Features of Cd and Ni accumulation by <i>Larix sukaczewii</i> Dyl. under technogenesis
In industrially developed cities, sanitary-protective afforestation promotes the deposition of aeropollutants, as well as the absorption of toxicants from the soil. However, performing a function of a phytofilter for a long time negatively affects the vitality of the trees themselves. Among numerous pollutants of technogenic origin, the most toxic for living organisms, including plants, are heavy metals. Studying the absorption capacity of different species of woody plants and their reaction to increased concentrations of toxicants allows us to predict the stability of protective forest stands and design forestry measures. One of the species, widely used in creating sanitary protection zones in the Republic of Bashkortostan, is Larix sukaczewii Dyl., characterized by its resistance to extreme environmental factors. The adaptive abilities of this species to an increased content of heavy metals remain poorly studied. The aim of this study is to identify the features of the accumulation and distribution of heavy metals (Cd and Ni) by Larix sukaczewii and assess the relative living condition of the stand in Sterlitamak industrial center. We conducted studies in 2007-2018 in the city of Sterlitamak (53°38'00"N, 55°57'00"E), a major center of the chemical and oil refining industry of the Russian Federation. The establishment and description of trial plots was carried out according to generally accepted methods [Andreeva EN et al., 2002; Forest ecology..., 2007]. Permanent trial plot No. 1 measuring 20x50 m was located 1-2 km from sources of petrochemical and chemical pollution (See Fig. 1). Permanent trial plot No. 2 measuring 18x50 m was located in the area of conditional control (25-30 km from pollution sources). Trial areas were laid in coeval (about 55 years) and clean tree stands (See Table 1). The category of the vital state of an individual tree was determined by the auxiliary table (See Table 2). After summing up the number of trees by categories, we assessed the relative living conditions of the entire stand [Alekseev VA, 1990]. At a rate of 100% to 80%, the stand was assessed as “healthy”, at 79-50% as “weakened”, at 49-20% as “strongly weakened”, and at 19% and below as “completely destroyed”. The study of the soil saturation with absorbing roots was carried out by the method of soil sections 1x1x1 m in size [Root Methods, 2000]. The roots were sorted by fractions: less than 1 mm in diameter (absorbing roots), 1-3 mm (half-skeletal roots), and more than 3 mm (skeletal roots). We determined the weight of the roots in the air-dry state. To study the metal content, annually during the growing season, we performed repeated re-selection of needles, branches and roots. At the same time, we took soil samples from the depth of 0-20 cm. The samples were dried to an air-dry state and analyzed on a “ZEEnit-650” atomic absorption spectrometer (Germany) [Hill SJ, Fisher AS, 2017]. In soil samples, we determined gross and mobile forms of metals. Data are presented as arithmetic mean ± standard deviation (M ± SD). Differences were considered statistically significant atp < 0.05. We found that at the permanent trial plot No. 1, the trees are more weakened in comparison to the conditional control. External signs of oppression were expressed in a crown density decrease to 75-80%, an increase in the number of dead branches to 25-30%, and damage to the assimilation apparatus by chlorosis and necrosis (on average by 15%). Trees classified as “weakened” accounted for 55% of the stand, and deadwood reached 10%. The relative vital condition of the entire stand was characterized as “weakened” (See Table 3). In the area of conditional control, the forest stand corresponded to the “healthy” category. Moreover, the trees had a better formed crown (average density is 82%) and fewer dead branches on the trunk (21% on average). Needles damage by chlorosis was not more than 11%. The results of chemical analysis of soil samples revealed statistically significant differences (p < 0.05) between the content of the metals in the zone of influence of the industrial center of Sterlitamak and in the area of conditional control (See Table 4). Higher concentrations of cadmium and nickel were found at the permanent trial plot No. 1. The leached chernozem prevailed in the soil cover. The pH of the soil according to many years of research ranged from 6.55-6.90 units. The bulk of the absorbing larch roots was located at a depth of 0-30 cm (See Fig. 2). Moreover, in the control area, the root saturation of the soil reached 78.51 ± 4.10 g / m<sup>2</sup>. In the influence zone of Sterlitamak industrial center, we observed a decrease in the proportion of absorbing roots: in healthy trees 2 times and in weakened trees 2-3 times. Perhaps, this is due to higher concentrations of metals in the surface of soil layer at a permanent trial plot No. 1, which led to a significant restructuring of the larch suction root apparatus. Chemical analysis of plant samples showed that in the contaminated area the content of cadmium in needles and branches is statistically significantly (p < 0.05) higher by 5-6 times and that of nickel by 3-4 times. In underground organs, the content of elements in comparison to the conditional control increased 2.5-4 times (p < 0.05). Besides, in the underground part, nickel is mainly accumulated in absorbing roots, and cadmium in absorbing and semi-skeletal ones (See Fig. 3 and 4). The movement of nickel from absorbing roots to conductive roots is less expressed than cadmium. In the aerial part of trees, the metal content in the branches is higher than in the needles (See Fig. 5 and 6). At the same time, in healthy trees, the barrier function of the root system is preserved, which prevents the excess intake of pollutants in the aboveground organs. Despite the fact that the relative vital condition of the stand at the contaminated site is characterized as “weakened”, there is no decrease in the quality class. It is not required to carry out operational forestry activities in the sanitary-protective larch plantations. Timely sanitary felling and further monitoring are recommended. The paper contains 6 Figures, 4 Tables and 38 References. The Authors declare no conflict of interest.