1. Introduction

A large proportion of the world’s mineral and energy resources are found in forested regions, which are consequently subjected to severe disturbance by surface mining. Reforestation is potentially the most beneficial reclamation measure because it partially mitigates some of the impacts across the landscape and increases biodiversity, soil ecosystem functions and biomass productionover time. Additionally, it is important to recognise that the legacy impacts of mining can extend far into the future, long after the first phase of reforestation is deemed successful (Dutta and Agrawal, 2003, Macdonald et al., 2015, Parrotta and Knowles, 2001, Pietrzykowski, 2015a, Pietrzykowski and Krzaklewski, 2018, Singh et al., 2002). From an ecological point of view, reclamation is a process of restoring the ecosystem and the integration of restoration strategies into mine planning is important for sustainable mining. (Bradshaw, 2000, Maiti and Ahirwal, 2019). The ecosystem is a basic ecological unit, constituting an integrated system of biotic and abiotic elements in which all trophic levelscontain a set of species ensuring circulation of matter and energy flow (originally definition formulated by Tensley, 1935 and developed by Golley, 1993). In fact reforestation of mine land is based on ecological restorationconcept as a holistic approach to long process of reconstructing mine degraded land which considers all the fundamental characteristics (physical, chemical and biological) that the environment must present to be considered as restored. Thus the stages of ecological restoration include a complex mixture of technical and biological processes, which have the potential to return a site to close to pre-mining conditions (Miao and Marrs, 2000, Maiti and Ahirwal, 2019).

Forest restoration is of significant interest and is a current global need (Angel et al., 2005, Burton and Macdonald, 2011, Macdonald et al., 2015, World Resources Institute, 2014). The planning of reforestation and sustainable forest management efforts is closely related to the understanding of tree species adaptation to reclaimed mine soil (Baumann et al., 2006, Macdonald et al., 2015, Walker, 2002). Thus, in recent years, interest has grown regarding soil establishment and development, microtopographic and soil heterogeneity and tree species selection and interactions with understory vegetation, soil biotaand soil microbial processes (Baumann et al., 2006, Farnden et al., 2013, Frouz et al., 2001, Frouz et al., 2013, Heinsdorf, 1996, Kuznetsova et al., 2010, Macdonald et al., 2015, Moss et al., 1989, Pietrzykowski and Socha, 2011, Torbert and Burger, 2000, Walker, 2002, Zipper et al., 2011a). Forests are structurally complex ecosystems containing a diversity of plant communities dominated by long-lived trees; thus, understanding and assessing the response of trees growing on reclaimed land to environmental stress under stressful site conditions is a crucial issue in the context of forest ecosystem dynamics. Furthermore, there is a need to ensure the resilience of reclaimed ecosystems in the face of large-scale natural and anthropogenic disturbances, which are connected to climate change (Johnson and Miyanishi, 2007, Macdonald et al., 2015, Malmsheimer et al., 2008, Pietrzykowski, 2015b, Pietrzykowski and Krzaklewski, 2018).

The aim of this review was to present the issues related to soil reconstruction and species composition in terms of the reforestation of reclaimed post-mine sites based on an assessment of tree adaptation, taking into account the following criteria: i) tree growth characteristics, ii) aboveground biomass and root system development, and iii) nutrient supply. This review provides examples of tree species that are important in the reforestation of post-industrial sites in central and eastern Europe, which include the Scots pine(Pinus sylvestris L.), European larch (Larix decidua Mill.), European oak (Quercus robus L.) and alders (Alnus incana (L.) Moench. and Alnus glutinosa (L.) Gaertn.).

2. Soil reconstruction under various post-mine site conditions

The main aim of reclamation is to reconstruct the soil as the basic element of the terrestrial ecosystem (Frouz et al., 2013, Hüttl and Bradshaw, 2000, Pietrzykowski et al., 2010a). Soil reconstruction methods may be divided into technical methods, in which the entire soil profile is reconstructed by topsoiling using organic and organic-mineral horizons previously removed from sites occupied by mining, or biological methods, which involve the incorporation of plant biomass via green manure (Pietrzykowski et al., 2017). The technology of reconstructing the entire soil profile is limited by the availability of rock overburden that is beneficial in terms of its soil formation properties. An example is the reclamation of the Rhine River basin in Germany, where loess in the upper Quaternary horizons is several metres deep. In this region, loess is mixed with sand and gravel at a ratio of 1:3, which produces substrate termed forest gravel (Forstkies) that is suitable for reforestation, mainly because of its suitable water-air properties (Topp et al., 2001). In general, the layering of cover materials with different textures creates interfaces that can influence soil watercapacity and moisture availability for plants (Macdonald et al., 2015, Zettl et al., 2011). In the mentioned reclamation effort in the Rhine River basin in Germany, the habitat heterogeneity at sites reclaimed with forest gravel can also be increased by the inclusion of patches of other lower-quality materials, even acidic Miocene substrates (Topp et al., 2001). However, acid-forming materials could also be used, mainly because of the oxidation of pyrite, and the utilisation of this substrate, which is dominant in many overburden sites associated with lignite mining, is an important strategy. Thus, the use of attenuated substrate containing good-quality material, such as loess, may reduce the negative effects associated with acid mine drainage, increase the biodiversity of specific microflora in the soil, and provide other benefits for not only forest species, including having different effects on plant succession.

If no good-quality soil is available, as is the case with extremely poor sandy substrates on sand mine casts, the soil reconstruction technology in the phase of biological reclamation could involve the application of green manure, i.e., humus-generating crops that fix atmospheric nitrogen. In the reclamation of former sand casts in Upper Silesia in southern Poland, before reforestation, yellow lupine (Lupinus luteus L.) is grown, and its biomass is incorporated into the soil in a two-year cultivation cycle combined with N-P-K mineral fertilisation. Additionally, following the reforestation of former sand casts, the lupine Lupinus polyphyllus L. is sown between rows of trees to supplement the soil with nitrogen (Pietrzykowski and Krzaklewski, 2009, Pietrzykowski et al., 2017).

Special treatment of deposits with neutralisers is required in areas made of Miocene acidic sandy formations occurring, e.g., in extensive stretches of the Lower Lusatia basin in eastern Germany, as well as in western Poland along the same coal steam, which has coal-bearing deposits associated with the former mine Przyjaźń Narodów in Łęknica (Katzur and Haubold-Rosar, 1996, Krzaklewski and Pietrzykowski, 2001). Acidic formations in coal deposits that have accumulated in the subsurface horizons of dumps also occur in parts of dumps in central Poland in the Bełchatów basin and in the southwestern Turoszów basin (Krzaklewski et al., 1997). Formations that naturally accompany mineral deposits, such as Neogene bog lime, in the Bełchatów basin may be used as a neutraliser in acidic formations (Pietrzykowski, 2010). In the Lower Lusatia basin, alkaline ash from the combustion of lignite from a nearby power station is used as a neutraliser (Katzur and Haubold-Rosar, 1996). The reclamation treatment and the doses and variations of the materials used for neutralisation are adapted to the specificities of different post-mine sites, their geological structure, the deposit mining and dumping technology, and the local availability of substrates (Pietrzykowski, 2015a).

In addition to the variability in soil characteristics among some mine soils, organic matter (OM) recently derived from vegetation occurs with fossil OM in coal and lignite (Vindušková et al., 2015). The contribution of such geogenic fossil carbon (OC) content to mine soil properties and its role in nutrient cycling and the tree nutrient supply are still poorly understood (Vindušková et al., 2015, Vindušková and Frouz, 2013). Poor mine soil properties occur in addition to deficiencies in nutrients and disturbances to their quantitative ratios, which results in an inadequate supply of nutrients for trees (Heinsdorf, 1996, Pietrzykowski et al., 2013). This in turn affects the growth, health and stability of the introduced tree stands (Katzur and Haubold-Rosar, 1996, Knoche et al., 2002, Ochał et al., 2010, Pająk et al., 2011, Pietrzykowski, 2010, Zipper et al., 2011b).

Additionally, an unfavourable characteristic of reclaimed mine soils is the potential for rapid changes in their properties, especially acidity (pH), which is evident, e.g., in the areas composed of Carboniferous rocks deposited on coal dumps and Neogene deposits that compose lignite mine dumps (Aldag and Strzyszcz, 1980, Katzur and Haubold-Rosar, 1996). As previously mentioned, strong acidification occurs in these deposits as a result of pyrite and marcasiteweathering. The above-listed features of the reclaimed mine soils indicate that the resulting habitat conditions make the introduction of forest trees difficult and are significantly different from those of natural forest habitats (Knoche et al., 2002, Pietrzykowski and Krzaklewski, 2007a, Pietrzykowski, 2014). Thus, under such conditions, reforestation plans should consider increasing the diversity of the species composition, including through the introduction of pioneer species, which may subsequently be replaced with target (climax) species (Pietrzykowski, 2014). However, which reforestation strategy should be used remains disputable, and questions remain about the role and the timing of the introduction of early successional and climax species to reclaimed post-industrial sites (Frouz et al., 2015a, Zipper et al., 2011a).

3. Pioneering versus climax tree species in reforestation strategies

The identification of optimal reforestation strategies and species compositions on post-industrial sites is inextricably linked to the assessment of the adaptation of individual tree species to the habitat conditions on reclaimed landand the impacts of these species on the soil substrate (Baumann et al., 2006, Chodak and Niklińska, 2010, Frouz et al., 2013, Frouz et al., 2015a, Frouz et al., 2015b, Kuznetsova et al., 2011, Pietrzykowski et al., 2014b). Depending on the differences in the adaptation of tree species to stressful post-industrial sites, they are classified as pioneering or target, i.e. climax or late successional tree species (Frouz et al., 2015a, Pietrzykowski, 2015a). This classification is based on the understanding of the classical succession theory by Clements, in which the first stage of succession involves ecosystem colonisation by pioneering species (Odum & Barrett, 2005). According to this theory, pioneering species prepare the habitat for the more demanding late successional species. Under natural conditions, pioneering tree species are better able to tolerate high insolation, temperature fluctuations in open areas, a lack of organic matter and water deficits. These species often have small seeds that are easily dispersed by wind, while climax species tend to have a capacity for vegetative propagation, and their large seeds are dispersed from the parent tree in many different manners (Jansen et al., 2002, Johnson et al., 1997, Oddou-Muratorio et al., 2011, Wagner et al., 2010. Reforestation strategies based upon successional theory aim to imitate the process of primary succession. Regardless of the reclamation treatments used, the process of ecological succession takes place independently. From the viewpoint of reclamation efficiency, it would be best to create habitat conditions in which this process proceeds in a predictable and desired direction (Bradshaw, 1983). Only then would it be right to say that succession may be managed (Bradshaw, 2000, Krzaklewski, 1993, Marrs and Bradshaw, 1993).

The dynamics of natural tree succession on newly colonised sites are dependent, among other things, on the seed bank, seed dispersal, seed germination, seedling survival and the distribution of microhabitats suitable for tree development (Frouz et al., 2015b, Johnson and Miyanishi, 2007, Macdonald et al., 2015). For example, the zoochory or dispersal of seeds by animals is particularly important for the long-distance migration of seeds (Jansen et al., 2002, Johnson et al., 1997). The seeds, however, must find appropriate microhabitats, and only under such conditions may the species achieve reproductive success. If a seedling later grows under the canopy of trees introduced during reforestation, for example, it will face considerable competition from older trees. On the other hand, gradually invading late successional species may use the cover of pioneering species in the stand and benefit from the better soil conditions created by the pioneers (Kunstler et al., 2006). No extensive, detailed studies on the mechanism of pioneering species replacement by late successional species in the stands on reclaimed land have been conducted thus far (Frouz et al., 2015a). Elucidating the mechanism of this process is vital for many reasons; it may, for example, be helpful in understanding the interactions among species, which include competition and facilitation. Conducting such studies makes it possible, therefore, to learn about the natural dynamics of reconstructed forest ecosystems and the succession of communities in areas undergoing anthropogenic transformation, which show some similarities to the process of succession after large-scale natural disturbances, such as in conflagration sites or sites that have suffered from a hurricane (Johnson and Miyanishi, 2007, Michaletz and Johnson, 2007, Pickett, 1987). These investigations are important not only from the viewpoint of the development of basic cognisance but also in a practical sense to identify an appropriate reclamation strategy and a suitable method for reforestation.

Three basic reforestation strategies are utilized in central and eastern Europe: reforestation with pioneering species, reforestation with target species and reforestation with a combination of pioneering and target species, such as oaks(Quercus spp.), and the significant addition of alders (Alnus spp.) (Frouz et al., 2015a, Frouz et al., 2015b, Pietrzykowski, 2014) based on their phytomeliorationfunction and N-fixing ability (Krzaklewski, 2009, Wójcik, 2002). In general, the strategy of reforestation with pioneering species pertains to the early stages of natural succession and the so-called forecrop forest, which is later replaced by target vegetation (Marrs and Bradshaw, 1993, Pietrzykowski, 2014). It should be mentioned that the use of pioneering herbaceous and woody vegetation for the biological restoration of soil on reclaimed land was proposed in the early twentieth century in the first doctoral thesis on this subject written by Heuson in 1928 (Pflug, 2013). Currently, most researchers question the sequential and orderly sequence of successional stages (Frouz et al., 2015a, Frouz et al., 2015b). A larger role in the dynamics of succession is now attributed to disturbances to the ecosystem, the availability and distribution of seeds and the location of microhabitats during the process of biotope colonisation by both early and late successional species (Frouz et al., 2015a, Johnson and Miyanishi, 2007). Nevertheless, under natural conditions in the temperate zone in post-disaster forest areas or fallow agricultural land, photophilous pioneering species, which are less demanding than other species in terms of nutrient requirements and more resistant to temperature fluctuations, are the first to appear on a large scale. More sensitive and shade-tolerant tree species called climax or late successional species, which constitute the target stands, appear in greater numbers and grow under their cover (Frouz et al., 2015a, Pietrzykowski, 2014).

At afforested mine sites, decisions about replacing the introduced monocultureswith pioneering species by stands consisting of more demanding species, including hardwoods, may be made after restoring is completed (Pietrzykowski, 2014). Such decision making particularly concerns post-mine sites with relatively fertile soils. In recent decades, the replacement of pine monocultures in more fertile habitats by mixed stands has been vital in forestry practices throughout central and eastern Europe (Knoche, 2005, Knoche and Ertle, 2010, Pietrzykowski, 2014). The need to transform monocultures into mixed deciduous forests at post-mine sites is also connected with the desire to achieve greater biodiversity and stability, i.e., resistance to damage by insects and fungi and a balance between the production of litter and soil organic matter, in forest habitats (Fischer et al., 2002, Pietrzykowski, 2014).

In central European reclamation practices, the introduction of target species normally occurs after intensive agricultural cultivation (Bender, 1995), and biodynamic reforestation with an admixture of alders has a phytomelioration function and a favourable impact on the growth of the target species and soil-formation processes. Another important aspect is the low competitiveness of alders in comparison with that of the main species (e.g., oaks). As a result, after several years of favourable protection and soil-formation impacts, alders spontaneously disappear, which reduces the effort required for remediation (Pietrzykowski, 2015b). An example of the practical application of alders is the reclamation of the Adamów lignite mine dump (Central Poland) and Turów (southern Poland) (Nietrzeba-Marcinonis, 2007, Wójcik, 2002). This topic is discussed in more detail in the subsequent sections. On the other hand, a study by Frouz et al. (2015a) mentions that the spontaneous establishment of late successional tree species English oak and European beech is significantly better at unreclaimed post-mine sites covered by successional woody vegetation than at sites reclaimed by planting alder.

4. Tree stand productivity and biomass as criteria of reclamation success

Tree stands, which dynamically transform the biotope to create a specific microclimate and shape the properties of the emerging soils over time, are the basic element in reconstructed forest ecosystems apart from the soil. A correct habitat diagnosis and the compliant adaptation of stand species composition have an impact on the stability and dynamics of recreated forest ecosystems (Andrews et al., 1998, Gale et al., 1991, Heinsdorf, 1996, Krzaklewski and Pietrzykowski, 2007, Pietrzykowski et al., 2013). The suitable selection of species adapted to the habitat conditions at a reclaimed facility means the identification of the best possible combination of tree species and initial soil conditions.

5. Tree root development on mine soils

In addition to the significant aboveground biomass in forest ecosystems, there is also a large amount of underground biomass that plays an extensive role in the circulation of matter and energy (Schlesinger, 1978, Wardle et al., 2004, Waring and Schlesinger, 1985). Assessments of tree root systems, including their biomass, which has been divided into thick and fine root fractions, vitality, range, density, and distribution in the soil profile, are important for the evaluation of the condition of trees and their adaptability to different habitat conditions (Nielsen & Hansen, 2006). This is particularly important in the case of post-mining areas reclaimed for forestry for the evaluation of strategies and adaptation to novel ecosystem habitat conditions (Rodrigue i in. 2002, Pietrzykowski, 2008, Pietrzykowski et al., 2010b). Understanding the participation of root biomass in comparison to that of aboveground tree biomass and the alteration of the root system in varied and compacted mine soils is crucial for managing stand stability. Additionally, root biomass studies are important in the context of the assessment of CO2 sequestration in new forest ecosystems (Pietrzykowski and Daniels, 2014, Waisel et al., 1991). While issues related to the aboveground biomass of plant communities and the productivity of ecosystems have been extensively researched (Lieth & Whittaker, 1975), evaluating belowground tree stand biomass is much more difficult methodically and is very labour intensive (Böhm, 2013). The root system of even a single tree may be vast. Additionally, the root hairs and fine roots that the tree uses for water and mineral intake are short lived and decay on an annual basis (Miller et al., 2006). Therefore, the methods applied in direct research (digging out, cylinder methods, counting in volumes of soil, the introduction of underground cameras and indirect methods based on estimation using empirical formulas) usually provide only approximate results (Vogt et al., 1998, Waisel et al., 1991).

To date, a several studies have been conducted on the development of root systems in post-mine habitats in central European conditions and reclaimed mined sites in general (Pietrzykowski, 2008, Pietrzykowski et al., 2010b). One of the first works in this field was a publication containing an analysis of the development of the root systems of plants following natural succession in the dump of the Piaseczno sulphur mine in southern Poland (Fabijanowski and Zarzycki, 1969). This study was conducted on a reclaimed sand open-cast mine in Upper Silesia in southern Poland. It indicated an expansion of the root system and thus biological soil depth in a chronosequence from 5 to 25 years on reclaimed sites and those undergoing natural succession Pietrzykowski (2008). It also showed deformation and modification of the Scots pine root systems growing at post-mine sites, including the dump of the reclaimed Bełchatów open-cast lignite mine and Szczakowa open-cast sand mine (in the Upper Silesian Industrial Region) (Pietrzykowski et al., 2010b) (Fig. 1). The cited work indicated that modification of the pine tap roots at the dump was due to the heavily compacted soils. On the other hand, the root systems of pines growing on open-cast sand mines in areas with shallow groundwater showed modifications involving the disappearance of the main tap root system and an increased share of fine roots in the form of bunches in the groundwater table range. In addition, lateral roots were better developed, as they provide the main support for the stabilisation of the tree (Pietrzykowski et al., 2010b). Anyway this is an open question about possibilities of using of native tree species, with probably more desirable characteristics in these environments, versus not native species, but there is still limited data of root system reaction. In some case using of shrubs as green alder (Alnus viridis) could be better solution for bio-stabilization e.g. for slopes (Pietrzykowski et al., 2015, Pietrzykowski et al., 2018). Although still the main goal of afforestation is introducing of trees and shrubs could be considered as admixture.

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Fig. 1. Root system of Scots pine growing on sand mine cast with high groundwater level –visible modifications involving the disappearance of the mainsystem of pile and an increased share of fine roots in the form of bunches in the zone of the groundwater table (Photo by M. Pietrzykowski, 2009).

6. Tree nutrient supply and stand stability

The response of tree species to reclaimed post-mine sites and their adaptive strategy must consider nutrient biogeochemical cycles and tree stand nutrient supply and stability (Baumann et al., 2006, Frouz et al., 2013, Pietrzykowski, 2010, Pietrzykowski et al., 2013, Šourková et al., 2005). The dynamics of biogeochemical cycles significantly impact the condition of the nutrient supply available to trees, as the health and stability of tree stands reflect how well they are nourished. The basic criterion in the assessment of tree nutritional status is the content of individual elements in the photosynthetic apparatus. Auxiliary criteria in the assessment of trophic conditions for trees in forest habitats may also include biometric features of the photosynthesis apparatus, such as the length and weight of the needles and growth characteristics of the tree stands (dbh and height) (Ochał et al., 2010, Pietrzykowski et al., 2013, Baule and Fricker, 1970). Despite the considerable significance of this problem, few works have been published on the adaptive strategies of woody species, including those regarding the assessment of nutritional strategies in post-mine habitats. Data on Scots pine, one of the main species used in the reforestation of sites in central Europe, were published by Baumann et al., 2006, Heinsdorf, 1996, Pietrzykowski et al., 2013. The works mentioned above clearly show that threshold values provided for the content of macronutrients in the needles of Scots pine growing in natural habitats should be supplemented by data from post-mine sites. The observed macronutrient content at such sites was often lower than the values recognised as representing a deficit, but this pine species showed no signs of stunted growth or needle chlorosis. This confirms the considerable adaptability of this species to the extreme habitat conditions at mine sites (Ochał et al., 2010, Pietrzykowski et al., 2013, Stolarska et al., 2006). These works also indicated that nitrogen, followed by phosphorus, was the scarcest element in the new forests and that the values for this element was lower than those in the literature. It would seem that a shortage in the supply of nitrogen should require supplementary fertilisation. However, this treatment should be considered at reclaimed mine sites only in cases of the greatest deficits, and each single dose must not be too large because of the danger of leaching and nitrogen losses and the possibility of disturbances to the quantitative ratios of macronutrients in the initial soils (Pietrzykowski et al., 2013). In this context, the most important goals for the emerging ecosystem should be to achieve the efficient circulation of nutrients and stability of stands as key modifiers of the newly forming forest habitats. Currently, it is even believed that obtaining maximum stand growth in terms of biomass by achieving a “perfect” level of tree nutrients at reclaimed sites is unfavourable (Knoche et al., 2002, Pietrzykowski et al., 2013). After a period of good growth of trees and biomass at an early phase, a decrease may occur during later stages of development. Therefore, it is necessary to monitor forest stands and emerging ecosystems over time.

Not only is the content of each macronutrient in the photosynthetic apparatus essential in the assessment of the nutritional conditions of forest stands but also their relative proportions (Marschner, 1995, Baule and Fricker, 1970). A balanced nutrient supply means that each component occurs in proportion to the other nutrients. As a result, nutrient uptake is not disturbed and ensures the optimal growth of trees. This does not mean, however, that the proportions of elements in plant biomass are fixed and well defined: they display a natural range of variability (Marschner, 1995, Baule and Fricker, 1970). Research by Pietrzykowski et al. (2013) showed that the ratios of macroelements in pine needles were disturbed. It was found that in comparison with the controls in natural habitats, the N:P ratio was not disturbed in Quaternary sands mixed with Neogene clays at the Piaseczno sulphur open-cast dump (southern Poland), where pine obtained the highest growth parameters (Pietrzykowski et al., 2010a). However, in the case of many other studied habitat variants, including Quaternary and Neogene deposits subjected to liming and Carboniferous deposits, the ratio was too low. Other ratios of N:K, N:Ca and N:Mg were unfavourable in all habitat types at post-mine sites. This confirms the fact that the harmonious supply of nutrients has been disturbed, particularly in the case of nitrogen. The occurrence of an unbalanced supply of nutrients may lead to physiological disorders and affect tree growth (Baule and Fricker, 1970). However, studies on the growth of restored forests at reclaimed post-mine sites in the Lusatia region indicated that this growth and the disrupted nutrient supply in the stand did not show a clear correlation in the first decades of growth (Heinsdorf, 1996). In the stands aged 20 to 40 years, no negative symptoms or inhibited growth were reported, while in recent years, a phenomenon of dying and thinning among the pine stands has been reported in this region (Knoche & Ertle, 2010) and in the nearby region of Przyjaźń Narodów in Łęknica in western Poland (unpublished personal data) due to infection with Heterobasidion annosum (Fr.). This phenomenon will likely be the most important problem at these afforested mine sites and will likely be influenced by the high pH as a consequence of liming, resulting in disturbance to the nutrient supply.

7. Growth of selected tree species at reclaimed mine sites