Abstract/Summary

Habitat restoration requires realistic goals. To naturally regenerate European lowland calcareous grassland,whose extent has severely declined, over a century may be required for vegetation to become indistinguishablefrom that of old calcareous grassland. Progress of natural regeneration can be characterized using memberspecies of the reference vegetation as indicators of favourable site condition. Chronosequence studies havesuggested that calcareous-grassland species differ predictably in their ability to colonize ex-arable land, withsome usually colonizing early on, and others in later stages. If such patterns are affected by gradually-attenuatingestablishment limitation, this would have important implications for restoration practice and indication ofprogress. Particularly, late-colonizing species might be better indicators of favourable site conditions than earlycolonizers.To explore these aspects, we have reanalysed chronosequence data previously used to investigate causalmechanisms affecting calcareous-grassland restoration progress. We carried out an indicator species analysis todetermine which species are indicative of particular stages of natural regeneration. Using correlation analyses,we tested whether species colonization patterns matched those found by previous chronosequence studies thatwere geographically more limited or relied on more informal approaches to determine species order of colo-nization. Correlation analyses were also used to test whether order of colonization could be explained by es-tablishment limitation or by dispersal limitation, or by established plant strategies that underlie such limitations.We identified 30 species as indicative of particular stages of natural regeneration, including nine that spe-cifically indicate old calcareous grassland. Correlation results confirmed high congruence with species order ofcolonization in previous chronosequence studies, and indicated that establishment limitation plays a role inshaping species order of colonization, potentially mediated through differential stress tolerance. We failed todemonstrate a role of dispersal limitation in shaping order of colonization. Based on our results, we derived threecategories of indicator species for passively-restored calcareous grassland, mirroring the regeneration stageduring which these species usually colonize. This includes a category labelled by us as‘old-grassland indicators’that achieve notable abundance only in old grassland. We conclude by discussing how such a categorization canbenefit the measurement of restoration progress, the tentative identification of old grassland and its conserva-tion, e.g. through linking agri-environment payments to the occurrence of old-grassland indicators, thus fos-tering positive change in farmer attitudes towards old grassland.