Trema is a genus of evergreen trees in the family Cannabaceae (order Rosales), closely related to the hackberries (Celtis). The genus is distributed across subtropical and tropical regions of southern Asia, northern Australasia, Africa, South and Central America, and parts of North America, with species typically growing as small trees reaching 10–20 metres in height.
The leaves are alternate, simple, 7–15 cm long, ovate-acuminate to lanceolate with a long pointed tip and evenly serrated margins. The fruit is a small drupe, 3–5 mm in diameter.
Taxonomically, Trema was previously placed in the elm family Ulmaceae, or alongside Celtis in the Celtidaceae. Molecular phylogenetic analysis demonstrated that Celtidaceae is best treated as part of the hemp family, Cannabaceae, where Trema now sits.
One of the ecologically notable features of several Trema species is their ability to enter into nitrogen-fixing symbiosis with rhizobia bacteria — an unusual trait for a non-legume. Species with this capacity are sometimes recognised under the separate genus Parasponia. Trema orientale is widely used in land reclamation projects across southern Asia for precisely this reason, as it tolerates poor soils well. Some species in the genus have also become invasive on Pacific Ocean islands.
Distribution
Trema species occur across subtropical and tropical regions of southern Asia, northern Australasia, Africa, South and Central America, and parts of North America. Trema orientale has also naturalised as an invasive species on some Pacific Ocean islands.
Taxonomy Notes
Trema was historically placed in the elm family Ulmaceae, or within Celtidaceae alongside Celtis. Molecular genetic analysis demonstrated that Celtidaceae are best subsumed into the hemp family, Cannabaceae, which is now the accepted placement for the genus.
Ecology
Several Trema species are unusual among non-legumes in being capable of symbiotic nitrogen fixation with rhizobia bacteria; these species are sometimes segregated as the genus Parasponia. Trema species also serve as larval food plants for hepialid moths of the genera Aenetus (e.g. A. splendens) and Endoclita (e.g. E. malabaricus), whose larvae burrow into the trunk.
Cultural Uses
Trema orientale is widely planted for land reclamation across southern Asia, where its tolerance of degraded, nitrogen-poor soils — a result of its nitrogen-fixing capacity — makes it a valuable pioneer species.
