Assessment of the Composition of Rain Deposition in Coniferous Forests at the Northern Tree Line Subject to Air Pollution

This study is devoted to assessing the interbiogeocenotic and intrabiogeocenotic variation and long-term dynamics of the composition of rain deposition in coniferous forests that are exposed to contamination from the Severonikel Copper-Nickel Smelter Complex (Murmansk oblast), the largest source of pollutant emissions into the atmosphere in Northern Europe. Spruce forests with dwarf shrubs and green mosses and pine forests with dwarf shrubs and lichens, which are common in the boreal zone, are studied at different stages of pollution-induced degradation. The chemical composition of rainwaters is characterized by a significant intra- and intrabiogeocenotic variation. Long-term data confirm that the content of elements is significantly higher below the crowns than between them, with the concentrations and deposition of elements being significantly higher in spruce forests than in pine ones. This difference is determined by different sorption capacities of tree crowns and different levels of leaching of elements from them. The fallout of the main pollutants with rainwaters exceeded the critical levels even in the background areas and was significantly higher in defoliating forests and pollution-induced sparse forests. It is shown that the excess of the critical levels is much more pronounced in rainwaters than in snow ones. The determination of the intra- and interbiogeocenotic differences in the composition of atmospheric fallout allows an earlier and more accurate diagnosis for the excess of critical loads at all degradation stages. The long-term dynamics of the fallout of chemical elements with rainwaters in pine and spruce forests is characterized by a high variability both below and between tree crowns. An increase in nickel concentrations in the background areas from 2013 to 2017 is revealed, which is explained by the growth in the content of pollutants in aerosols spreading over considerable distances.