leaves contain flavone glycosides characterised by spectroscopic and chemical methods as the 7-O-β-ᴅ-glucuronopyranosyl--β-ᴅ-glucopyranosides of acacetin, apigenin, diosmetin and luteolin.Similar species
Although similar in general appearance to the honey locust, the black locust lacks that tree's characteristic long branched thorns on the trunk, having instead pairs of short prickles at the base of each leaf; the leaflets are also much broader than honey locust. It may resemble Styphnolobium japonicum, which has smaller flower spikes and lacks spines.Taxonomy
The black locust is a plant from the subfamily of Faboideae in the family of legumes and is a relative of the pea and bean.The black locust is commonly referred to as "false acacia" after its species name "pseudoacacia", although it is not particularly closely related to the acacia, which belongs to the mimosa subfamily. Both species are similar in the form of their feathered leaves and thorns, but the flower shapes are very different. Confusion between species of both genera is almost impossible in higher latitudes, since acacias are native to subtropical and tropical areas and do not thrive in the cooler climates favoured by the black locust.
History
The tree was identified in 1607 at Jamestown by British colonists, who used the timber to build houses. The tree was named for its resemblance to Ceratonia siliqua, known as the "Old World Locust". Jesuit missionaries apparently fancied that this was the tree that supported St. John in the wilderness, despite its being native to North America.Introduction in Europe
The genus is named after the royal French gardeners Jean Robin and his son, who brought the plant to Europe in 1601, in what may be regarded as a reintroduction. A black locust planted by Jean Robin in 1601 still grows in the Square René Viviani in Paris, and is believed to be the oldest living tree in that city. Another was planted in 1636 by Vespasien Robin at the Jardin des plantes and can still be viewed today. It was introduced into Britain in 1636.According to research by Hungarian naturalist József Ernyey, the proliferation of the Black Locust in the mid-18th century was closely linked to the Seven Years' War. Recognizing the tree's rapid growth and dense habit, the military administration of the Habsburg Monarchy utilized the species as a defensive tool. Under orders from Queen Maria Theresa, Robinia pseudoacacia was planted near fortifications to create physical obstacles that hindered the movement of enemy infantry and the positioning of artillery.
The species saw a second major wave of planting following World War I. In the wake of territorial losses and environmental concerns regarding the increasing aridity of the Great Hungarian Plain, the Hungarian government launched large-scale afforestation programs. The Black Locust was favored for its ability to stabilize sandy soil and reverse erosion, leading the state to override private property rights to enforce its planting.
Distribution
The genus Robinia is native to North America, although traces of it are found in Eocene and Miocene rocks in Europe.The species itself is native to the eastern United States, but its exact native range is not accurately known as it has been cultivated and is now found in all 48 lower states, British Columbia, and eastern Canada. The native range is thought to be two separate populations, one centered about the Appalachian Mountains from Pennsylvania to northern Georgia, and a second westward focused around the Ozark Plateau and Ouachita Mountains of Arkansas, Oklahoma and Missouri.
The tree has been naturalized in temperate North America, Eurasia, and Africa. Humans have distributed the tree for landscaping, with its range now including Canada, Southern South America, Europe, temperate regions in Asia, Northern and Southern Africa, Australia, and New Zealand.