A comment on “International principles and standards for the ecological restoration and recovery of mine sites”—useful but limited

International standards should be the basis for good practice in ecological restoration worldwide but need to be adapted to the particular conditions of each region. We welcome the publication of the Principles and Standards presented by Young et al. (2022) summarizing knowledge available for ecological restoration of various postmining sites. However, the content of the Standards is based on experience mostly gained from North America and Australia. Although we share many points, Europe, which has been rather neglected both in examples and references in the Standards, differs to some extent from the other continents. Therefore, the Standards cannot be fully adopted there.

Europe is a largely human-made landscape mostly depleted of pristine ecosystems and the targets of restoration are often semi-natural ecosystems (Welzholz & Johann 2007; Lundholm & Richardson 2010; Nunes et al. 2016). Most of the European landscape has been subject to rather intensive human impact since at least the beginning of the Neolithic period nearly 8,000 years ago (Poschlod 2015; Roberts et al. 2018), while North America and Australia were largely covered by more or less natural vegetation until the nineteenth century (Gajewski et al. 2019). Natural, undisturbed vegetation still survives there in some places, often near postmining sites, and may well represent reference and target sites with a high diversity. By contrast, in Europe, a variety of semi-natural habitats have developed under continuous human pressure, for example hay meadows and pastures in the temperate zone or pseudo-steppes and agro-silvopastoral systems in the Mediterranean zone (Marañon 1988), which harbor the greatest levels of biodiversity in both temperate (Wilson et al. 2012) and Mediterranean Europe (Buisson et al. 2020). Thus, the potential natural vegetation, that is, the theoretical terminal successional vegetation corresponding to site conditions (Somodi et al. 2021), may not always be the primary target of restoration. Instead, an alternative state related to semi-natural habitats of the European cultural landscape is often preferred (Siles et al. 2010; Tischew et al. 2014).

We are convinced that too much emphasis is placed on restoration of late- or terminal- successional stages in the Standards. In Europe, this would mean coniferous forests in the north, evergreen woodlands or maquis in the south, steppes in the east, and deciduous broadleaved woodlands in the rest of Europe (Bohn et al. 2000). However, initial and early successional stages shortly after a disturbance by mining, or intermediate successional stages with still limited tree establishment can be of great conservation value (Baasch et al. 2012; Alday et al. 2022; Münsch & Fartmann 2022). The European landscape is also heavily eutrophicated causing altered soil dynamics and shifts in species composition (Bobbink et al. 2010). Conversely, mining can create nutrient-poor sites serving as surrogate habitats for competitively weak species which are in steep decline in the surrounding landscapes (Jefferson 1984; Kirmer et al. 2008). These species are often rare and endangered Red List specialists (Twerd et al. 2021). However, a precondition for this is that neither nutrient-rich organic material, usually topsoil, nor saplings or seeds of less specialized and competitive species are introduced into these nutrient-poor mining sites. In central Europe, Řehounková et al. (2020) demonstrated that out of 271 Red List species occurring at various spontaneously restored postmining sites, a great majority (223) avoided closed woodlands, created within late successional stages. In this sense, we do not agree with the statement in the Standards that “the goal should be to achieve the highest level of recovery possible at a site” (p. 27), but would recommend aiming at the highest level of biodiversity and ecosystem functions possible for a specific-successional stage.

We also think the strong bond between some special soils, such as serpentine, gypsum or metalliferous outcrops, and specialized biota occurring on and in mined resources has been overlooked in the Standards. There is a long evolutionary history of adaptation of plants and soil microbiota to the particular edaphic substrates (Rajakaruna 2004; Hulshof & Spasojevic 2020). These substrates, if available after mining (e.g. not depleted or covered by topsoil), may constitute important habitats for unique plant communities and other associated fauna and microflora (Ballesteros et al. 2014; Boisson et al. 2017).

We are also convinced that spontaneous (passive) restoration of postmining sites, that is, an approach relying on natural succession without intentional human influence, is rather underestimated in the Standards. In a worldwide meta-analysis of 74 different successional series covering at least several decades at postmining sites, Prach and Walker (2020) clearly demonstrated that spontaneous succession used as a restoration tool was successful in reaching the target in 54% of cases, while it was partly successful in 34%, and failed in only 12% of cases. The “success of succession” was defined as the ability to reach a target, that is, the establishment of particular potential natural vegetation or another targeted state desirable for restoration, within 100 years, and the presence of weeds and aliens was unimportant in the target stages. This work also proved that spontaneous succession at postmining sites was the most successful in the temperate zone and the least in Mediterranean-type ecosystems. This means that in Europe we can often rely on spontaneous succession in its temperate zone if well-preserved or semi-natural habitats or their fragments occur in the surrounding, which should also be considered in restoration projects. Similarly, we are convinced that the combination of assisted and spontaneous succession should be especially considered in Mediterranean ecosystems. The only European study presented in the Standards from Kiruna in northern Sweden, that is, from the boreal zone of Europe, is not really representative of European conditions. We are aware of the great diversity of scenarios and regional constraints for restoration worldwide and thus, we miss long-term representative case studies in postmining sites (Regueiro & Alonso-Jimenez 2021), particularly in Europe (e.g. Jefferson 1984; Borgegård 1990; Prach et al. 2013; Tischew et al. 2014).

Spontaneous succession generally selects species which are adapted to a disturbed site much better than those selected by people in assisted restoration or reclamation (Bradshaw 2000). It must be appreciated that the authors of the Standards mention the use of natural restoration, natural regeneration and succession (sensu spontaneous succession) but we miss clear recommendations of how to apply the concept of spontaneous succession under different environmental conditions and in various landscapes and regions, that is, including those where native ecosystems or their fragments are missing (Prach et al. 2020). On the other hand, we are aware that spontaneous succession is not the best option everywhere and assisted succession could be an option under certain circumstances, for example lack of appropriate seed sources, invasive species abundant in the surroundings, extreme site conditions such as low pH (Costigan et al. 1981), but it could be applied whenever possible, also to save labor and money.

We would further like to highlight the importance of long-term monitoring of restored sites, especially if selected as case studies for a broader audience and encourage in the Standards a larger representation of case studies based on long-term rather than envisaged monitoring. Short-term data can lead to misinterpretation of the restoration results because quite different trends may occur after a decade or later as documented by Pospíšilová et al. (2022). Also, short-term results may promote unnecessary management at higher economic and even environmental costs (Oliveira et al. 2011). Therefore, we recommend careful selection of case studies which give evidence of a particular phenomenon or trends and are documented by representative, if possible statistically tested, data obtained by means of long-term monitoring.

We appreciate the inclusion of a chapter dealing with social aspects of restoration projects. The acknowledgement of the culture and rights of indigenous people is a topic of major concern in several continents, however lesser so in Europe. The Saami are the only recognized indigenous people of Europe living in the northern part, that is, Norway, Sweden, and Finland (Tuori 2015). Thus, the case study located in Kiruna, northern Sweden represents a very specific example. On the other hand, we miss a general focus on legislation concerning mining and restoration, which we regard to be one of the main barriers to the application of scientific approaches to the restoration of European mining sites (Cortina-Segarra et al. 2021). The diverse legislation in European countries makes the implementation of procedures and standards even more complicated. Therefore, a brief overview of the current state-of-the-art across the different continents with some recommendations for the practitioners would contribute to more realistic targets, for example definition of a minimum restored area of a postmining site for the purpose of nature conservation. We hope that the Nature Restoration Law, currently under negotiation among the European Union countries, will help in this respect in the near future.

In conclusion, Standards need be adapted to the European context, and modified to make it a more practical document after a broad discussion with a balanced selection of representatives across the biomes, regions and public and private sectors (FAO et al. 2021). The leading role in this adaptation process should be coordinated by a well-qualified team serving as a restoration hub at the European level. In the meantime, the Standards as presented by Young et al. (2022) should in Europe be regarded as a tentative, prevailingly academic-oriented framework with some substantial limitations.