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Abstract
The legumes (Fabaceae) Genista monspessulana and Spartium junceum are major invaders in several other parts of the world, but not yet so in South Africa. We determine their current distributions in South Africa at different spatial scales, assess population structure (soil seed banks and size at reproduction) evaluate current management activities, and provide recommendations for control (including assessing the feasibility of nation-wide eradication). G. monspessulana occurs at nine localities in three quarter-degree cells, covering a total of 22.7ha. S. junceum is much more widespread, occurring in 33 quarter-degree cells and is frequently cultivated in private gardens. All naturalised or invasive populations are in disturbed areas, mostly along roadsides. Once established, G. monspessulana and S. junceum accumulate large, persistent soil-stored seed banks, ranging in size between 909 and 22,727 (median 1970)seeds/m2 and 0 and 21,364 (median 455)seeds/m2 for the two species respectively. Both species resprout vigorously after cutting and stump herbicide application (60% of G. monspessulana and 43% of S. junceum plants resprouted) which necessitates regular follow-ups. We estimate that over 10years, at a cost of about ZAR 81,000 (1 ZAR=0.114 US$ as on 6 October 2012), G. monspessulana could be extirpated from South Africa. S. junceum is far more widespread and coupled with low effectiveness of control, abundance of seeds and seed longevity, eradication is unfeasible. We recommend that control methods used for S. junceum be improved to prevent resprouting, and that areas are managed to limit the movement of seeds and avoid further spread and establishment. Further studies are required to understand why these two species have failed to replicate the invasiveness shown in other parts of the world.
Highlights
The distributions of many invasive plant species are to a large extent, determined by the extent of human usage of the species (e.g. Gravuer et al, 2008; Lavergne and Molofsky, 2004; Procheş et al, 2012; Wilson et al, 2007, 2011)
This paper focuses on two species introduced for horticulture: G. monspessulana and S. junceum
The earliest record of G. monspessulana in South Africa is from Newlands on the Cape Peninsula from 1904 (Pretoria National Herbarium)
Summary
The distributions of many invasive plant species are to a large extent, determined by the extent of human usage of the species (e.g. Gravuer et al, 2008; Lavergne and Molofsky, 2004; Procheş et al, 2012; Wilson et al, 2007, 2011). Legumes have been introduced globally for multiple uses, predominantly in agriculture and horticulture Their nitrogen-fixing ability, drought resistance and the large number of edible species make them the second most important agricultural plant family after grasses (Graham and Vance, 2003). Legumes are popular in horticulture for their showy flowers, hardiness and ability to thrive in nutrient-poor soils. For these reasons, and because of the long seed dormancy in many species, legumes are among the most notorious contributors to the naturalised flora of the world (Binggeli, 1996; Paynter et al, 2003; Pyšek, 1998; Richardson and Rejmánek, 2011; Sánchez-Blanco et al, 2012). A group of legumes known collectively as brooms
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