330 Mori et al.: Carbon Uptake and Air Pollution Mitigation of Different Evergreen Shrub Species In Mediterranean areas, the use of drought- tolerant shrubs can considerably increase carbon sequestration by local vegetation (Bombelli and Gratani, 2003). Unfortunately, the degree of drought tolerance and the mechanisms adopted to cope with drought have not been studied for most shrub species commonly planted in the urban environment. Particular interest should be given to determining which species are capable of tolerating (rather than avoiding) drought, thus maintaining high CO2 assimila- storage, plants may play a role in decreasing local air pollution. Particulate matter (PM) is the prin- cipal component of anthropogenic air pollut- ants. PM includes a mixture of elements, heavy metals, black carbon, polycyclic aromatic hydro- carbons, and other substances suspended in the atmosphere (Bell et al. 2011). Plants can adsorb and absorb PM, as well as other noxious gaseous pollutants such as Sox tion during dry periods (Bussotti et al. 2013; Fini et al. 2014). Shrub species are suggested as valid complements, especially where there is not enough space for tree growth or where local legislation imposes specific distances for tree plantation along streets or buildings. In parallel with CO2 assimilation and carbon al. 2000; Nowak et al. 2006; Bealey et al. 2007), and pollution-capturing efficiency is species- specific (Dzierzanowski et al. 2011; Sæbø et al. 2012; Mori et al. 2015a). Generally, it has been found that coniferous species intercept higher amounts of particulates due to their complex leaf structures compared to broad-leaved species (Beckett et al. 2000; Freer-Smith et al. 2005). In addition, evergreen plants are able to intercept air pollutants during winter as well (Freer-Smith et al. 2005), when the concentration of air pollut- ants is usually higher (Pikridas et al. 2013). Spe- cies with large leaf areas and dense canopies can create higher air , NOx , and O3 turbulence and consequently have a higher impaction of suspended particles compared to species with low-dense canopies (Mori et al. 2015b). Trees can also act nega- tively, increasing air pollution concentrations at the pedestrian level by reducing air circulation (Buccolieri et al. 2009; Salmond et al. 2013). Leaf anatomical and ultrastructural traits, such as texture, surface roughness, presence of ©2016 International Society of Arboriculture (Nowak et pubescence, and the quantity and size of stomata are positively related to the retention capacity of leaves toward PM (Huixia et al. 2013). Leaf surfaces are temporary retention sites for pol- lutants, the presence of which is influenced by meteorological factors (Nowak et al. 2006). Thus, an interesting aspect of the interaction between plants and air pollutants is how deposition of pol- lutants on leaves changes over time. For example, Anicic et al. (2011) found a seasonal accumula- tion, from May to September, of Cr, Fe, Ni, Zn, and Pb on leaves of Aesculus hippocastanum and Tilia spp., but similar information about shrub species in dry and warm climates is still lacking. Indeed, most of the research in the last 15 years has been carried out on temperate tree species (Beckett et al. 2000; Nowak et al. 2002; Nowak and Crane 2002; Pugh et al. 2012). Elevated con- centrations of heavy metals are common in urban areas as a result of both a wide range of human activities and natural processes (Lu et al. 2010). As heavy metals are strongly associated with PM (Duzgoren-Aydin et al. 2006), especially with the coarse fraction having diameters up to 10 µm (PM10 ) (Araujo and Nel 2009), sev- eral studies have measured some heavy metals as descriptors of air pollutants (Fowler et al. 2003; Mori et al. 2015a). To improve knowl- edge of the benefits of shrubs for urban land- scape, three experiments were carried out to characterize the following seven shrub species regarding their capacity for CO2 uptake and conditions of non-limiting moisture availability (Exp. 1) and drought stress (Exp. 2), while a third experiment (Exp. 3) aimed to iden- tify the species with highest leaf deposition of trace metals (Cd, Cu, Ni, Pb, and Zn) (Vibur- num tinus was not included in this experi- ment), to study the relationships between trace metal depositions trends and meteorological parameters, and to identify the possible sources responsible for the presence of metals on leaves. storage and air pollution sequestration: Vibur- num tinus subsp. lucidum L. cited as Viburnum lucidum, Viburnum tinus subsp. tinus L. cited as Viburnum tinus, Arbutus unedo L., Photinia × fraseri Dress., Laurus nobilis L., Elaeagnus × ebbingei L., and Ligustrum japonicum Thunb. CO2 uptake and storage were evaluated under
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