Arboriculture & Urban Forestry 42(5): September 2016 Table 2. Leaf area (m2 ) of the six shrub species used for trace metals analysis at planting date (autumn 2010). Mean SD Viburnum lucidum Arbutus unedo Photinia × fraseri Laurus nobilis Elaeagnus × ebbingei Ligustrum japonicum 0.41 0.30 0.35 0.44 0.50 0.20 0.04 0.01 0.05 0.09 0.06 0.05 In 2011, leaf trace metal deposition was mea- sured three times (June, August, and October) on leaves formed in the current growing season in order to avoid any confusing age-related effects. Samplings were conducted aſter at least 10 rain- less days. Two replicates of 10 fully expanded leaves per species per belt (40 leaves in total) were col- lected, including leaves at different heights. The leaf area of every sample was measured. Healthy leaves of each species were collected at a distance of at least 1.5 m from the adjacent species in order to reduce the influence of position. During the experi- ment, 72 samples were collected and immediately taken to the laboratory for subsequent analysis. In the laboratory, leaves were washed in 50 ml of a solution composed of distilled water and HCl 0.01 M (pH 2.0), per 30 minutes, placing samples on an orbital shaker. The concentrations of Zn, Cd, Pb, Ni, and Cu in the solutions of the differ- ent samples were directly analyzed by Inductively Coupled Plasma – Optical Emission Spectrometer – Dual Vision (Optima 2000, Perkin Elmer, Waltham, Massachusetts, U.S.). The results were expressed as weight of metals per unit of leaf area. To measure the quantity of metals washed off from the canopy by precipitations, two containers (50 cm × 100 cm) were placed at the base of four replicate shrubs of each species. Control contain- ers were placed at the same distance from the street in an area without vegetation. Rainwater in the containers was collected, measured, and stored in the dark at 0°C from June to October 2011, after each significant rain event (>5 mm). In November 2011, each rainwater sample was mixed until complete re-suspension of the sedi- ments was reached and greater homogeneity within the sample was obtained; 20 ml of each sample was collected and fully evaporated at 50°C to collect the dry matter. The dry matter was then digested with 5 ml of a mixture of HNO3 (65%) : 333 HClO4 (68%), (5:2) v/v at 180°C per 60 minutes using a heating digester DK 6 (VELP® Scientifica, Usmate, Italy) to obtain the metal concentration, which was, for each sample, multiplied by the total volume of collected rainwater in order to obtain the total quantity of each metal. The pri- mary aim was not to compare the effect of species but to compare the effect of presence or absence of shrubs on the metal content in runoff water. Plant height and crown diameter were mea- sured on six shrubs (three per belt) per species in May, July, and October 2011. On the same sam- pling dates, leaf area index (LAI) was measured by a ceptometer (AccuPAR LP-80, Decagon Devices, Inc., Pullman, Washington, U.S.) at 1 m above the ground. In October, the total leaf area of four shrubs per species was measured by destructive harvest. The depositions of trace metals on the whole plant were then calculated by multiplying the deposi- tions per unit leaf area and the whole plant leaf area. Morphology of the leaf surface and presence of particulate matter on the abaxial and adaxial sur- faces were observed on ten leaves per species from one sampling event in July using an ESEM (electron scanning environmental microscope) Quanta 200, (Fei Corporation, Eindhoven, The Netherlands), operating in low-vacuum mode (chamber pressure was kept at 1 Torr) and 25 kV. Leaves to be observed by ESEM were harvested using separate disposable gloves for each species; leaves were harvested from the petioles, avoiding contact with the leaf blade. Aſter harvesting, leaves were carefully placed in disposable paper bags and immediately processed. Meteorological parameters (rainfall, wind speed, wind direction, relative humidity, and air- temperature) were measured continuously from June to October 2011 at 1.5 m height, using an iMetos® SD meteorological station (Pessl Instru- ments, Weiz, Austria) positioned between the vegetation belts (15 m distant from each belt). Statistical Analysis Growth parameters and water relationships were analyzed using one-way (Exp. 1) or two- way ANOVA (Exp. 2). Leaf gas exchange data were analyzed using repeated measures ANOVA. Mean separation was carried out using Duncan’s MRT (SPSS statistics 19, IBM Company, New York, U.S). Differences in trace metal deposi- ©2016 International Society of Arboriculture
September 2016
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