Style Persistence, Pollen Limitation, and Seed Set in the Common Prairie PlantEchinacea angustifolia(Asteraceae)

Abstract

Pollen limitation of seed set in flowering plants has important ramifications for the population dynamics, evolution, and conservation of plant populations. I conducted a pollen addition and exclusion experiment demonstrating that style persistence signifies pollen limitation in the narrow-leaved purple coneflower Echinacea angustifolia, a species native to the North American prairie and plains. I developed a measure of style persistence, SP, a novel way to quantify pollen limitation in individual plants during the flowering season. Using this measure, I investigated the relationship between pollen limitation and seed set over two years in 19 and 27 natural remnant populations in an agricultural landscape. Population mean rates of seed set per plant varied from 0% to 54% in 1997 and from 0% to 63% in 1998. I found that pollen limitation reduced annual reproductive fitness within and among the populations studied. An analysis of the relationship between floret production and the rate of seed set provided no evidence that resource limitation influenced the rate of seed set. I estimated annual fecundity per plant as the product of the rate of seed set per floret, a pollen-limited process, and floret production per plant, likely a resource-limited process. Population means of individual annual fecundity ranged from 0 to 182 in 1997 and from 0 to 156 in 1998 and were predicted by population means of SP and the rate of seed set, but not by floret production. The effect of pollen limitation, as quantified by SP, overrides the strong, fundamental relationship between fecundity and floret production. This finding shows that populations consisting of large plants with large floral displays do not necessarily produce more seeds per plant.