During mineral deposit exploitation, dumps from overburdened rocks are formed, which leads to ecosystem destruction and environment pollution [1,2]. Reforestation of mining industry dumps is carried out to reduce secondary pollution of the environment during wind and water erosion and to restore biological productivity [3,4]. The study of the condition of tree stands makes it possible to evaluate the results of dump reforestation and determine the prospects for the elimination of accumulated environmental damage. The purpose of the study was to examine the state of birch stands, evaluate the metal accumulation features in woody plants and characterize soil cover formation after reforestation of the dump of the Kumertau brown coal deposit between 1980 and 2017.
The objects of research were birch (Betula pendula Roth) stands planted in 1980–1984 after the reforestation of the dump of the Kumertau brown coal deposit. Estimation of the life state of birch stands was carried out by the Alekseev method . Selection of plant and soil samples was carried out during the vegetation period (from June to August). For the determination of metal content, the plant and soil samples were ashed in a muffle furnace at a temperature of 450 C. To determine the mobile forms, soil samples were extracted with an ammonium acetate buffer with a pH 4.8. The content of copper, zinc, lead and cadmium was determined. The content of the metal total and the active forms in the soil was determined by atomic absorption spectrometry in the Contr-AA (Analitic) . The metal content in plant samples was determined on a STA (MU 08–47/136) using an inversion voltammetric method. Ecotoxicological assessment of soils was determined by the maximum allowable concentration (MAC) of metals. The humus content was determined from the concentration of organic matter (GOST 26213–91) with subsequent transfer to the total carbon. The acidity and the total absorbed bases were determined by the Kappen method (GOST 27821–88). The composition of the absorbed bases (Ca and Mg ) was determined by the CINAO method (GOST 26487–85). The content of active phosphorus and potassium was determined by the Machigin method with CINAO modification (GOST 26205–91). The nitrogen content was determined by the Cornfield method. The obtained data were processed with conventional statistical methods.
Birch plantations growing on the dump (L = 93.3%) and in control soil (L = 93.6%) are characterized as `healthy'. Geologically, the dumps are characterized by great heterogeneity and are represented by sedimentary rocks  – conglomerates, sandstone, limestone, ancient alluvial sands and pebbles, Permian and Tertiary clay. These contain a wide range of chemical elements: copper, zinc, cobalt, calcium, manganese and others. Of these elements, the greatest danger to the environment is presented by copper, zinc, lead, and cadmium. In dumps and control soils under birch stands, the content of the total forms of copper, zinc and cadmium corresponds to MAC, with the exception of the control soils, where one finds that zinc exceeds MAC 2.2 times (Table 1). The content of metal active forms in dumps is much lower than the background level and MAC.
The morphological markers of soil formation under the canopy of birch stands on dumps are noted. In birch stands, the upper 1.5 cm of soil consists of a litter of fallen leaves and branches: a humus-accumulative horizon with a total thickness of up to 10 cm develops deeper.
A feature of woody plants growing on dumps is the ability to accumulate chemical elements from the soil in various organs (roots, branches, leaves), which reduces the spread of pollutants in the environment [8–11]. Birch accumulates the largest amount of zinc (in bark and shoots) and lead (in shoots and roots). The greatest amount of copper accumulates in the birch root system, which indicates the barrier role of roots in the transit of copper to the above-ground organs (Table 2).
It was found that zinc in birch organs is distributed as follows: branches bark leaves roots. The maximum lead content is observed in the branches: there is no excess of MAC. In all organs of birch, cadmium exceeded MAC.
After the reforestation of dumps, soil is formed. In soils under birch stands on dumps (Table 3), a high content of total carbon (21%) was found, which is due to the presence of brown coal particles in the soils. In soils under birch stands, the substrate reaction varies from weakly alkaline to weakly acidic. For 30 years in the soils under birch stands, the content of exchange forms of calcium and magnesium and active forms of phosphorus has decreased, which is associated with their leaching out of soils. In soils under birch stands on dumps, there is a decrease in acidity (pH = 5.2), which is consistent with the results of other researchers .
The life state of birch stands on dumps is estimated as `healthy'. It is established that in soils on dumps and in control soils, the content of active forms of metals is comparable with the total content of elements. The greatest average amount of metals is noted in perennial parts of plants (root system, bark, and branches), and the smallest – in assimilation organs. The metal accumulation in birch roots is an adaptive reaction aimed at the survival of this tree species in the pessimal conditions of the dumps. The life state of birch stands proves the success of the reforestation of the dumps of the Kumertau brown coal deposit. The formation of soils under the canopy of birch stands on dumps is marked. During the formation of soil under the canopy of birch stands on dumps, there are positive changes in parameters such as the content of total carbon, phosphorus and exchange forms of calcium and magnesium and the level of acidity. This indicates the contribution of birch stands to the biological remediation of industrial dumps.