Colletia is a genus of flowering shrubs in the family Rhamnaceae, within the order Rosales, first described by Antoine Laurent de Jussieu in 1789 (authorship "Comm. ex Juss."). The genus comprises around five accepted species of spiny shrubs, all native to southern South America, with a native range spanning Argentina, Bolivia, southern Brazil, Chile (including the Juan Fernández Islands), Ecuador, Peru and Uruguay.
Colletia species are non-legume nitrogen fixers, a trait shared with a handful of other woody genera outside the pea family that can host nitrogen-fixing symbionts in their roots, allowing them to establish on nutrient-poor soils. Several species carry common names referencing their formidable spines — Colletia paradoxa is known as crucifixion thorn, thorn of the cross, or anchor plant, and Colletia hystrix as pink crucifixion thorn — reflecting the flattened, cross- or anchor-shaped thorny branchlets that are one of the genus's most distinctive features.
The taxonomic history of Colletia is tangled with several other spiny South American Rhamnaceae genera: species once described under Colletia have since been reclassified into Discaria, Retanilla, Scutia and Trevoa, and even into the unrelated genus Celtis, underscoring how much the group's thorny, often leafless shrub architecture has historically complicated species-level classification within the family.
Distribution
Colletia is native to southern South America, with recorded native localities across Argentina, Bolivia, southern Brazil, Chile (including the Juan Fernández Islands), Ecuador, Peru and Uruguay. Occurrence records also show scattered naturalized or cultivated plants in the United States, United Kingdom and New Zealand.
Ecology
Colletia species are non-legume nitrogen fixers, allowing them to colonize nutrient-poor soils in their native South American habitats.
Taxonomy Notes
Colletia's circumscription has shifted over time: species originally described in the genus have since been transferred to the related Rhamnaceae genera Discaria, Retanilla, Scutia and Trevoa, and one (as C. iguanaea) turned out to belong to the unrelated genus Celtis.
