Experimental manipulation of plant density and its effect on pollination and reproduction of two confamilial montane herbs

The density of a plant population is expected to influence reproductive suc cess through changes in the quantity and quality of pollination service, or because both density and reproduction respond to quality of the local envi ronment. We reported previously that seed set in sparse natural populations of Delphinium nuttallianum and Aconitum columbianum was lower than in near by dense populations, whereas quantity of pollination service was equivalen t. To explore the hypotheses that environmental quality or pollination qual ity are lower in sparse natural populations, leading to lower seed set, we manipulated density using arrays of potted plants. In three replicate exper iments with D. nuttallianum, pollinator visitation rate and seed set were i ndistinguishable in sparse and dense arrays, consistent with the interpreta tion that environmental quality contributed to the earlier result in natura l populations of this species. In three replicates with A. columbianum, vis itation rate tended to increase with density, and seed set increased signif icantly, in contrast to our earlier result. One element of pollination qual ity, the degree of within-plant selfing, did not vary between sparse and de nse arrays. These results highlight the complexity of mechanisms by which p opulation parameters may influence plant reproductive success, and the temp oral variation that characterizes pollination service.