Many studies have investigated competition for resources between invasive and native plants, but few have examined competition for pollination. Flowering shrubs may affect pollinator visitation to co-flowering herbs directly, via competition or facilitation, or indirectly, via shading, and subsequently influence reproductive success of herbs. Shrubs may also directly reduce reproductive success of herbs via reduced light availability. The goal of this study was to determine the effect of the invasive shrub Lonicera maackii (Amur honeysuckle) on pollinator visitation and reproductive success of two native forest understory herbs, Geranium maculatum and Hydrophyllum macrophyllum, and the mechanism of this effect. I compared pollinator visitation rate, stigmatic pollen loads, fruit set, and seed set of native species among three experimental treatments: L. maackii removed (no shade, no flowers), L. maackii flowers removed (shade, no flowers), and L. maackii intact/flowering (shade and flowers). I also conducted a hand pollination experiment to assess pollen limitation of reproductive output of native herbs to determine the mechanism of the interaction between L. maackii and native herbs.

Results/Conclusions

Lonicera maackii significantly reduced the amount of light reaching the understory. The presence of L. maackii reduced fruit and seed set of G. maculatum in both flowering and de-flowered treatments compared to the removal treatment. Results suggest that this pattern is due to a pollinator-mediated effect, although hand pollinations were unsuccessful. Geranium maculatum stigmas in flowering plots received on average fewer conspecific pollen grains, and stigmas in both de-flowered and flowering plots contained higher percentages of L. maackii pollen. These results are consistent with two potential mechanisms of pollinator-mediated reductions in fruit and seed set in G. maculatum: competition for visits or interspecific pollen transfer. The bloom of H. macrophyllum and L. maackii did not overlap, but reduced light availability due to the presence of L. maackii shrubs caused a significant reduction in seed set of H. macrophyllum compared to the removal treatment.