MULTIYEAR STUDY OF POLLEN LIMITATION AND COST OF REPRODUCTION IN THE ITEROPAROUS SILENE VIRGINICA

We investigated whether pollen deposited onto stigmas limited female r eproductive success in the hummingbird-pollinated, short-lived, iterop arous, Silene virginica (Caryophyllaceae). The study was conducted ove r a 4-yr span in a population occurring in a woodland area and over a 3-yr span in a second population occurring in a nearby open meadow. We contrasted average fruit set, seed set per fruit, and total annual se ed production (in only the woodland site) between open-pollinated cont rol plants and hand-pollinated experimental plants. We also followed s urviving individuals in subsequent years and repeated the same treatme nts on plants when they flowered. All plants were monitored annually f or survival and reproduction at both sites; growth was monitored at on ly the woodland site because of extensive herbivory in the meadow popu lation. Lack of pollen deposited onto stigmas significantly limited re productive potential at the level of percentage fruit set throughout t he study for both sites. In contrast, pollen deposition onto stigmas e xhibited considerable site and year heterogeneity in its effect on see d production per fruit. In the cumulative test of pollen limitation, h owever, we detected no difference between total annual seed production between our open-pollinated control and hand-pollinated experimental plants in the woodland site during the 4-yr study. A weak negative tre nd was detected between fruit set per plant and average seed set per f ruit among all plants in the woodland site, suggesting a limited role for an intraplant compensation mechanism. No significant trade-off was detected in probability of survival and flowering between the control and hand-pollinated experimental treatment groups at either site. In addition, no cost was detected in future growth and reproduction in th e woodland population, Similar total seed production among individuals in the two treatment groups explains in part, why no difference was o bserved in future survival, growth, and reproduction between the contr ol and hand-pollinated treatment groups.