Arboriculture & Urban Forestry 38(5): September 2012 to perform and cheap field diagnostic techniques are preferred to expensive laboratory measurements (Halvorson et al. 1996). Assessment of soil quality requires evaluation of the current state of an indicator in comparison with known or desired values (Karlen et al. 1997; Burger and Kelting 1999). However, estab- lished norms for urban soil properties that accurately reflect a soil’s inherent productive or environmental filtering potential do not ex- ist. Currently, it is very difficult to evaluate urban soil quality as related to soil function criteria. Multivariate statistical approaches such as principal component analyses (PCA) provide non-subjec- tive means to extract and weight information in complex univari- ate data sets and are appropriate first steps towards soil quality assessments (Halvorson et al. 1995; Wander and Bollero 1999). In this study, physical, chemical, and biological properties contributing to urban soil quality were evaluated in relation to tree performance. The specific objectives were to 1) sample 84 plots in five locations in the western suburban area of Chicago, Illinois, U.S.; 2) characterize soils (17 physical, 17 chemical, and 14 biological responses) and trees (9 performance responses); 3) establish MDS and an urban soil quality index (USQI) for assess- ing urban soil quality; 4) test the MDS and USQI for predicting urban tree performance; and 5) discuss the mechanism(s) driv- ing soil quality and tree performance in these urban landscapes. MATERIALS AND METHODS Site Description Sampling occurred at four residential homeowner’s associa- tions (sites): Arboretum Estates in Glen Ellyn, IL; Baker Hill in Glen Ellyn, IL; River Oaks in Warrenville, IL; and Stonebridge in Aurora, IL; and at The Morton Arboretum in Lisle, IL; all located approximately 20 km west of Chicago, IL (Appendix 1). The four residential homeowner associations were selected from a larger group based on similarities in human impacts, soil management, topography, parent material, and age. Surveys of all trees in roadside planting strips were performed at each of the four homeowner’s associations. Street trees were located in the space between the sidewalk and the road. In a few plots without sidewalks or with sidewalks up to road, street trees were in the front yard within 10 m of the street. On each site, approximately 25% of the street tree population was sampled. The major species representing each site were identified and at least nine and up to 26 trees per species were then randomly selected from that population for sampling. The Morton Ar- boretum was included as a fifth site to provide information on older, less-disturbed urban landscapes of similar parent materi- al, topography, and management. Twelve arboretum trees were randomly selected from the collections list to match Fraxinus and Quercus genera found in the homeowner’s associations. Soils at the residential locations are classified as urban land, built up areas and deep, gently rolling to nearly level, moderately to poorly drained soils that have clayey subsoil and formed in glacial till (Mapes 1979). Typical native soils in these areas and at the arboretum include forest (e.g., Markham series) to prairie (e.g., Ashkum series) soils. The Markham series are fine, illitic, mesic Mollic Oxyaquic Hapludalfs, consisting of very deep, mod- erately well drained soils on Wisconsin, U.S., till plains formed in thin layers of loess or silty material and in the underlying silty clay loam till. Ashkum series are fine, mixed, superactive, mesic 215 Typic Endoaquolls, consisting of very deep, poorly drained soils on till plains formed in colluvial sediments and in the underlying silty clay loam till. Mean annual precipitation is about 890 mm, and mean annual air temperature is about 10°C (Mapes 1979). Soil management and human impacts at the residential sites were attained from interviews with managers from each home- owner’s association. All sites were developed in a similar fash- ion, which is typical of the region. At site development, topsoil (c.a., 0 to 25 cm) was removed, and the subsoil was graded and compacted to standard densities to support the infrastruc- ture (c.a., 1.5 to 1.7 Mg m-3 ). Nominal depths (3 to 6 cm) of topsoil were replaced on the sites after construction. The ages of urban landscapes were inferred from manager interviews and tree cores from the sites. Site disturbance at the arbore- tum was relatively minimal and trees were planted in soils that were not truncated, buried, graded, or compacted. Tree plant- ing and care on all sites was performed by qualified arborists. Tree fertilization and irrigation was performed according to standards and best management practices during the tree es- tablishment period (two to three years). Tree fertilization was not performed on any of the trees in the three years leading up to sampling. Granular NPK fertilizer was applied annually in the spring, following manufacturer’s label, to turf areas in the residential associations, but not in the arboretum. No trees re- ceived irrigation, herbicides, or pesticides in any of the sites, including the arboretum trees, over at least the past five years. Trees in all sites were pruned as needed to develop ideal form and remove dead wood, typically at three- to five-year pruning cycles. Tree ages ranged from 5- to 86-years-old, trunk diam- eters from 7 to 80 cm, and heights from 4 to 27 m (Appendix 1). At each individual tree plot the following was measured and recorded: address, GPS coordinates, slope, aspect, distance to nearest building, distance to nearest hardspace, estimation of plantable space, and the percent of that space covered by ce- ment, tar, rock, mulch, grass, and herbaceous/shrub plants (Ap- pendix 1). In estimating plantable space, it was assumed that tree roots were able to grow beneath sidewalks, but researchers did not extend this beyond buildings, driveways, and roads. Maxi- mum plantable space was set at 3,848 m2 (35 m radius), which is approximately 10 times the maximum canopy surface area. Soil Assessment The eighty-four plots were sampled between July and August in 2010. All soil sampling was performed at random locations under the tree drip line. Many of the field assessments of soil quality were adapted from Doran’s soil test kit procedure (Sar- rantonio et al. 1996) and the field book for describing and sam- pling soils (Schoeneberger et al. 2002). Given the uncertainty and heterogeneity of urban soils, the study includes a wide and exhaustive range of soil properties from which a small subset is selected for relating tree performance. Certain soil properties are known to be highly variable (e.g., moisture and tempera- ture) or influenced by other soil properties (e.g., penetration re- sistance); however, these properties were included in the initial characterization given reoccurrence in the literature to assess to soil quality, ease of measurement, and potential ability to cor- relate with more time-consuming or expensive soil assessments. Soil physical observations were performed on a 25 cm × 25 cm × 20 cm deep excavation on each plot. Soil color (hue, value, ©2012 International Society of Arboriculture
September 2012
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