Woody vegetation can promote pollinator biodiversity in semi‐natural grasslands

Semi‐natural grasslands are biodiversity hotspots which host a high concentration and diversity of flowering plants and insect pollinators. Conservation policies commonly support management practices which limit woody encroachment to reduce competition with flowering plants and the floral resources they provide to pollinators. However, trees and shrubs provide nesting habitat and foraging resources for pollinators in addition to other environmental benefits such as microclimatic buffering. To design policies that maximize the combined benefits of trees, shrubs, and flowering plants in semi‐natural grasslands, it is necessary to identify the thresholds at which woody vegetation begins to negatively affect pollinators. Using a multi‐year dataset of 812 semi‐natural grasslands across Sweden, we found butterfly diversity peaked at 24% shrub cover before declining and was not negatively affected by deciduous hardwood trees until approximately 50% cover. Abundance of butterfly species which use trees as host plants increased with tree cover up to 33% and butterfly abundance increased with shrub cover up to 24%–29%, whether or not they used shrubs as host plants. Bumblebees were largely unaffected by grassland characteristics. These results suggest that conservation policies should incorporate allowances for moderate tree and shrub cover in grassland habitats with limited risk of adversely affecting pollinator communities.