Restoration can help to counteract biodiversity loss and to secure ecosystem functioning. In grassland ecosystems, a variety of restoration methods have been applied. Studies assessing their effectiveness often include categorical comparisons that ignore variation in restoration intensity, especially within one method. To account for the complexity of grassland restoration, a continuous metric that assesses restoration intensity would facilitate deeper analyses which may identify leverage points for successful restoration. Therefore, we propose a continuous multi-dimensional Restoration Intensity Index (RII) to quantify the degree of intervention and the follow-up management. The suggested RII includes the components (i) site preparation intensity, (ii) species introduction quality, and (iii) management appropriateness. We evaluated the RII using vegetation data from 77 restored grasslands covering a gradient in soil moistures and restoration methods across three regions in Germany. The restoration methods included adapted management, cultivar or regional seed mixtures, and transfer of directly harvested seed material. The RII was positively correlated with the overall and target plant species richness and Hill-Shannon diversity. Furthermore, species introduction quality and thus seed origin and seed composition were key components to promote total and target plant species richness, and to increase the proportion of forb cover. The time since restoration intervention did not influence grassland recovery, except in interaction with site preparation intensity. Thus, the RII is a promising tool enabling comparison of complex measures. It indicates that increased efforts in restoration intervention and follow-up management are decisive for restoring and maintaining species-rich grasslands in the long-term.
