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-incompatible species. In 1993, 1994, and 1995, we examined 13-17 populations, with densities ranging from one to 39 flowering ramets per square meter and sizes ranging from 25 to 27,000 flowering ramets. In 1993, stigmas received fewer pollen grains than in 1994 or 1995. In 1993 and 1995, population size and fruit set were significantly and positively related, and in 1993, both population density and stigma pollen load were significantly and positively related to fruit set. Stigma pollen load was not significantly related to population density or size. Our results indicate that the effects of population density and population size on plant reproduction in A. canadensis represent a range of outcomes influenced by differences between years in the quality (self vs. outcross) of pollen moved. We hypothesize that small populations typically have poor fruit set because they consist of relatively few genets and infrequently receive large amounts of outcross pollen, particularly when they are relatively close to large populations in protected areas.