©2023 International Society of Arboriculture 92 directions and then calculated using the equations presented in Moser et al. (2015). Site quality was assessed by a single primary investigator at each sample plot using the RUSI in the spring and fall of 2017 (Table 2). The RUSI uses cli- matic, urban, soil physical, soil chemical, and soil biological factors to provide an index (0 to 100) of urban site quality (Scharenbroch et al. 2017). Embed- ded in each of these main factors are 3 parameters. Individual parameters were assessed in the field and scored on a 0 to 3 scale using the scoring functions described in Scharenbroch et al. (2017). Observed scores were summed, divided by the maximum pos- sible score, and then multiplied by 100 to compute the RUSI score. Soil Collection, Treatment, and Analyses During each site visit, 20 soil cores 2.5 cm wide by 15 cm deep were randomly collected throughout each sample plot. Cores were composited by plot, placed in individually labeled plastic bags, and kept on ice in in each city from planting data and tree inventories. A target tree age of 5 to 12 years old was selected to avoid trees that might still be under transplant stress. The most common species planted in this age cohort in all 3 cities was Tilia spp. and thus was chosen as the tree species for this experiment. Sample plots were defined as a single tree and the surrounding 9.3-m2 circular or rectangular planting area. In Stevens Point and Green Bay, 15 plots were rectangular shaped between the street and the sidewalk, with the other 15 plots circular shaped (not bound by a sidewalk). In Milwaukee, all of the study sites were rectangular shaped between the street and sidewalk. Field Assessments Urban tree performance was assessed by a single pri- mary investigator using urban tree growth and health metrics (Table 1). Tree performance evaluations were done independently of the site assessments to limit bias. The urban tree health metrics included tree con- dition (TC), tree condition index (TCI), and urban tree health (UTH), as used by Scharenbroch et al. (2017). Tree health was also assessed by measuring leaf chlorophyll contents of 12 leaves per tree using a SPAD meter (SPAD-502, Konica Minolta, Tokyo, Japan). These 12 leaves were collected on 4 sides of the tree from equally distributed branch tips through- out the bottom, middle, and top of the crown. Growth metrics included total tree height measured with a height pole and diameter at breast height, which was measured at 1.37 m and marked to ensure accurate follow-up readings. Crown volume was calculated by measuring the crown radius in each of the 4 cardinal Scharenbroch et al: Towards an Improved Rapid Urban Site Index Table 1. Mean (n = 90), standard deviations (SD), minimum values, and maximum values for tree diameter (DIA), tree height (HT), tree crown volume (CV), leaf greenness (SPAD), tree condition (TC), tree condition index (TCI), and urban tree health (UTH). Property Mean SD Minimum Maximum DIA (cm) 12.6 3.88 5.48 21.8 HT (cm) 624.0 148.0 328.0 1,002.0 CV (m3) 55.7 42.9 4.39 211.0 SPAD 38.4 6.86 20.5 57.1 TC 2.19 0.34 1.5 3.0 TCI 67.0 8.57 44.4 83.3 UTH 88.5 8.48 48.0 100.0 Table 2. Mean (n = 90), standard deviations (SD), minimum values, and maximum values for scores of precipitation (PPT), growing degree days (GDD), exposure (EXP), traffic (TRA), infrastructure (INF), surface (SUR), estimated rooting area (ERA), penetration (PEN), A-horizon (HOR), texture (TEX), structure (STR), wet aggregate stability (WAS), soil organic matter (SOM), electrical conductivity (EC), pH, and Rapid Urban Site Index (RUSI). Property Mean SD Minimum Maximum PPT 1.69 0.51 1.00 3.00 GDD 1.34 0.47 1.00 2.00 EXP 2.77 0.45 1.00 3.00 TRA 2.86 0.49 0.00 3.00 INF 0.72 0.45 0.00 1.00 SUR 1.89 0.40 1.00 3.00 ERA 2.31 0.73 0.00 3.00 PEN 1.63 0.84 0.00 3.00 HOR 2.42 0.51 1.00 3.00 TEX 2.46 0.58 1.00 3.00 STR 2.19 0.57 0.50 3.00 WAS 2.42 0.54 1.00 3.00 SOM 2.65 0.48 2.00 3.00 EC 1.12 0.79 0.00 3.00 pH 1.93 0.27 1.00 3.00 RUSI 67.6 5.47 51.1 81.1
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