Optimizing yield and flower resources for pollinators in intensively managed multi-species grasslands

Abstract

Intensively managed grasslands, typically containing a few high-yielding species, play a key role in providing biomass feedstock for animal feed and bioenergy production. However, these grasslands often contain few resources for flower-visiting insects, which provide pollination services for crops. In this study, we investigate if both high yield and floral resources for pollinators can be achieved in intensively managed grasslands by varying species diversity and composition, and cutting frequency. Four perennial grassland mixtures containing 3, 5, 11 and 13 species of grasses and/or forbs were established at three sites varying in surrounding landscape, and plots were managed with three cutting strategies (four cuts per year, two cuts per year and no cut). Biomass production was measured as annual herbage yield. Every 2–4 weeks throughout the flowering season, flower abundance, richness, temporal stability and flower-visiting insects were monitored. The 11- and 13-species mixtures, designed to enhance pollinators, produced similar or higher herbage yield than the 3-species mixture under a two-cut strategy or the 5-species mixture under a four-cut strategy. The 3- and 5- species mixtures had a high accumulated flower abundance due to excessive flowering of lucerne under the two-cut strategy and white clover under the four-cut strategy. Interestingly, flower abundance was not significantly reduced under the two-cut strategy compared to no cut. As expected, the 11- and 13- species mixtures presented a higher diversity of flowers during the flowering season. Flower temporal stability was generally higher in 11- and 13-species mixtures under the two-cut strategy, while it was highest in the 5-species mixture under the four-cut strategy. Pollinator profiles (visits by different functional groups of insects) were plant-species specific, and varied very little among sites. Legume species mainly attracted large bees (honey bees and bumblebees), while the most abundant forbs attracted other insect groups, in particular syrphids and other flies. Our results suggest that multi-species grasslands can be designed to support a high diversity of pollinators without compromising herbage yield. In particular, adding forbs to the grass-legume seed mixtures and managing grasslands with a two-cut strategy may increase flower resources available for a larger range of wild pollinators.