Floral resources in Swedish grasslands remain relatively stable under an experimental drought and are enhanced by soil amendments if regularly mown

1. One of the main reasons why insect pollinators are declining is a lack of floral resources. In agricultural landscapes, remaining seminatural grasslands play a key role for providing such resources. However, droughts pose an increasing threat to the abundance and continuity of flowers. Soil amendments are a novel management tool for Swedish grasslands aiming to increase carbon sequestration and soil water holding capacity. In this study, we examined how drought is affecting floral resources (i.e. floral units, nectar quantity and nectar continuity) in grasslands with different mowing regimes, and if soil amendments could mitigate potential negative drought effects. 2. In summer 2019, we set up an experiment combining rain-out shelters ('drought'), soil amendments ('compost') and different mowing regimes ('mown' vs. 'abandoned') in four extensively managed Swedish grasslands (48 plots, size 2 m(2)). Between May and August 2021, we counted the floral units nine times in each plot. We derived values for the nectar sugar production per floral unit from an existing database. 3. We observed a decrease in floral units under drought in the mown, but not in the abandoned plots. Nectar quantity and continuity over the season were not significantly affected by drought across both mowing regimes-in the abandoned plots the nectar provision even extended slightly in duration (towards late summer). The compost treatment had positive effects on the floral units, nectar quantity and continuity (extending it towards early summer) in the mown, but not in the abandoned plots. The plant species in our study reacted differently to the treatments. Most of the nectar was provided by only few species (mainly Lathyrus pratensis, Vicia cracca and Anthriscus sylvestris). 4. The results are species specific, thus other plant communities might respond differently. However, our experiment shows that nectar provision (based on database values) in grasslands with a native plant community and natural soil conditions remains relatively stable under drought. We also found that soil amendments increase floral resources in managed grasslands.

Automated summary

One of the main reasons for the decline of insect pollinators is the lack of floral resources. Drought poses an increasing threat to the abundance and continuity of flowers in agricultural landscapes. Soil amendments are a novel management tool for Swedish grasslands aiming to mitigate the negative effects of drought on floral resources.