©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(2): March 2023 103 The urban factors in the RUSI model are traffic (TRA), infrastructure (INF), and surface (SUR). The TRA score was based on the number of lanes and amount of parking available on the street. More lanes and less parking indicate more traffic, likely faster-moving automobiles, and more of an “urban” impact (e.g., road salts, recent soil disturbance) on the site. The INF score was based on the distance to the nearest hard-space or building from the main stem of the tree. The SUR score was based on the type of ground covering for the majority (greater than 50%) of the rooting area for the tree. Soil physical factors include texture (TEX), structure (STR), and penetration (PEN). Texture reflects the relative par- ticle size distribution and is determined by the feel method. Structure is the shape of the soil aggregates present. Meth- ods for assessing soil texture by the feel method and structure shape are described in Schoeneberger et al. (2012) and Scharenbroch and Watson (2014). Penetration was assessed by recording the depth and ease that the core sampler went into the soil when collecting samples. The soil chemical factors were pH, electrical conductivity (EC), and soil organic matter (SOM). Soil pH and EC were measured on homogenized subsamples at each site using a handheld combination pH/EC meter. For this research, the Oakton PCTestr 35 (OAKTON Instruments, Vernon Hills, IL, USA) was used. Soil organic matter was estimated using the Color Chart for Estimating Organic Matter in Mineral Soils of Illinois (Alexander 1971). The soil biological factors were estimated rooting area (ERA), depth of the A-horizon or topsoil (HOR), and wet aggregate stability (WAS). Estimated rooting area was an evaluation of the surface permeable space for root growth. The ERA score was increased by 1 to a maximum of 3 if a breakout area of at least 50 m2 was present within 2 m of the tree. The HOR was the depth of the A-horizon or topsoil via visual inspection. The A-horizon was distinguished by darker color, a more well-developed structure, and a greater abundance of fine roots compared to the underlying horizon. Wet aggregate stability is an estimate of the strength of the aggregates to resist degradation (Nimmo and Per- kins 2002). A modified field method was used to assess WAS. A total of 5 aggregates 2 to 5 mm in diameter were placed on a 1-mm screen. The aggregates were soaked in water for 30 seconds. After 30 seconds the screen was agi- tated (i.e., a vigorous swirl) for another 30 seconds. The number and amount of aggregates left after the soak and swirl were volumetrically estimated and scored. Equation S4. Rapid Urban Site Index (RUSI) = (∑s/3n) × 100 Table S4. Parameters and scoring functions for the Rapid Urban Site Index (RUSI) model. Precipitation = PPT; growing degree days = GDD; exposure = EXP; traffic = TRA; infrastructure = INF; surface = SUR; texture = TEX; structure = STR; penetration = PEN; A-horizon = HOR; estimated rooting area = ERA; wet aggregate stability = WAS; soil organic matter = SOM; electrical conductivity = EC; sand = S; sandy clay = SC; silt = SI; sandy loam = SL; clay = C; loam = L; clay loam = CL; loamy sand = LS; sandy clay loam = SCL; silty clay = SIC; silt loam = SIL; silty clay loam = SICL; coarse fragment (>2 mm in diameter) = CF; massive = M; single-grained = SG; platy = PL; angular blocky = ABK; subangular blocky = SBK; granular = GR. RUSI Units 0 1 2 3 PPTa mm yr -1 < 500 500 to 750 751 to 1,000 > 1,000 GDD d < 1,000 1,001 to 2,500 2,501 to 4,000 > 4,000 EXP # 0 1 to 2 3 to 4 5 TRA n/a > 4 lanes 2 to 4; no parking 2 to 4; parking < 2 lanes INF m < 1 1 to 5 6 to 10 > 10 SUR n/a nonpermeable or bare patchy vegetation thick vegetation organic mulch TEX n/a no soil; CF > 75% S, SI, C; CF = 50% to 75% LS, SCL, SICL, CL, SC, SIC; CF = 25% to 49% SL, SIL, L; CF < 25% STR n/a M, SG, PL ABK SBK GR PEN cm < 5 5 to 20 20 with max effort 20 with min effort
March 2023
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