Interactions between a native and non-native annual of coastal sage scrub: Can Emmenanthe penduliflora suppress Centaurea melitensis?

Centaurea melitensis (Maltese star-thistle or tocalote) is native to Mediterranean regions of Europe. It has colonized areas with similar climates throughout the world, including southern California. It is most common in disturbed areas and grasslands in southern California, but is also found in coastal sage scrub and chaparral. We conducted this study to assess the potential for a native, fast-growing species to displace C. melitensis in these systems. We planted C. melitensis and Emmenanthe penduliflora, a native annual that is common after fires, in monoculture and mixture at two densities in a greenhouse study with a replacement-series design. The mass of E. penduliflora decreased with increasing plant density, but this decrease did not depend on the identity of the competitor. The mass of C. melitensis also decreased with plant density, and this decrease did not depend on the identity of the competitor either. There were indications that C. melitensis suppressed or delayed flowering in E. penduliflora, but this result was not strong. Results of our study suggest that C. melitensis and E. penduliflora are fairly equivalent in their ability to inhibit each other’s growth. The ability of E. penduliflora to displace C. melitensis, therefore, may depend entirely on the relative densities of the two species in the field. Further field studies of interactions among C. melitensis, E. penduliflora, and native shrub species are needed to fully assess the desirability of using a fast-growing native annual, such as E. penduliflora, to control C. melitensis in chaparral and coastal sage scrub.