An assessment of the invasive status of Cytisus scoparius and Ulex europaeus in South Africa

An assessment of the invasive status of Cytisus scoparius and Ulex europaeus in South Africa
Philani Mbatha1 , Syd Ramdhani1, Sershen Naidoo1, John R. Wilson2 3, Kanyisa Jama2
1 University of KwaZulu-Natal; 2 Invasive Species Programme, SANBI; 3 Centre for Invasion Biology, Stellenbosch University

Cytisus scoparius  and  Ulex europaeus are  recognised invasive plants in  many countries. Both species have been recorded in South Africa since the 1930’s. However, empirical data on their invasion in South Africa are scarce. To shed light on this, the present study characterised known and newly identified populations of both species in terms of demographics and reproductive potential. Individual plants were counted and growth parameters (height, crown and basal stem diameters) were measured. Flowering, fruiting and seed set timing was recorded and measures of reproductive capacity were based on seed set, seed bank load and germinability (for C. scoparius). Seventeen (four invasive and 13 naturalized) populations of C. scoparius and three U. europaeus (one invasive and two naturalized) populations were identified, varying in size from 96 to 6841 individuals over 2.4 ha for C. scoparius and 52 to 1883 individuals over 1 ha for U. europaeus. These populations were found along roadsides and in natural and commercial forests. In both species individuals ≥0.5 m were reproductively active. Seed bank density and germinability for C. scoparius varied widely across sites. Dormancy breaking treatments applied to C. scoparius seeds showed that they exhibited physical dormancy, which once broken resulted in ±100% germination. Growth parameters appeared to be poorly related to reproductive output (R2  < 0.5). Competitive ability, relative growth rate, potential spread, management options and associated costs of both species are still to be determined. Both species appear to pose a substantial threat to South African biodiversity.