214 Scharenbroch and Catania: Soil Quality Attributes as Indicators of Urban Tree Performance Arboriculture & Urban Forestry 2012. 38(5): 214–228 Soil Quality Attributes as Indicators of Urban Tree Performance Bryant C. Scharenbroch and Michelle Catania Abstract. Soil quality assessments are needed to improve a professional’s ability to manage urban soils and trees. This research was conducted to identify which soil properties are most useful for relating information on urban tree performance. In total, 48 soil properties were measured at 84 sites from five urban landscapes in the western suburbs of Chicago, Illinois, U.S. Key physical, chemical, and biological properties to be included in a minimum data set (MDS) for assessing urban soil quality were identified using statistical approaches and practical considerations. The MDS included: texture, bulk density, wet-aggregate stability, pH, electrical conductivity, soil organic matter (SOM), and particulate organic matter. The MDS was used to establish an urban soil quality index (USQI). The MDS and USQI were highly correlated with tree size attributes of height, trunk diameter, crown area, and age. Correlations between the MDS and USQI with trunk diameter growth rate, height growth rate, foliar N, and chlorophyll content were often significant, but less strong. Among the MDS parameters, SOM, pH, and texture appear to be the most informative measures for soil quality relat- ing to urban tree performance. Soil quality and tree performance increased logarithmically following site disturbance, with a plateau after 50 years. Key Words. Minimum Data Set; Organic Matter; pH, Texture; Tree Growth; Urban Site Index. Soil quality is defined as the capacity of soil to function (Karlen et al. 1997). Some important soil functions include: water and solute flow and retention, physical stability and support, reten- tion and cycling of nutrients, buffering and filtering of toxic materials, and maintenance of biodiversity and habitat (Larson and Pierce 1994; Doran et al. 1996). The term soil quality also refers to the effects of human use and management on these soil functions (Doran and Jones 1996; Seybold et al. 1999). As a consequence of anthropogenic influences and manage- ment practices, the quality of urban soils is commonly impaired. Urban soils often have high bulk densities and low porosities, poor soil structure, altered water status and redoximorphic fea- tures, elevated pH and salinity, environmental contaminants, re- duced organic matter contents, and altered microbial populations (e.g., Short et al. 1986; Craul 1999; Scharenbroch et al. 2005; Scheyer and Hipple 2005; Pouyat et al. 2007). Degraded soil con- ditions constrain urban tree growth and health (Craul 1992; Wat- son and Neely 1994; Neely and Watson 1998; Watson et al. 2008). Assessment and improvement of urban soil quality is impera- tive for the establishment, growth, and longevity of urban trees. Urban tree performance (e.g., establishment, growth, lon- gevity) is influenced by interactions of edaphic, genetic, cli- matic, and anthropogenic factors (Kozlowski 1971a; Kozlows- ki 1971b; Harris et al. 1999). Foresters commonly express the quality of the site in terms of site index or the average height of dominant trees at age 50. Techniques for estimating site qual- ity have emphasized analysis of soil properties because site quality within moderately broad geographic areas appear to be controlled more by soil characteristics than by climatic fac- ©2012 International Society of Arboriculture tors (Colie and Schumacher 1953). Soil quality indicators are commonly used to predict or assess forest stand productiv- ity (Burger and Kelting 1999; Woolery et al. 2002). To date, no such approaches have been developed for urban trees and soils. Indices of forest soil quality are most useful if they incorpo- rate soil physical, chemical, and biological properties, and are sensitive to management-induced changes, easily measured, inexpensive, relevant across sites and over time, and adaptable for specific ecosystems (Schoenholtz et al. 2000). Soil physi- cal properties, especially those with direct impact on soil mois- ture, have been shown to have large impacts on tree growth (e.g., Zahner and Stage 1966; Schoenholtz et al. 2000; Wool- ery et al. 2002; Galvez et al. 2004; de Castilho et al. 2006). The importance of soil chemical properties to tree growth is represented by the voluminous literature showing the stimulat- ing effects of fertilizers on growth of trees (e.g., Himelick et al. 1965; Schoenholtz et al. 2000; Udawatta and Henderson 2003; Galvez et al. 2004; Hamel et al. 2004). Soil biological factors influence both soil tilth and fertility (Knoepp et al. 2000), and need be considered for their impacts on urban tree growth. Minimum data sets (MDS) of soil parameters and methods to use for assessing soil quality have been identified (e.g., Ar- shad and Coen 1992; Doran and Parkin 1994). These indica- tors should: correlate well with ecosystem processes, integrate soil properties and processes, be accessible to many users, be sensitive to management and climate, and when possible, be components of existing databases (Doran and Parkin 1994). Minimum data sets should include soil physical, chemical, and biological properties (Gregorich et al. 1994). Relatively easy
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