The effects of plant density upon several determinants of reproductive success in Cistus landanifer (Cistaceae)

1. C. landanifer is a selfincompatible shrub which forms a major component of many Mediterranean habitats. It typically occurs in large, dense mono-species stands, though in some areas individuals may be more sparsely distributed. The plant produces prominent flowers which attract a wide diversity of pollinators. A proportion of these flowers subsequently develop into fruits which are subject to parasitism and seed predation. 2. C. landanifer populations were surveyed at three sites. Information relating to plant pollination success, reproductive effort and fruit parasitism was recorded. This data was plotted against plant density in order to determine density-dependant patterns of reproductive success. 3. Overall, pollination success declined significantly as the distance to the nearest cospecific increased. Reproductive effort and fruit parasitism showed the opposite pattern. Pollination success and reproductive effort per fruit increased with fruit height, but only in dispersed plants. 4. Measures of reproductive success were the product of a significant interaction between pollination success and reproductive effort. This means that dispersed plants were able to compensate for reduced pollination success by increasing reproductive effort. 5. These results indicate that overall reproductive success is the accumulated result of several different processes which may each respond to plant density in different ways. In order to effectively manage plant populations it is necessary to understand in greater detail the dynamic interactions between these processes.