Pollen limitation of reproductive success in two sympatric alpine willows (Salicaceae) with contrasting pollination strategies

We compared the extent of pollen limitation on female reproductive success of Salix lanata L., an entirely insect-pollinated willow, and S. lapponum L., which is 50:50% insect : wind pollinated (ambophilous). Supplemental hand-pollination significantly increased seed number per fruit by nearly 50% in the insect-pollinated willow, but had no significant impact on seed number in the dually pollinated species. Fruit set was not affected by the treatment in either of the species. These results demonstrate that pollen limitation on reproductive success is most pronounced in the species that depends entirely on insects for pollination. In general, pollinator visitation was highest to S. lapponum, but bumble bees were only observed on S. lanata, suggesting that the quantity and quality of pollinator visitation differed between the species. Our results empirically support the hypothesis that a dual pollination strategy is most effective in alpine environments with low and infrequent pollinator activity and high wind speeds.