156 Dadea et al.: Tree Species as Tools for Biomonitoring and Phytoremediation in Urban Environments 2011). Key pollutants are the so-called traffic- related elements (TREs)—i.e. Cd, Mn, Cu, Mo, As, Sb, Zn, together with Pd, Pt, and Rh, which are regarded as emerging contaminants (Duong and Lee 2011; Fujiwara et al. 2011). Table 1 sum- marizes the main inorganic marker elements associated with various emission sources or processes (Duong and Lee 2011; Fujiwara et al. 2011; Calvo et al. 2013). TREs can precipitate directly on ground surfaces, or first accumulate in the atmosphere and then deposit on ground surfaces through rainwater transport (Gunawar- dena et al. 2013). For this reason, the presence of TREs not only in airborne particulate matter, but also in road dust, soils, vegetation, river sediments, and other related matrices, represent a risk factor for human health (Fujiwara et al. 2011 and references therein; Sawidis et al. 2011). The importance of this phenomenon needs to be viewed against the background of a gen- eral growth of large cities throughout the world and coupled with increasing vehicular traffic (Amato et al. 2009; Gunawardena et al. 2013). In such complex environments like urban- industrial areas, it is essential to identify plant spe- cies that can be used to assess the status and trends related to human health (Burger 2006) and are also potentially suitable for environmental quality improvement. Bioindication and phytoremedia- tion are applied aspects of the uptake of chemical elements from soil to plants. According to Markert (2008) and Pellegrini et al. (2014), a bioindicator is an organism that contains information on the quality of the environment. In urban and indus- trial areas, vascular plants have been studied as bioindicators and bioaccumulators of trace ele- ments. Evergreen species were used as a passive sampler and as indicators for airborne trace ele- ment in anthropogenic and remote sites. Also, the foliage of common deciduous trees was used for long–term monitoring of trace metal concentra- tions (Pellegrini et al. 2014 and references therein). Phytoremediation has emerged as a potential tool to rehabilitate contaminated lands by remov- ing pollutants from the environment or rendering them harmless (Pulford and Watson 2003). It is a “soſt” remediation treatment that makes use of the ability of some plant species to accumulate certain elements, including TREs, in amounts ©2017 International Society of Arboriculture exceeding the nutrient requirements of plants (Baker et al. 2000; Gupta et al. 2000). Ornamental tree and shrub species can also provide this eco- system service in urban-industrial environments. Accordingly, urban planners could prioritize their potential in environmental quality improvement and maintenance, together with their stress tol- erance, when selecting trees for urban-industrial areas or heavy-traffic roads. This strategy needs species-specific information, including their bio- indication and phytoremediation potential, their biological characteristics, and any potential eco- system disservices (Pataki et al. 2008; Lyytimäki and Sipilä 2009; Lyytimäki 2014); the latter, for example, might include the reduction of air flow by street trees (Vos et al. 2013; Nowak et al. 2014) or the production of pollen and its interaction with air pollutants (Traidl-Hoffmann et al. 2003). Therefore, researchers carried out a literature review by focusing on the following information: 1) The general potential of trees for bioindica- tion and phytoremediation in urban environments; 2) the tolerance of selected urban tree species against stress (as a continuous negative impact by biotic or abiotic factors on the plant, which causes damage and/or loss of productivity); and 3) recommendations for urban plan- ners with regard to urban forestry. The general bioindication and/or phytoreme- diation potential with regard to heavy metals are outlined first. Secondly, the review focuses on nine tree species that are commonly planted along streets in cities of central Europe, and provide comprehensive information on their role and potential for bioindication and phy- toremediation. With this review, researchers provide a comprehensive basis for urban plan- ning and urban forest management, also with regard to the potential role of those tree spe- cies in environmental quality improvement. Phytoremediation with Regard to Heavy Metals: Strategies and Application The dramatic increase of land surfaces and soils affected by pollution, salinization, and non- sustainable land use (Daily 1995) has led to the study and implementation of various technologies for cleaning contaminated sites. However, conven-
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