Factors that extend flowering phenology for pollinators in prairie restorations

Abstract

Tallgrass prairies contain plant species that flower sequentially throughout the growing season, and having a range of flowering phenologies is assumed to be important for pollinators. However, it is poorly known what factors cause extended flowering and how to best manage for it in restorations. Here, we test the hypotheses that extended flowering in restorations is caused by 1) seed mixes of high forb abundance relative to grasses, 2) greater abundance of forbs that flower especially early and late in the growing season, 3) greater richness of early and late-flowering forbs, and 4) mowing during the growing season. We tested hypotheses with two separate restoration experiments, the first in which seed mixes with different grass: forb ratios were compared at a site in Iowa, USA (grass: forb ratio experiment), and the second in which early and late-flowering forbs were added (or were not added) to restorations that were subsequently mowed or not mowed at three sites in Iowa and Minnesota, USA (forb addition experiment). In both experiments, community floral volume was estimated at least monthly throughout the growing season and flowering niche breadth was calculated as a measure of extended flowering phenology. In addition, in the grass: forb ratio experiment visitation by pollinators was recorded. Plots sown with intermediate grass: forb ratios had greater flowering niche breadth, flower production, and pollinator diversity, and the increased species richness of these seed mixtures best explained positive effects on flowering niche breadth. Adding a greater number of forb species in the forb addition experiment increased flowering niche breadth and flower production, especially when forbs were added as transplants to bypass the seedling recruitment stage. Mowing reduced flower production, but it increased flowering niche breadth, primarily in the forb additions with greatest species richness. Our results indicate that extending flowering phenology is possible in restorations when seed mixes with intermediate grass: forb ratios are used and a large number of early and late-flowering forb species are added as transplants. These factors also increase flower production, which was a significant predictor of the number and diversity of pollinators that visited flowers.