DOI: 10.5061/dryad.2v6wwpzt1
title: The relative effectiveness of different grassland restoration methods: A systematic literature search and meta-analysis

description: 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.
description: We followed the guidelines of
 the Collaboration for Environmental Evidence (Pullin & Stewart,
 2006) and the ROSES
 standard (see ROSES form in Table S1) to conduct our systematic literature
 search. By doing so, we ensure repeatability of our search and screening
 process (Romanelli, Meli, Naves, Alves, & Rodrigues,
 2021; Romanelli, Silva, et al., 2021). We prepared a protocol that was peer-reviewed
 and published (Slodowicz,
 Humbert, & Arlettaz, 2019). As some points of the original protocol had to
 be amended due to some unexpected issues, the following section contains
 the updated protocol. Systematic literature
 search We formulated our research question according to
 the PICO-structure (population, intervention, control, outcome): Do
 different seed addition methods for the restoration or re-creation of
 species-rich grasslands differ in their effectiveness to enhance the
 diversity of plants? (See Table S2 for details on the question
 components). Based on the question components, we developed an initial
 search string that went through a scoping process. This initial string was
 used in a search in the Web of Science database. We compared the search
 result with the reference lists of two reviews on the same topic
 (Hedberg &
 Kotowski, 2010; Kiehl, Kirmer, Donath, Rasran, & Hölzel,
 2010). To achieve adequate
 sensitivity, we adapted the search string, until no references of the
 reviews were missed. We used the final search string as a template for our
 database searches and adapted it accordingly to the requirements of the
 respective databases: (grassland* OR meadow* OR pasture*) AND (restor* OR
 seed addition OR seed transfer OR hay transfer OR sow* OR strew*) AND
 (*diversity OR enhance* OR success OR richness OR establish*). We
 conducted the database searches between 26 November 2019 and 16 March 2020
 in Web of Science Core Collection, Scopus, Directory of open access
 journals (DOAJ) and eThOs. In addition, we used the ‘Publish or perish’
 software (Harzing,
 2007) to search articles
 in Google Scholar and retained the first 1000 hits. A detailed overview of
 the search string development and database searches can be found in the
 supplementary material (Table S3, Appendix S1).
 To complement the database
 search, we looked for other publications and grey literature in Google,
 organisational websites and through direct requests to authors. The
 searches and requests were done in English, French, German and Polish. We
 removed duplicates automatically using the JabRef Reference Manager
 (JabRef Development Team,
 2021). Article screening
 Screening was done on title,
 abstract and full-text level by two reviewers (DS, AD). A third reviewer
 (JYH, co-author of this paper) checked for inclusion consistency using
 Cohen’s Kappa (Pullin
 & Stewart, 2006)
 on a subsample of 500 articles from each reviewer, respectively. A Kappa
 score of &gt; 0.6 indicated high consistency between reviewers. At
 title and abstract level, we included all restoration studies that were
 conducted within temperate Europe. At full-text level, the articles had to
 fulfill our eligibility criteria for inclusion (Table 1). We distinguished
 three grassland habitat types: dry, wet and mesic. We considered grassland
 habitats to be “dry” if the substrate was coarse or sandy with low water
 retention capacity and low amount of nutrients in the soil
 (e.g., Wolff et al.,
 2017). We considered
 grasslands to be “wet” if they were peat or fen meadows
 (e.g., Klimkowska et al.,
 2010), or alluvial meadows
 (e.g., Schmiede et al.,
 2009). All other
 grasslands were considered mesic and therefore eligible populations. In
 our meta-analysis, we have intentionally excluded short-term studies of
 less than three years duration to reduce confounding factors. In effect,
 the first two years after restoration are typically characterized by a
 rise in species richness (Albert et al., 2019; Baasch, Engst, Schmiede,
 May, & Tischew, 2016; Freitag et al., 2021). This is often due to the presence of ruderal
 species, which have become dominant in the seed bank after perturbation
 (Valkó, Rádai, &
 Deák, 2022). Once the
 grassland species become more dominant, the number of ruderal species
 diminishes (Albert et al.,
 2019). This is reflected
 in a slight decline in species richness after the second year of
 restoration (Freitag et
 al., 2021). For this
 reason, we focused on the mid-term, thus ensuring that the plant community
 had become more stable. Yet, to identify research gaps in a later phase,
 we compiled a separate list of all excluded European studies at full-text
 screening. All articles on European grassland restoration excluded during
 the full-text screening step are provided in Table
 S4. Data extraction and
 moderators The geographic location of each restoration site
 was recorded and, if necessary, changed into decimal degrees. If the site
 coordinates were not provided, we looked for a locality (such as a city,
 village, or a protected area) in the site description of the respective
 article and determined the coordinates from Google Maps. As potential
 moderator variables (effect modifiers) we included the control type, type
 of study design, seed source, seed material, seed bed preparation (seed
 bed preparation is requisite for efficient seed addition), former
 land-use, restoration duration, number of experimental replicates, as well
 as vegetation survey plot size (see Table 2 for a detailed definition of
 each moderator). These moderator variables are either linked to applied
 aspects of grassland restoration (e.g., seed source), which are relevant
 for practitioners, or to experimental aspects (e.g., control type). As
 response variables, we extracted the mean plant species richness and a
 measure of variance (which was converted to standard deviation if
 necessary) from the restored and control plots. We extracted these from
 summary tables, calculated it from raw data or extracted it from the
 figures using the WebPlotDigitizer (Rohatgi, 2021). We contacted the authors by e-mail to request
 missing data if relevant data was missing or not extractable from the
 study. A list of data sources used in the study is provided in the Data
 sources section. References 
 Albert, Á. J., Mudrák, O.,
 Jongepierová, I., Fajmon, K., Frei, I., Ševčíková, M., … Doležal, J.
 (2019). Grassland restoration on ex-arable land by transfer of
 brush-harvested propagules and green hay. Agriculture, Ecosystems and
 Environment, 272(November 2018), 74–82. doi:
 10.1016/j.agee.2018.11.008 Baasch, A., Engst, K., Schmiede, R., May, K.,
 & Tischew, S. (2016). Enhancing success in grassland restoration
 by adding regionally propagated target species. Ecological Engineering,
 94(2016), 583–591. doi:
 10.1016/j.ecoleng.2016.06.062 Freitag, M., Klaus, V. H., Bolliger, R., Hamer,
 U., Kleinebecker, T., Prati, D., … Hölzel, N. (2021). Restoration of plant
 diversity in permanent grassland by seeding: Assessing the limiting
 factors along land-use gradients. Journal of Applied Ecology, 58(8),
 1681–1692. doi: 10.1111/1365-2664.13883
 Harzing, A. W. (2007).
 Publish or Perish. Retrieved from
 https://harzing.com/resources/publish-or-perish/
 Hedberg, P., &
 Kotowski, W. (2010). New nature by sowing? The current state of species
 introduction in grassland restoration, and the road ahead. Journal for
 Nature Conservation, 18(4), 304–308. doi:
 10.1016/j.jnc.2010.01.003 JabRef Development Team. (2021). JabRef --- An
 open-source, cross-platform citation and reference management software.
 Retrieved from https://www.jabref.org/
 Kiehl, K., Kirmer, A.,
 Donath, T. W., Rasran, L., & Hölzel, N. (2010). Species
 introduction in restoration projects - Evaluation of different techniques
 for the establishment of semi-natural grasslands in Central and
 Northwestern Europe. Basic and Applied Ecology, 11(4), 285–299. doi:
 10.1016/j.baae.2009.12.004 Klimkowska, A., Kotowski, W., Van Diggelen, R.,
 Grootjans, A. P., Dzierz̈a, P., & Brzezińska, K. (2010).
 Vegetation Re-development After Fen Meadow Restoration by Topsoil Removal
 and Hay Transfer. Restoration Ecology, 18(6), 924–933. doi:
 10.1111/j.1526-100X.2009.00554.x
 Pullin, A. S., &
 Stewart, G. B. (2006). Guidelines for systematic review in conservation
 and environmental management. Conservation Biology, 20(6), 1647–1656. doi:
 10.1111/j.1523-1739.2006.00485.x
 Rohatgi, A. (2021).
 Webplotdigitizer: Version 4.5. Retrieved from
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 Romanelli, J. P., Meli, P.,
 Naves, R. P., Alves, M. C., & Rodrigues, R. R. (2021). Reliability
 of evidence-review methods in restoration ecology. Conservation Biology,
 35(1), 142–154. doi: 10.1111/cobi.13661
 Romanelli, J. P., Silva, L.
 G. M., Gonçalves, M. C. P., Naves, R. P., de Almeida, D. R. A., de
 Resende, A. F., & Rodrigues, R. R. (2021). Repeatability of the
 searching process in reviews of restoration outcomes. Restoration Ecology,
 29(8), 1–10. doi: 10.1111/rec.13496
 Schmiede, R., Donath, T. W.,
 & Otte, A. (2009). Seed bank development after the restoration of
 alluvial grassland via transfer of seed-containing plant material.
 Biological Conservation, 142(2), 404–413. doi:
 10.1016/j.biocon.2008.11.001 Slodowicz, D., Humbert, J. Y., &
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 protocol. Environmental Evidence, 8(1), 1–7. doi:
 10.1186/s13750-019-0174-2 Valkó, O., Rádai, Z., & Deák, B. (2022).
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 Hölzel, N., & Schneider, S. (2017). Status and restoration
 potential of heathlands and sand grasslands in the southwest of
 Luxembourg. Tuexenia, 37(1), 179–200. doi:
 10.14471/2017.37.007 
 Data
 sources Albert, Á.-J.,
 Mudrák, O., Jongepierová, I., Fajmon, K., Frei, I., Ševcíková, M., …
 Dolezal, J. (2019). Grassland restoration on ex-arable land by transfer of
 brush-harvested propagules and green hay. Agriculture, Ecosystems and
 Environment, 272(November 2018), 74–82. doi:
 10.1016/j.agee.2018.11.008 Auestad, I., Austad, I.,
 & Rydgren, K. (2015). Nature will have its way: Local vegetation
 trumps restoration treatments in semi-natural grassland. Applied
 Vegetation Science, 18(2), 190–196. doi: 10.1111/avsc.12138
 Baasch, A., Engst, K., Schmiede, R., May, K., & Tischew,
 S. (2016). Enhancing success in grassland restoration by adding regionally
 propagated target species. Ecological Engineering, 94(2016), 583–591. doi:
 10.1016/j.ecoleng.2016.06.062 Chevalier, R., Le Bris,
 C., Swiderski, C., Baron, S., Isselin-Nondedeu, F., Lesage, C., &
 Michau, F. (2018). Restauration de la
 biodiversité floristique des bordures de champs par semis de fleurs
 sauvages : premiers résultats obtenus en Beauce. Sciences Eaux &
 Territoires, Numéro 25(1), 52. doi: 10.3917/set.025.0052
 Dolnik, C.,
 Jansen, D., & Rickert, B.-H. (2020). Praxisleitfaden BlütenMeer
 2020. Molfsee. Retrieved
 from
 https://www.stiftungsland.de/fileadmin/pdf/Bluetenmeer2020/20-2841_Praxisleitfaden_Naturschutz_Internet.pdf Freitag, M., Klaus, V. H., Bolliger, R., Prati, D., Schäfer, D., Hinderling, J., … Hölzel, N. (2021). Data and Code for the manuscript Freitag et al. “Restoration of plant diversity in permanent grassland by seeding: assessing the limiting factors along land-use gradients.” doi: https://doi.org/10.25829/bexis.30915-10 Fritch, R. A., Sheridan, H., Finn, J. A., Kirwan, L., & hUallacháin, D. Ó. (2011). Methods of enhancing botanical diversity within field margins of intensively managed grassland: A 7-year field experiment. Journal of Applied Ecology, 48(3), 551–560. doi: 10.1111/j.1365-2664.2010.01951.x Gentili, R., Montagnani, C., Gilardelli, F., Guarino, M. F., & Citterio, S. (2017). Let native species take their course: Ambrosia artemisiifolia replacement during natural or “artificial” succession. Acta Oecologica, 82, 32–40. doi: 10.1016/j.actao.2017.05.007 Hovd, H. (2008). Occurrence of meadow herbs in sown and unsown ploughed strips in cultivated grassland. Acta Agriculturae Scandinavica Section B: Soil and Plant Science, 58(3), 208–215. doi: 10.1080/09064710701593046 Prach, K., Jongepierová, I., Řehounková, K., & Fajmon, K. (2014). Restoration of grasslands on ex-arable land using regional and commercial seed mixtures and spontaneous succession: Successional trajectories and changes in species richness. Agriculture, Ecosystems and Environment, 182, 131–136. doi: 10.1016/j.agee.2013.06.003 Smith, B. M., Diaz, A., & Winder, L. (2017). Grassland habitat restoration: Lessons learnt from long term monitoring of Swanworth Quarry, UK, 1997-2014. PeerJ, 2017(11), 1997–2014. doi: 10.7717/peerj.3942 Veen, G. F., van der Putten, W. H., & Bezemer, T. M. (2018). Biodiversity-ecosystem functioning relationships in a long-term non-weeded field experiment. Ecology, 99(8), 1836–1846. doi: 10.1002/ecy.2400 Woodcock, B. A., Vogiatzakis, I. N., Westbury, D. B., Lawson, C. S., Edwards, A. R., Brook, A. J., … Mortimer, S. R. (2010). The role of management and landscape context in the restoration of grassland phytophagous beetles. Journal of Applied Ecology, 47(2), 366–376. doi: 10.1111/j.1365-2664.2010.01776.x Woodcock, B. A., Edwards, A. R., Lawson, C. S., Westbury, D. B., Brook, A. J., Harris, S. J., … Mortimer, S. R. (2010). The Restoration of Phytophagous Beetles in Species-Rich Chalk Grasslands. Restoration Ecology, 18(5), 638–644. doi: 10.1111/j.1526-100X.2008.00472.x Woodcock, B. A., Westbury, D. B., Brook, A. J., Lawson, C. S., Edwards, A. R., Harris, S. J., … Mortimer, S. R. (2012). Effects of seed addition on beetle assemblages during the re-creation of species-rich lowland hay meadows. Insect Conservation and Diversity, 5(1), 19–26. doi: 10.1111/j.1752-4598.2011.00132.x Note that the three Articles by Woodcock, Vogiatzakis, et al., (2010), Woodcock, Edwards, et al., (2010) and Woodcock et al., (2012) shared several study sites, which we included in our meta-analysis. Their effect sizes are summarized in Fig. 3 as “Woodcock et al., 2010”.
description: All analyses were performed
 with R version 4.1.1 (R
 Core Team, 2021) using the
 <em>metafor</em> (Viechtbauer, 2010) and <em>puinform</em> packages
 (van Aert,
 2018).<br>