Casuarina cunninghamiana in the Western Cape, South Africa: Determinants of naturalisation and invasion, and options from management
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Alien species that are desirable and commercially important in some parts of the landscape but damaging invaders in other parts present a special challenge for managers, planners, and policy-makers. Casuarina cunninghamiana (Casuarinaceae), native to the eastern and north-eastern coasts of Australia, has been cultivated in South Africa for more than a century. This study explores the invasion ecology of C. cunninghamiana in the south-western part of the Western Cape. We examined differences between naturalized and non-naturalized populations (e.g. the roles of propagule pressure, land use and bioclimatic suitability), assessed invasion risk, and provide recommendations for control. Naturalisation was observed in 81% of the populations surveyed. In climatically suitable areas, propagule pressure and distance to water bodies and water courses were significant predictors of naturalisation — naturalisation was most likely to occur within 100 m from the nearest planted individual and close to water bodies and water courses. The species has also naturalized in regions with suboptimal bioclimatic conditions, but then only very close (b10 m) to planted trees. Based on our findings we recommend: 1) the immediate removal of female trees from within 100 m of water bodies and water courses; 2) all future sales and plantings to be restricted to male plants; and 3) the development of a management strategy (potentially including biological control) to control existing invasions and limit future spread. If these steps are taken, we believe it would be possible to maintain the beneficial uses the species has in some locations without incurring substantial negative impacts in other locations.