The relative effectiveness of different grassland restoration methods: A systematic literature search and meta-analysis

1. Active grassland restoration has gained in importance to mitigate the dramatic decline of farmland biodiversity. While there is evidence that such operations are generally effective in promoting plant diversity, little is known about the effectiveness of the different methods applied. Restoration methods can differ in intensity of seed bed preparation, seed source and technique of seed application.

2. In this systematic literature search and meta-analysis, we screened the literature for studies of the restoration of mesic grasslands in temperate Europe. We focused on active restoration experiments that included a treatment and lasted more than three years. We evaluated the influence of restoration factors on plant species richness relative to non-restored controls.

3. We found 187 articles that investigated the outcome of operations aimed at actively restoring mesic temperate grasslands. Most articles focused on plants, with only 9.6% dealing with other organisms (e.g. beetles, pollinating insects). Many papers had to be excluded due to incomplete data, too short study duration and/or lack of an adequate control. This resulted in 13 articles fulfilling our criteria for inclusion, yielding a total of 56 data points for the meta-analysis.

4. Restoration actions increased plant species richness by, on average, 17.4%, compared to controls. The seed source explained a significant amount of variation in plant species richness: seeds originating from a speciose donor grassland had a positive effect. This effect was even enhanced when combined with a commercial seed mix, whereas commercial seed mixes alone had no significant effect. We did not observe any effect of other factors, such as the type of seed bed preparation or the seed application method.

5. A seed-source obtained from species-rich grasslands seems to be key to efficient grassland restoration in mesic grasslands of temperate Europe. Even though seeds from a speciose donor grassland should be preferred over commercial seeds, associating natural and commercial seed mixes increases plant species richness. This systematic literature search further revealed two major research gaps in grassland restoration ecology: a deficit in long-term investigations as well as a deficit in studies focusing on non-plant organisms.