338 Mori et al.: Carbon Uptake and Air Pollution Mitigation of Different Evergreen Shrub Species number of artificial variables (factors), indicated that approximately 85% of the variance may be explained by the first two factors. Factor 1, which accounted for 58% of total variance, showed posi- tive loadings with Cu, Ni, and Zn. In factor 2 (27% of total explained variance), Cd and Pb were dominant metals. Correlation analysis between leaf metal depositions showed that Cd is correlat- ed with Pb (r = 0.51), Cu with Ni and Zn (respec- tively r = 0.92 and 0.83), and Ni with Zn (r = 0.77). DISCUSSION Under Contrasting Water Availability Experiments 1 and 2 were aimed at investigating CO2 Experiments 1 and 2: CO2 assimilation and storage of seven widely used Figure 8. A) Accumulation of Cu and Zn; B) Ni and Pb; C) and Cd expressed as mg (metal) / plant, from June to October 2011 in the water collected at the base of Viburnum lucidum, Arbutus unedo, Photinia × fraseri, Laurus nobilis, Elaeagnus × ebbingei, Ligustrum japonicum, and control. Values are means (mg/plant). Different letters within the same sampling date indicate significant differences among species at P < 0.05 (*) using Duncan’s MRT. Metal Source Identification Cluster analysis, which is generally used to group variables (metals) based on their similarity/ dissimilarity, grouped the metals in two principal clusters. The first cluster included Cu and Ni, while Cd, Zn, and Pb were located in the second (Table 5). Factor Analysis (Table 5), which has the aim of describing variability among variables by a lower ©2016 International Society of Arboriculture unit leaf area when water is not limiting, and drastic reduction of stomatal conductance, transpiration, and CO2 assimilation aſter the onset of stress. Other species (i.e., A. unedo and V. tinus), instead, have evolved a conservative water-use strategy, consist- ing in relatively low leaf gas exchange during the favorable periods, which was only slightly depressed during stress (Moreno-Gutiérrez et al. 2012). These findings have important applications for lation and carbon storage are highly correlated sustainable and benefit-based planning of urban green areas. In moist and favorable sites, or where irrigation water is readily available, CO2 assimi- evergreen shrub species under either optimal mois- ture availability (Exp. 1) or water stress (Exp. 2). The results of these experiments corroborate the idea that the “carbon benefit” of shrubs is largely species- specific and dependent on a species-planting site interaction, as previously observed with tree species (Nowak and Crane 2002; McPherson and Simpson 2003). In particular, the ecological behavior of the species and the mechanisms evolved to cope with environmental stress are key determinants of carbon assimilation and carbon storage (Penuelas et al. 2001; West et al. 2012). The data highlight differ- ent growth strategies by the seven species investi- gated, which may be critical for proper use of these shrubs in the landscape. Some shrub species (i.e., E × ebbingei, V. lucidum, and L. japonicum) displayed a profligate/opportunistic water use (in the sense intended by Moreno-Gutiérrez et al. 2012), charac- terized by high water use and CO2 assimilation per Uptake
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