Gahnia is a genus of sedges in the family Cyperaceae, order Poales, commonly known as sawsedge or saw-sedge. The genus comprises roughly 40 accepted species distributed across a wide Indo-Pacific range that includes China, Southeast Asia, New Guinea, Australia, New Zealand, and numerous Pacific Islands from Hawaii to Fiji and the Marquesas.
Plants in this genus are typically coarse, perennial tussock-forming sedges. The common names "sawsedge" and "saw-sedge" refer to the sharply toothed margins of the long, strap-like leaves, which can cause cuts on skin and are a characteristic feature of the genus. Species vary considerably in size and fruit colour, from the red-fruited Gahnia sieberiana and yellow-fruited G. xanthocarpa to forms with black or dark nutlets.
Gahnia is an ecologically important genus across its range. In New Zealand, species such as G. setifolia (Māpere, razor sedge) and G. xanthocarpa are widespread components of lowland and coastal forest margins and scrub. In Australia, species like G. radula (thatch saw-sedge) and G. sieberiana (red-fruited saw sedge) occur across eastern and south-eastern states, often in heath, sclerophyll forest, and wetland margins. G. aspera extends from Australia into the Pacific as far as Hawaii.
Etymology
The common names "sawsedge" and "saw-sedge" refer to the sharply serrated leaf margins characteristic of the genus, which can lacerate skin on contact.
Distribution
Gahnia is native to China, Southeast Asia, New Guinea, Australia, New Zealand, and a number of Pacific Islands including Hawaii, Fiji, New Caledonia, and the Marquesas. In Australia, species occur across all states. In New Zealand, several species are endemic or near-endemic to the North and South Islands.
Ecology
Species of Gahnia typically grow in heath, scrub, forest margins, wetland edges, and coastal areas. Many species form persistent tussocks that provide structural habitat for ground-dwelling invertebrates and nesting birds. Gahnia sieberiana and related taxa are notable components of heath and open sclerophyll communities in eastern Australia.
Conservation
Gahnia lanaiensis was federally listed as an endangered species in the United States as a rare Hawaiian endemic from Lanaʻi Island. Research published in 2010 suggested that these plants are actually Gahnia lacera, a New Zealand species inadvertently introduced to Lanaʻi in the early 20th century, raising questions about the conservation status of the listed population.
