Syzygium australe (J.C.Wendl. ex Link) B. Hyland (Myrtaceae) in South Africa: current distribution and invasion potential

Résumé

Syzygium australe (Australian brush-cherry; the names Eugenia australis and Syzygium paniculatum have been misapplied to this species in some regions) is native to Australia.It has been introduced and used as an ornamental plant in several regions outside its native range and is invasive in Hawaii and New Zealand.The species was first recorded in South Africa in 1968 and has become a popular and widely traded and planted ornamental species.The first reports of naturalisation in the country appeared in the first decade of the 21 st century; the species was subsequently flagged as a priority for investigation and potential regulation as an invasive species.In this paper we mapped the current distribution of S. australe in South Africa, determined its introduction status, and modelled its potential distribution.We also investigated whether cultivated plants are producing fertile seeds and compared such seeds with those produced by plants growing outside cultivation.We recorded S. australe at 268 sites across the country, clustered primarily in the Western Cape province.Naturalised populations have established at three sites, all in the Western Cape.Surveys of these established populations revealed ~ 4000 plants covering an area of ~ 7 ha (representing ~ 2 ha condensed canopy area).These populations were flourishing in riparian habitats in urban areas.Species distribution models suggest that S. australe has the potential to expand its current range in South Africa, primarily in coastal regions.Seeds of both cultivated and naturalised plants showed similar high levels of germinability (both 100%).Building on these findings, we conducted a risk analysis using the Risk Analysis of Alien Taxa Framework, and found S. australe to be of high invasion risk in South Africa.We recommend that all populations outside cultivation be controlled, and that propagation and trade be prohibited.However, except where they occur near riparian habitats, garden plantings do not need to be prioritised for immediate control, and can rather be phased out over time.

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