Optimal pollinator attraction strategies in Trollius ranunculoides Hemsl. (Ranunculaceae) at different altitudes: increased floral display or promotion of nectar output?

For alpine plant species, patterns of resource allocation to functional floral traits for pollinator attraction can be highly significant in adaptation to low pollinator abundance and consequent pollen limitation. Increased pollination can be achieved either through a larger floral display or production of more pollen rewards. In this study, variation in resource allocation to different components for pollinator attraction was studied along an altitudinal gradient in Trollius ranunculoides, an obligate self-incompatible out-crosser of the Qinghai-Tibet Plateau. We compared resource allocation to conspicuous yellow sepals (which mainly provide visual attraction) and degenerate petals (which provide the major nectar reward) between populations at four altitudes. Furthermore, we investigated the contribution of sepals and petals to pollinator attraction and female reproductive success in an experiment with sepal or petal removal at sites at different altitudes. At the level of single flowers, resource allocation increased to sepals but decreased to petals with increasing altitude. Consistent with these results, sepals contributed much more to visitation rate and seed set than petals, as confirmed in the sepal or petal removal experiment. Sepals and petals contributed to female reproductive success by ensuring visitation rate rather than visitation duration. To alleviate increasing pollen limitation with increasing altitude, resource allocation patterns of T. ranunculoides altered to favour development of sepals rather than petals. This strategy may improve pollination and reproductive success through visual attraction (sepal) rather than nectar reward (petal) over a gradient of decreasing pollinator abundance.