Family: Casuarinaceae
Casuarina glauca, commonly known as gray sheoak, longleaf ironwood, saltmarsh ironwood, or swamp oak, is a fast-growing evergreen tree native to eastern Australia. It has a distinctive appearance, with thin, jointed, needle-like branchlets that resemble pine needles and small woody cone-like fruits. Gray sheoak is well adapted to coastal and wetland environments and tolerates salt spray, flooding, drought, and poor soils. Because of its hardiness, it has been widely planted outside its native range as a windbreak, for erosion control, shoreline stabilization, and occasionally as an ornamental or shade tree.
Despite these uses, gray sheoak poses significant environmental risks in Hawaiʻi and other tropical and subtropical regions. It spreads readily by wind-dispersed seeds and can form dense stands that crowd out native plants. As a nitrogen-fixing species, it alters soil conditions in ways that favor invasive plants over native ecosystems adapted to low-nutrient soils. Its dense canopy and thick leaf litter reduce light and space for understory species, degrading habitat quality. Once established, gray sheoak is difficult to control, making early prevention and removal critical to protecting native coastal and wetland ecosystems.
High Risk Traits:
- Highly invasive outside its native range, including Hawaii, Florida, and parts of the Caribbean
- Produces abundant wind-dispersed seeds, allowing rapid spread over long distances
- Forms dense, monotypic stands that displace native vegetation
- Nitrogen-fixing species, enabling it to alter soil chemistry and nutrient cycling
- Tolerant of a wide range of environmental conditions, including drought, flooding, saline soils, and poor substrates
- Fast growth rate, allowing it to quickly dominate disturbed areas
- Allelopathic leaf litter, which can suppress germination and growth of other plant species
- Reduces habitat quality for native wildlife by simplifying vegetation structure
- Difficult to control once established, with resprouting and persistent seed banks
- Commonly spreads from intentional plantings (windbreaks, landscaping, erosion control)
Low Risk Traits:
- Requires open or disturbed conditions for optimal establishment (less competitive in intact native forests)
- No known human or livestock toxicity issues
- Specific symbiotic relationships (e.g., nitrogen-fixing bacteria) may limit establishment in some soils
