COMPETITIVE EFFECTS OF CENTAUREA-MACULOSA ON THE POPULATION-DYNAMICS OF ARABIS-FECUNDA

We used a perturbation experiment coupled with demographic analysis to study competition between two short-lived taprooted perennial plants, the rare native Arabis fecunda and an aggressive exotic, Centaurea ma culosa. Centaurea maculosa was removed from randomly selected plots at two sites where these species occur together and the fate of mapped A . fecunda individuals was followed in treatment and control plots for two years at one site and four years at the other. We analyzed differe nces between treatment and control in recruitment, survival, individua l growth and fecundity separately. Estimates of equilibrium population growth (lambda) were obtained from matrix projection models to evalua te the overall competitive effects of C. maculosa on A. fecunda. Over the course of the study lambda was significantly higher in treatment p lots compared to controls, indicating that C. maculosa had a negative effect on A. fecunda populations. Recruitment was significantly higher in treatment plots during the first two years of the study. Survival, growth and fecundity did not differ between treatment and control. Th ese results indicate that the main effect of removing C. maculosa on p opulations of A. Secunda was enhanced recruitment resulting from incre ased seedling establishment. Competitive effects of C. maculosa on A. fecunda varied between sites and among years, indicating that long-ter m studies are required to fully understand the effects of competition on population dynamics.