Can Labile Soil Carbon be Used to Improve the Rapid Urban Site Index Assessment Tool for Tree Plantings in the Urban Landscape?.

Urban trees have the capacity to provide valuable services such as energy savings, removal of air pollutants, carbon sequestration, and aesthetic value. Large, older urban trees are desired to receive full benefits of these services. An urban site assessment tool will aid managers to understand site conditions, and match species tolerances to increase urban forest diversity and urban tree health. The Rapid Urban Site Index (RUSI) is an in-field assessment tool to assess soil and site quality for urban tree planting spaces. The RUSI model includes 15 site and soil properties and was tested on 360 tree planting spaces in nine cities throughout the USA. The RUSI model was significantly associated with urban tree health metrics (P<0.0001; R2 = 0.18 to 0.40); but, less than half of the variation was explained with the RUSI model. The objective of this research is to identify a more sensitive indicator of soil biological properties to improve the RUSI model and explain more of the variation in tree health in urban planting sites. This project assessed four labile C assessments: Solvita respiration, respiration via NaOH base traps, potassium permanganate oxidizable C (POX-C) and particulate organic matter (POM). The RUSI model, urban tree health, microbial biomass C and labile C assessments were measured on 45 urban tree planting sites in Milwaukee, Green Bay, and Stevens Point, WI. The data is being analyzed to determine if any of the labile C assessments are associated with microbial biomass C and urban tree health. Future research will include these labile C assessments in the RUSI model to see if it can improve its ability to relate tree health in urban planting sites. A practical and accurate site index for the urban forests can improve urban tree diversity and health, maximizing urban forest ecosystem services.