Woody plants in the cycle of heavy metals: features of regional conditions and species specificity

Woody plants as forests and urban greening systems frame are long-lived organisms capable of extracting chemical elements and absorbing from the atmosphere, fixing them in the phytomass and then returning them to the biogeochemical cycle active links. As for the general cycle schemes, their flow features reveal regional specificity. The space of the Middle Volga forest-steppe-steppe region, which is an ecotone, is based on sedimentary rocks, confined to temperate continental climate, and is characterized by significant technogenesis manifestation. Its natural biogeochemical features are: enrichment of natural environments by Ca, local manifestation of chloride-sulfate salinization in relief depressions. Technogenesis contributes to the introduction of additional heavy metals into natural environments.
The ecological space determines the specifics of woody plants development. Its natural dendroflora has a limited set of species adapted to periodic summer droudhts, extreme winter frosts. Some of them grow here on their natural areas borders. A significant number of introduced arboreal species and varieties are used in man-made plantations (protective forest belts, urban greening) and some turned to be bioinvaders. The accumulation of the inorganic elements sum (ash) in the woody plants leaves, totally from 5 to 14 % to dry mass, evaluated for a large group as integral indicator, does not allow us to speak about higher ash content in the leaf mass of local or introduced species.
The elemental analysis performed for 25 tree species showed that the most active accumulators of metals with relatively high clarkes in soils and a comparably low representation in technogenic pollution fluxes (Ti, Mn, Fe, Sr, Rb) are woody plants from  Aceraceae and Ulmaceae. The same role for technogenic elements (Cr, V, Co, Ni, Cu, Zn, Pb) is played by Salicaceae and Ulmaceae families. Plants from Pinaceae and Oleaceae families showed weak metal-accumulating ability. The active Cu accumulation from the soils of the corresponding habitats seems to be  inherent feature for all families  that form the region dendroflora, with the exception of conifers.
Some named onward species had the maximum metal storage capacity in relation to elements with high clarke content in soils: Padus avium (Mn, Fe, Sr), Sambucus racemosa (Mn, Fe, Rb), Acer platanoides (Ti, Mn), A. tataricum (Ti). , Fe), Corylus avellana (Fe, Sr), Tilia platyphyllos (Fe, Sr). The lesser amounts of Fe, Ti, Mn, Sr, Rb were accumulated  by Fraxinus lanceolata, Larix sibirica, Pinus sylvestris. The accumulators of technogenic elements are the species of the genus Ulmus: U. glabra (V, Ni, Cu), U. laevis (V, Ni, Cu), U. pumila (V, Zn, Cu); as well as Salix alba (V, Co, Ni, Cu, Zn) and Populus  nigra (especially Zn, at the middle level - Cu). Species of the Acer genus are characterized by an average level of technogenic elements accumulation. The least quantity of  technogenic elements were accumulated by Cerasus fruticosa, Larix sibirica, Pinus sylvestris. In general, the dendroflora of the region is actively involved in the circulation of heavy metals, which is associated not only with their metal-accumulating ability, but also with deciduous specifics.