The effects of population size and density on the reproductive output of Anemone canadensis L. (Ranunculaceae)

We studied the effects of population density and population size on stigma pollen load and fruit set in Anemone canadensis, a xenogamous self-incompat ible species. In 1993, 1994, and 1995, we examined 13-17 populations, with densities ranging from one to 39 flowering ramets per square meter and size s ranging from 25 to 27,000 flowering ramets. In 1993, stigmas received few er pollen grains than in 1994 or 1995. In 1993 and 1995, population size an d fruit set were significantly and positively related, and in 1993, both po pulation density and stigma pollen load were significantly and positively r elated to fruit set. Stigma pollen load was not significantly related to po pulation density or size. Our results indicate that the effects of populati on density and population size on plant reproduction in A. canadensis repre sent a range of outcomes influenced by differences between years in the qua lity (self vs. outcross) of pollen moved. We hypothesize that small populat ions typically have poor fruit set because they consist of relatively few g enets and infrequently receive large amounts of outcross pollen, particular ly when they are relatively close to large populations in protected areas.