HERBIVORY, GROWTH, SEED PRODUCTION, AND RESPROUTING OF AN EXOTIC INVASIVE SHRUB CYTISUS-SCOPARIUS

Cytisus scoparius (L.) Link (Scotch broom) is a non-native shrub species now naturalized in California, currently occupying over 250,000 ha including several protected areas. In this paper, the impact of biocontrol agents and of general herbivory on two C. scoparius populations (one in the Sierra foothills and one along the California northern coast) were assessed and the productivity characteristics, seed production, and resprouting capabilities of this exotic species were determined in the field. The growth period occurs in May/June with dieback or no growth discernable from August to March. Estimated mean total number of viable seeds/shrub/year was 9650. The number of pods and seeds produced in the drought year 1988 was an order of magnitude lower than those produced in 1987 or 1989. In spite of the introduction of biological control agents, no significant vertebrate or invertebrate herbivory was found at any life stage of the foothills population; however, vertebrate herbivory significantly decreased biomass of the northern coast C. scoparius. Cutting at the end of the dry season significantly decreased the rate of resprouting. Implications of this study for biocontrol are discussed.