Arboriculture & Urban Forestry 46(2): March 2020 91 Table 4. Tukey multiple comparison test for variables mean and source of variation. Variables Source of variation Coverage Level 20 40 60 80 100 Time of day Season Morning Midday Evening Autumn Spring Summer PM10 (mg/m3 Max 0.053ab 0.062b 0.064b 0.045a 0.06b 0.079b 0.040a 0.053b 0.110c 0.037b 0.025a ) Mean 0.033a 0.037ab 0.039b 0.032a 0.041b 0.045b 0.026a 0.038b 0.076c 0.021b 0.012a Noise (dBA) Max 73.4b 77.1d 75.7c 71.6a 71.1a 74.5b 73.4a 73.4a 73.5a 74.5b 73.3a Min 57.5a 57.6a 57.8a 58.2a 57.1a 59.0c 56.4a 57.5b 57.8b 58.6c 56.5a Temperature (°C) Ground 21.3b 19.6a 19.2a 19.3a 18.8a 13.1a 23.7c 22.1b 15.0a 19.4b 24.4c UV (nm) RH (%) 1.5 m 20.1b 19.0a 18.8a 18.9a 18.5a 12.7a 22.6c 21.9b 15.0a 19.0b 23.2c 377.0b 275.0ab 234.0ab 153.0a 144.0a 50.2a 632.0b 28.1a 41.2a 230.0b 439.0c Results with significant differences are colored grey. Subscripts with different letters indicate significant differences (probability < 0.05). than 2.5 kilometers. With a larger spatial extent and point pair separations, it’s likely that significant dif- ferences would be found. On the other hand, it must be taken into consideration that a quantity of particu- late material is raised from ground level by the pas- sage of different types of automobiles, for which raised-level readings could be expected in times of greater traffic. In numerical terms, if the percentage reduction between the smallest and largest tree coverage is compared, a decrease in PM10 of 6.4% and 0.3% for maximum and average measurements respectively can be observed (Table 1). According to the studies carried out by McDonald et al. (2007) and Vos et al. (2013), the reduction of PM10 associated with canopy coverage can fluctuate between 2% and 26%, which coincides with the maximum value of the variable recorded in this study. For the season of year, the largest values of partic- ulate matter and relative humidity were recorded in autumn, followed by spring, and then summer (Tables 1 and 4). The high values in particulate matter are explained by two reasons: one is the climatic condi- tions (known as thermal inversion phenomenon), and the second is the use of firewood as a fuel to heat homes and the relative humidity of summer condi- tions in a continental Mediterranean climate (MMA 2011). On the other hand, regarding the time of day when measurements were recorded, maximum values of particulate matter and relative humidity were recorded in the morning and the evening, whereas at midday lower values were recorded. The temperature and ultraviolet radiation mea- surements showed maximum values in summer, fol- lowed by spring, and then autumn (Tables 1 and 4). When time of day is considered, the maximum value was observed at midday, followed by evening, and then morning, similar to the result found by Scott et al. (1999). The measurements for these variables, as with particulate matter and relative humidity, showed a decreasing tendency as the tree coverage increased. However, with the measurements of the noise, no ten- dency related to season, time of day, or canopy cover- age was observed. This finding is consistent with the studies of Posada et al. (2009) and Pudjiwati et al. (2013), which state that tree and vegetation coverage must be significant so as to reduce noise levels, a sit- uation that is not present in this study. The analysis of the season when measurements were made showed maximum values for all variables in the autumn season, with the exception of tempera- tures and ultraviolet radiation. Even though the presented results confirm existing studies and our hypothesis, the challenges encoun- tered in finding the relationships between the vari- ables require that the study should be enhanced to include more plots, other areas within the city, and other times of day and months of year. This is ©2020 International Society of Arboriculture 54.4a 59.6bc 60.2bc 58.5b 61.2c 79.0c 45.9a 51.4b 71.0c 55.3b 49.9a
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