158 Dadea et al.: Tree Species as Tools for Biomonitoring and Phytoremediation in Urban Environments et al. (1977) defined those plants as “hyperac- cumulators,” which contain more than 1 mg Ni g–1 dry weight (Rascio and Navari-Izzo 2011). Accumulators are able to quickly trans- locate trace elements with a concentration of 100–1,000 fold higher than those found in non-accumulator species to the shoots and to the leaves, in particular, to the shoots and leaves (Baker 1981; Salt et al. 1998). Consequently, effective phytoremediation depends on the plant’s ability 1) to grow fast in nutrient-poor soils, 2) to develop a dense and/or deep root system, and 3) to show metal-tolerance traits (Baker et al. 1994; Brooks et al. 1994; Rascio and Navari-Izzo 2011; Sarma 2011). Furthermore, accumula- tors are genetically adapted and often strictly bound to their natural habitats and selec- tive for specific metals. Urban trees, in par- ticular, are considered efficient in pollutant removal thanks to their root systems, their large total leaf area, and their high transpi- ration (Pulford and Watson 2003; Rosselli et al. 2003; Meers et al. 2007; Unterbrunner et al. 2007; Brunner et al. 2008; Domínguez et al. 2008; Papa et al. 2012; Ugolini et al. 2013). Experiments have shown that toler- ance against heavy metals can be induced by gradual acclimatization to metal stress (Dickinson et al. 1991; Turner and Dick- inson 1993; Punshon and Dickinson 1997; Dickinson et al. 2002; Pulford and Watson 2003). Trees have the potential to enhance the remediation of brownfields, landfills, and other contaminated sites by absorbing, transforming, and accumulating a number of contaminants (Nowak and Dwyer 2007). REVIEW METHODOLOGY This review focuses on trees in urban areas, considering their capacity to withstand stress by heavy metals. Species-specific information on the phytoremediation potential and/or phy- toremediation techniques will be identified. In particular, researchers consider the suitability for bioindication and heavy-metal stress tol- erance and hardiness for a selection of tree species. Additionally to the specific phytore- mediation suitability of species, information was compiled with regard to their origin and morphological characteristics (e.g., maximum height, root development, or life span). Ecosys- tem services related to environmental quality improvement as well as potential disservices were assessed with special regard to volatile organic compounds (VOCs) and ozone emission. In total, more than 134 literature sources, including books and manuals on dendrology, tree identification, and tree biology, as well as scientific papers, were reviewed, all extracted from the Scopus® and ScienceDirect® citation/ journal databases. For the current study, key words searched included “heavy metals” and the respective species name, as well as “urban areas” and/or “cities” together with “heavy met- als.” Figure 1 quantifies the revised literature sources differentiated into research topics. The selected species were Acer pseudoplatanus L., Ailanthus altissima P. Mill., Betula pendula Roth, Carpinus betulus L., Ginkgo biloba L., Platanus × hispanica Mill. Ex Muenchh. “Aceri- folia”, Quercus robur L., Robinia pseudoacacia L., and Tilia cordata Miller (for biological and phytogeographical characteristics, see Table 2). These species are frequently found in sev- eral medium-large cities in northern and northeastern Italy (Paludan-Müller et al. 2002; Comune di Padova 2006; Paoletti 2009; Comune di Bolzano, 2010; Semenzato et al. 2011; Comune di Merano 2012; Città di Torino 2013; Marziliano et al. 2013). Moreover, they are commonly planted in urban areas and along streets and highways throughout central and northwestern European cities (Pauleit et al. 2002; Pauleit et al. 2005; Sæbø et al. 2012; Sajdak and Velazquez-Marti 2012). Betula pen- dula, for example, occurs up to arctic (Sjöman et al. 2012) and near-arctic regions of Europe (Rosenvald et al. 2011; McBride and Douhoni- koff 2012). Researchers did not consider trees with invasive root systems that are not suitable for streets, such as Populus and Salix species (Trees for Cities 2014). Some species listed in Table 2 (e.g., Robinia pseudoacacia and Betula pendula), while not specifically indicated for phytoremediation purposes, are considered ©2017 International Society of Arboriculture
July 2017
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