Family: Fabaceae

Acacia robusta (synonym of Vachellia robusta), commonly called splendid acacia or ankle thorn, is a fast-growing tree native to southern and eastern Africa. It can reach heights of up to 20 meters and is known for its wide, spreading crown and pairs of stout, hooked thorns along its branches. The tree is well adapted to a variety of environments, thriving in savannas, woodlands, and riverbanks, and is valued in its native range for providing shade, fuelwood, fencing material, and browse for livestock. Its resilience to drought and ability to fix nitrogen also make it ecologically significant in supporting degraded soils.

Outside its native range, Acacia robusta has the potential to become invasive due to its rapid growth, thorny defenses, and high seed production. These traits allow it to outcompete native plants, form dense thickets, and hinder the movement of animals and people. Its strong thorns can also pose a hazard to livestock and humans. While it has not yet spread widely in Hawaiʻi or the Pacific, its history of naturalizing elsewhere suggests that it should be approached with caution before planting, as it could threaten native ecosystems if introduced.

High Risk Traits:

• Naturalized beyond native range: Reported as invasive in Australia and sparingly naturalized in Hawaii.
  
• Environmental weed: Potential to alter ecosystems, though specific impacts are not well-documented.
  
• Spines/thorns: Produces paired, straight stipular spines up to 11 cm long, which may pose a physical hazard.
  
• Tolerates a wide range of soils: Adaptable to sandy, clay, and loamy soils, including acidic to alkaline pH.
  
• Nitrogen-fixing: Could alter soil chemistry and outcompete native species.
  
• Dispersal by animals: Seeds survive gut passage and may be spread by herbivores.
  
• Persistent seed bank: Seeds can remain viable in the soil for extended periods.
  
• Fire-adapted: Tolerates and may benefit from fire, potentially increasing its spread in fire-prone areas.
  
• Coppicing ability: Resprouts vigorously after cutting or damage.

Low Risk Traits:

• No evidence of toxicity to humans: No reports of allergies or poisoning.
  
• Palatable to livestock: Foliage and pods are browsed, reducing unchecked growth in grazed areas.
  
• Limited unintentional dispersal: Large seeds lack mechanisms for accidental spread (e.g., burrs).
  
• Moderate seed production: No evidence of prolific seed output (>1000/m²).