Restoring plant diversity in lowland grasslands: Efficacy of different seed addition and soil preparation methods

The biodiversity of semi-natural grasslands has dramatically declined over the past century. We experimentally tested, at field scale, the efficacy of different assisted (active) restoration methods to enhance plant biodiversity in species-poor, extensively managed lowland meadows. Four restoration treatments, plus one control were randomly allocated to five meadows at 12 spatially-replicated study sites in the Swiss lowlands in 2019: 1) hay transfer from a species-rich donor meadow on a harrowed meadow; 2) the same as 1 but on a ploughed meadow; 3) sowing a directly harvested native seed mixture originating from a species-rich donor meadow on a ploughed meadow; 4) sowing a multiplied native seed mixture on a ploughed meadow; 5) control, i.e. with no soil disturbance and no reseeding. Vegetation surveys were performed before (2018) and after the restoration actions (2021 and 2023). After a pronounced biodiversity rise in 2021, plant species richness had stabilized by 2023 across most treatments with, on average, 29% more species compared to 2018 and 16% more species in restored compared to control meadows in 2023. Moreover, 90% of the restored meadows qualified for the result-based payment scheme in 2023. Harrowing before seeding was as effective as ploughing. While the multiplied seed mixture initially showed a stronger increase, this effect had decreased by 2023, reaching levels comparable to the other treatments. Beta diversity was higher in hay transfer treatments than in the control and other restoration treatments, indicating greater heterogeneity in community composition. Findings provide evidence-based guidance for cost-effective and practically feasible grassland restoration. • Rise of plant species richness after two years of restoration, then stabilization. • 16% more plant species in restored compared to control meadows. • 90% of restored meadows qualifying for the result-based payment scheme. • Harrowing before seeding as effective as ploughing.