Celtis is a genus of roughly 60 to 70 species of deciduous trees in the family Cannabaceae, known in English as hackberries or nettle trees. Most species are mid-sized trees of about 10 to 25 metres, though the largest occasionally reach 40 metres. The genus is almost cosmopolitan, with native species across temperate and tropical regions on every continent except Antarctica, and a fossil record reaching back to the Paleocene of North America and eastern Asia and the Miocene of Europe.
Hackberries are easily recognized by their alternate, simple, ovate-acuminate leaves with evenly serrated margins, typically 3 to 15 centimetres long, and by their characteristic warty or ridged bark on older trunks. Plants are monoecious: in early spring the bare branches produce small, inconspicuous flowers — longer, hairy male flowers and rounded, greenish female flowers — followed by small drupes 6 to 10 millimetres across. The fruits are dryish but sweet and sugary, and have long been gathered for food by people as well as eaten by birds and mammals.
Celtis has had a somewhat unsettled taxonomic history. The genus was traditionally placed in Ulmaceae alongside the elms, and some twentieth-century treatments segregated it into its own family Celtidaceae; the APG III system, followed by modern databases such as the GBIF Backbone, now places Celtis in an expanded Cannabaceae together with Cannabis, Humulus and several smaller tropical genera. The group is widely acknowledged to be taxonomically complex and in need of further revision, with morphologically similar species and many synonyms in circulation. Linnaeus published the genus in Species Plantarum in 1753, borrowing the name from a Latin word for the unrelated lotus tree of North Africa.
The genus has substantial ecological and cultural value. Hackberries form thickets that provide cover and persistent autumn-to-winter fruit for birds and mammals, and several Celtis species are important larval host plants for butterflies, including the hackberry emperor (Asterocampa celtis), Libytheana carinenta, beak butterflies (Libythea) and the io moth (Automeris io). Indigenous peoples of North America, including the Omaha, Dakota, Pawnee, Mescalero Apache and Tohono O'odham, pounded and ate the drupes, and the genus's tough, fine-grained wood has been used for cabinetry, woodworking and fence posts; leaves of some species have served as a natural sandpaper. In horticulture, hackberries are valued as tough, drought-tolerant shade and street trees, while the Chinese hackberry (C. sinensis) is a classic subject for bonsai.
Etymology
The genus name Celtis is borrowed directly from Latin, where it was originally applied to an unrelated North African tree — usually identified with the lotus tree of classical authors. Linnaeus reused the word when he established the modern genus in 1753, but did not document why he transferred the classical name to these trees.
Distribution
Celtis is a cosmopolitan genus, occurring naturally on every continent except Antarctica and ranging from temperate to tropical climates. In Europe, native Celtis are essentially limited to the Mediterranean basin, where C. australis is the principal species; Info Flora records both C. australis and the introduced North American C. occidentalis from Switzerland. North America is the centre of species richness for the temperate zone, with C. occidentalis through the east, C. laevigata across the south-central and southeastern United States and Mexico, and C. reticulata, C. pallida and C. ehrenbergiana in the arid southwest, where SEINet treats two species as native to Arizona. The genus extends through tropical Africa and Madagascar (where C. africana ranges across the Afromontane region), through Asia from China and Japan (C. sinensis) south into the tropics, and into the Australasian and Neotropical regions, reflecting its long fossil history.
Ecology
Hackberries are ecologically significant in many of the woodlands and savannas they inhabit. Their fleshy drupes ripen in autumn and persist after leaf-fall, providing critical winter food for birds and small mammals, while dense thickets of trees and shrubs supply shelter and nesting cover. Celtis species are particularly important to Lepidoptera: they are the larval host plants of beak butterflies (Libythea), the American snout (Libytheana carinenta), the hackberry emperor (Asterocampa celtis) and the io moth (Automeris io). Honeybees use the early-spring flowers as a minor source of pollen and nectar. Hackberries are also associated with their own pathogens — the wood-decay fungus Perenniporia celtis was first described from a Celtis host.
Cultivation
Members of Celtis are grown widely as shade, street and park trees because of their toughness and adaptability. Plants for a Future records that the genus succeeds in most soils, including dry gravels, but prefers a deep, fertile soil; species tolerate light to heavy textures and neutral to slightly alkaline pH, and across the genus PFAF gives a hardiness range covering USDA zones 3 to 11. Wikipedia notes that hackberries are particularly valued for their drought tolerance and are commonly featured in arboreta and botanical gardens. The Chinese hackberry, C. sinensis, is a classic bonsai subject, and several other species are routinely used in traditional and contemporary bonsai practice.
Cultural Uses
Celtis fruits have a long history of human use. In North America the small, sweet drupes were gathered by the Omaha, Dakota, Pawnee, Mescalero Apache and Tohono O'odham, often pounded together with fat and maize. The wood of hackberries is moderately hard and has been used in cabinetry and woodworking as well as for utilitarian items such as fence posts. Plants for a Future records that the rough leaves of some Celtis species were employed as a natural sandpaper for smoothing wood, and that bark fibres have been used for rope-making and as a raw material for rayon. As food for humans, however, the fruit is rated only moderately useful in modern compilations.
Conservation
There is no genus-wide conservation listing for Celtis. The IUCN Global Invasive Species Database currently lists no Celtis species in its archive, so no member of the genus is presently catalogued as a globally significant invader through that source; individual Celtis species may, however, be locally weedy outside their native range.
Taxonomy Notes
The genus Celtis was published by Linnaeus in Species Plantarum (1753) and is treated by the GBIF Backbone as an accepted genus in the family Cannabaceae, order Rosales. Historically, however, Celtis was placed in Ulmaceae, the elm family, and several twentieth-century authors segregated it (with Aphananthe, Pteroceltis, Trema and allies) into a separate family Celtidaceae. The APG III system folded these genera into an expanded Cannabaceae, and that placement is now followed by major checklists including GBIF and the Swiss Info Flora database. Older horticultural sources such as Plants for a Future still cite the Ulmaceae placement. Modern global tallies report approximately 60 to 70 accepted species, with regional treatments such as SEINet citing "about 60 species" worldwide; the group is widely acknowledged to be taxonomically complex and in need of revision, and many specific names appear as synonyms in current databases.
