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

Cytisus scoparius (L.) Link (Scotch broom) is a non-native shrub speci es now naturalized in California, currently occupying over 250,000 ha including several protected areas. In this paper, the impact of biocon trol agents and of general herbivory on two C. scoparius populations ( one in the Sierra foothills and one along the California northern coas t) 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 o f viable seeds/shrub/year was 9650. The number of pods and seeds produ ced in the drought year 1988 was an order of magnitude lower than thos e produced in 1987 or 1989. In spite of the introduction of biological control agents, no significant vertebrate or invertebrate herbivory w as found at any life stage of the foothills population; however, verte brate herbivory significantly decreased biomass of the northern coast C. scoparius. Cutting at the end of the dry season significantly decre ased the rate of resprouting. Implications of this study for biocontro l are discussed.